Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label hydrogeology. Show all posts
Showing posts with label hydrogeology. Show all posts

Guest Blog: How Speleothems Are Used To Determine Past Climates?

About author: Alex Graham is an undergraduate student at University of Newcastle, Australia. He is interested in Geology as a whole but his major interests include fluvial processes, karst systems and ocean science. During his visit to New Zealand, he has obeserved the glow worms in Waitomo Caves and spelunking in Nikau Caves.

Speleothems, more commonly known as stalactites or stalagmites, consist of calcium carbonate (calcite or aragonite) crystals of various dimensions, ranging from just a few micrometers to several centimetres in length, which generally have their growth axis perpendicular to the growth surface. Speleothems are formed through the deposition of calcium carbonate minerals in karst systems, providing archives of information on past climates, vegetation types and hydrology, particularly groundwater and precipitation. However, they can also provide information on anthropogenic impacts, landscape evolution, volcanism and tectonic evolution in mineral deposits formed in cave systems.


Stalagmite Formation
Rainfall containing carbonic acid weathers the rock unit (generally either limestone or dolomite) and seeps into the cracks, forming caverns and karst systems. The groundwater, percolating through such cracks and caverns, also contains dissolved calcium bicarbonate. The dripping action of these groundwater droplets is the driving force behind the deposition of speleothems in caves.
Core drilling of an active stalagmite in Hang Chuot cave.
Speleothems are mainly studied as paleoclimate indicators, providing clues to past precipitation, temperature and vegetation changes over the past »500,000 years. Radioisotopic dating of speleothems is the primary method used by researchers to find annual variations in temperature. Carbon isotopes (d^13C) reflect C3/C4 plant compositions and plant productivity, where increased plant productivity may indicate greater amounts of rainfall and carbon dioxide absorption. Thus, a larger carbon absorption can be reflective of a greater atmospheric concentration of greenhouse gases. On the other hand, oxygen isotopes (d^8O) provide researchers with past rainfall temperatures and quantified levels of precipitation, both of which are used to determine the nature of past climates.


Stalactite and stalagmite growth rates also indicate the climatic variations in rainfall over time, with this variation directly influencing the growth of ring formations on speleothems. Closed ring formations are indicative of little rainfall or even drought, where-as wider spaced ring formations indicate periods of heavy rainfall or flooding. These ring formations thus enable researchers to potentially predict and model the occurrence of future climatic patterns, based off the atmospheric signals extrapolated from speleothems. Researchers also use Uranium –Thorium radioisotopic dating, to determine the age of speleothems in karst formations. Once the layers have been accurately dated, researchers record the level of variance in groundwater levels over the lifetime of the karst formation. Hydrogeologists specialise in such areas of quantitative research. As a result, speleothems are widely regarded as a crucial geological feature that provide useful information for researchers studying past climates, vegetation types and hydrology.


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The Messinian Salinity Crisis


You will have heard of The Messinian Salinity Crisis no doubt. From learned articles, geology textbooks, probably lectures at your college or University. Or possibly not. This was not always the hot topic it is now. In fact, the very idea of this happening, was for a while, challenged, even ridiculed. It seemed too incredible that this could happen as it did and Dessication/Flood theories took time to gain traction. But, if you had heard about it, you would remember that The Messinian Salinity Crisis, was a time when the Mediterranean Sea, very much as we know it today, evaporated – dried out, almost completely.



You will have heard of the rates of desiccation, influx and yet more desiccation, repeated in endless cycles over tens, even hundreds of thousands of years. On a human temporal scale, this would have been a long drawn out affair, covering a time hundreds of generations deep, more than the span of Homo sapiens existence. In Geologic terms however, it was a string of sudden events. Of incredibly hot and arid periods followed by rapid ingress of waters, either via spillways through what is now modern day Morocco and the southern Iberian peninsular, or headlong through a breach in the sill between the Pillars of Heracles, the modern day Straights of Gibraltar.


There were prolonged periods of dessication, of desolate landscapes beyond anything seen today in Death Valley or The Afar Triangle. These landscapes were repeatedly transgressed by brackish waters from storm seasons far into the African and Eurasian interiors, or the Atlantic, and these in turn dried out. Again and again this happened. It had to be so because the vast deposits of rock salt, gypsum and anhydrites could not have been emplaced in a single evaporite event. The salt deposits in and around the Mediteranean today represent fifty times the current capacity of this great inland sea. You may have heard too of the variety of salts production, as agglomerating crystals fell from the descending surface to the sea floor, or as vast interconnected hypersaline lakes left crystalline residues at their diminishing margins, as forsaken remnant sabkhas, cut off from the larger basins, left behind acrid dry muds of potassium carbonates – the final arid mineral residue of the vanished waters.


Just under six million years ago, Geologic processes isolated what was left of the ancient Tethys ocean, the sea we know as the Mediterranean, home to historic human conflicts and marine crusades of Carthage, Rome, Athens and Alexandria, a Sea fringed by modern day Benidorm, Cyprus, Malta and Monaco. At a time 5.96 million years ago – evaporation outpaced replenishment. Indeed, just as it does today, but without the connecting seaway to replenish losses. Inexorable tectonic activity first diverted channels, then – sealed them. Cut off from the Atlantic in the West, water levels fell, rose briefly and fell again, and again. The mighty Nile - a very different geophysical feature of a greater capacity than today, and the rivers of Europe cut down great canyons hundreds and thousands of metres below present Eustatic sea and land surface levels, as seismic cross sections show in staggering detail. The cores taken at depth in the Mediterranean, show Aeolian sands above layers of salt, fossiliferous strata beneath those same salts, all indicating changing environments. The periods of blackened unshifting desert varnished floors and bleached playas, decades and centuries long, were punctuated often by catastrophic episodes, with eroded non conformable surfaces of winnowed desert pavement, toppled ventifracts, scours and rip up clasts. Species of fossilised terrestrial plant life, scraping an arid existence have been found, thousands of meters down, in the strata of the Mediterranean sea floor.
 


There is much evidence too, in the uplifted margins of Spain, France, and Sicily, of those hostile millennia when the sea disappeared. Incontrovertible evidence, painstakingly gathered, analysed and peer reviewed, demonstrates via the resources of statistical analysis, calculus and geophysical data that the Messinian Salinity Crisis was a period during the Miocene wherein the geology records a uniquely arid period of repeated partial and very nearly complete desiccation of the Mediterranean Sea over a period of approximately 630,000 years. But for the Geologist, the story doesn’t end there. The Geologists panoptic, all seeing third eye, sees incredible vistas and vast panoramas. Of a descent from the Alpine Foreland to the modern day enclave of Monaco, gazing out southwards from a barren, uninhabited and abandoned raised coast to deep dry abyssal plains, punctuated by exposed chasms, seamounts and ridges, swirling and shifting so slowly in a distant heat haze. A heat haze produced by temperatures far above any recorded by modern man and his preoccupation with Global Warming. An unimaginable heat sink would produce temperatures of 70 to 80 degrees Celsius at 4000M depth within the basins. 




Looking down upon this Venusian landscape, the sun might glint on remaining lakes and salt flats so very far away and so very much farther below. Hills and valleys, once submerged, would be observed high and dry – from above, as would the interconnecting rivers of bitter waters hot enough to slowly broil any organism larger than extremophile foraminifer. All this, constantly shimmering in the relentless heat. Only the imagination of the geologist could see the vast, hellish, yet breathtaking landscape conjured up by the data and the rock record. And finally, the Geologist would visualise a phenomenon far greater in scope and magnitude than any Biblical flood – The Zanclean Event.
Also known as The Zanclean Deluge, when the drought lasting over half a million years was finally ended as the Atlantic Ocean breached the sill/land bridge between Gibraltar and North West Africa. Slowly perhaps at first until a flow a thousand times greater than the volumetric output of the Amazon cascaded down the slopes to the parched basins. Proximal to the breach, there would be a deafening thunderous roar and the ground would tremor constantly, initially triggering great avalanches above and below the Eustatic sea level as the far reaching and continuous concussion roared and rumbled on, and on, and on. For centuries great cataracts and torrents of marine waters fell thousands of metres below and flowed thousands of kilometers across to the East. Across to the abyssal plains off the Balearics, to the deeps of the Tyrrhenian and Ionian seas, into the trenches south of the Greek Islands and finally up to the rising shores of The Lebanon. The newly proximal waters to the final coastal reaches and mountains that became islands, must have had a climatological effect around the margins of the rejuvenated Mediterranean. Flora and Fauna both marine and terrestrial will have recolonised quickly. Species may have developed differently, post Zanclean, on the Islands. And in such a short period, there must surely have been earthquakes and complex regional depression and emergence. Isostacy compensated for the trillions of cubic meters of transgression waters that now occupied the great basins between the African and Eurasian plates, moving the land, reactivating ancient faults and within and marginal to the great inland sea, a region long active with convergent movements of a very different mechanism.

Hollywood and Pinewood have yet to match the imagination of the Earth Scientist, of the many chapters of Earths dynamic history held as fully tangible concepts to the men and women who study the rocks and the stories they tell. The movies played out in the mind of the geologist are epic indeed and – as we rightly consider the spectre of Global Warming, consider too the fate of future populations (of whatever evolved species) at the margins of the Mediterranean and the domino regions beyond, when inexorable geologic processes again isolate that benign, sunny holiday sea. Fortunately, not in our lifetime, but that of our far off descendants who will look and hopefully behave very differently from Homo Sapiens.


Note: This blog is written and contributed by Paul Goodrich. You can also contribute your blog or article on our website. See guidelines here.

10 of the Best Learning Geology Photos of 2016

A picture is worth a thousand words, but not all pictures are created equal. The pictures we usually feature on Learning Geology are field pictures showing Geological structures and features and many of them are high quality gem and mineral pictures. The purpose is to encourage students and professionals' activities by promoting "learning and scope" of Geology through our blogs.
In the end of 2016, we are sharing with you the 10 best photos of 2016 which we have posted on our page.

P.S: we always try our best to credit each and every photographer or website, but sometimes it’s impossible to track some of them. Please leave a comment if you know about the missing ones.

1. Folds from Basque France

 Image Credits: Yaqub ShahYaqub Shah

2. Horst and Graben Structure in Zanjan, Iran


Image Credits: https://www.instagram.com/amazhda



3. A unique Normal Fault

4. The Rock Cycle
The
 rock cycle illustrates the formation, alteration, destruction, and reformation of earth materials, and typically over long periods of geologic time. The rock cycle portrays the collective system of processes, and the resulting products that form, at or below the earth surface.The illustration below illustrates the rock cycle with the common names of rocks, minerals, and sediments associated with each group of earth materials: sediments, sedimentary rocks, metamorphic rocks, and igneous rocks.


Image Credits: Phil Stoffer


5. An amazing Botryoidal specimen for Goethite lovers! 


Image Credits: Moha Mezane 
   

6. Basalt outcrop of the Semail Ophiolite, Wadi Jizzi, Oman

Image Credits: Christopher Spencer
Christopher Spencer is founder of an amazing science outreach program named as Traveling Geologist. Visit his website to learn from him


7. Val Gardena Dolomites, Northern Italy





8. Beautiful fern fossil found in Potsville Formation from Pennsylvania.
The ferns most commonly found are Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum.


Image Credits: Kurt Jaccoud


9. Snowball garnet in schist

Syn-kinematic crystals in which “Snowball garnet” with highly rotated spiral Si. 

Porphyroblast is ~ 5 mm in diameter.
From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures.



10. Trilobite Specimen from Wheeler Formation, Utah
The Wheeler Shale is of Cambrian age and is a world famous locality for prolific trilobite remains. 


Image Credits: Paleo Fossils

Groundwater Problems

Groundwater Problems

Since prehistoric times, groundwater has been an important resource that people have relied on for drinking, irrigation, and industry. Groundwater feeds the lushness of desert oases in the Sahara, the amber grain in the North American high plains, and the growing cities of sunny arid regions. 
Though groundwater accounts for about 95% of the liquid freshwater on the planet, accessible groundwater cannot be replenished quickly, and this leads to shortages. Groundwater contamination is also a growing tragedy. Such pollution, caused when toxic wastes and other impurities infiltrate down to the water table, may be invisible to us but may ruin a water supply for generations to come. In this section, we’ll take a look at problems associated with the use of groundwater supplies. 

Depletion of Groundwater Supplies 

Is groundwater a renewable resource? In a time frame of 10,000 years, the answer is yes, for the hydrologic cycle will eventually resupply depleted reserves. But in a time frame of 100 to 1,000 years the span of a human lifetime or a civilization groundwater in many regions may be a non-renewable resource. By pumping water out of the ground at a rate faster than nature replaces it, people are effectively “mining” the groundwater supply. In fact, in portions of the desert Sunbelt region of the United States, supplies of young groundwater have already been exhausted, and deep wells now extract 10,000-year old groundwater. Some of this ancient water has been in rock so long that it has become too mineralized to be usable. A number of other problems accompany the depletion of groundwater.

Effects of human modification of the water table.
  • Lowering the water table: When we extract groundwater from wells at a rate faster than it can be resupplied by nature, the water table drops. First, a cone of depression forms locally around the well; then the water table gradually becomes lower in a broad region. As a consequence, existing wells, springs, and rivers, and swamps dry up (figure above a, b). To continue tapping into the water supply, we must drill progressively deeper. Notably, the water table can also drop when people divert surface water from the recharge area. Such a problem has developed in the Everglades of southern Florida, a huge swamp where, before the expansion of Miami and the development of agriculture, the water table lay at the ground surface (figure above c, d). Diversion of water from the Everglades’ recharge area into canals has significantly lowered the water table, causing parts of the Everglades to dry up.
  • Reversing the flow direction of groundwater: The cone of depression that develops around a well creates a local slope to the water table. The resulting hydraulic gradient may be large enough to reverse the flow direction of nearby groundwater (figure below a, b). Such reversals can allow contaminants, seeping out of a septic tank, to contaminate the well.
  • Saline intrusion: In coastal areas, fresh groundwater lies in a layer above saline (salty) water that entered the aquifer from the adjacent ocean (figure below c, d). Because fresh water is less dense than saline water, it floats above the saline water. If people pump water out of a well too quickly, the boundary between the saline water and the fresh groundwater rises. And if this boundary rises above the base of the well, then the well will start to yield useless saline water. Geologists refer to this phenomenon as saline intrusion. 
  • Pore collapse and land subsidence: When groundwater fills the pore space of a rock or sediment, it holds the grains apart, for water cannot be compressed. The extraction of water from a pore eliminates the support holding the grains apart, because the air that replaces the water can be compressed. As a result, the grains pack more closely together. Such pore collapse permanently decreases the porosity and permeability of a rock, and thus lessens its value as an aquifer (figure below e, f).
Some causes of groundwater problems.
Pore collapse also decreases the volume of the aquifer, with the result that the ground above the aquifer sinks. Such land subsidence may cause fissures at the surface to develop and the ground to tilt. Buildings constructed over regions undergoing land subsidence may themselves tilt, or their foundations may crack. In the San Joaquin Valley of California, the land surface subsided by 9 m between 1925 and 1975, because water was removed to irrigate farm fields.

Natural Groundwater Quality 

Much of the world’s groundwater is crystal clear, and pure enough to drink right out of the ground. Rocks and sediment are natural filters capable of removing suspended solids these  solids get trapped in tiny pores or stick to the surfaces of  clay flakes. In fact, the commercial distribution of bottled groundwater (“spring water”) has  become a major business worldwide. But dissolved chemicals, and in some cases methane, may make some natural groundwater unusable. For example, groundwater that has passed through salt-containing strata may become salty and unsuitable for irrigation or drinking. Groundwater that has passed through limestone or dolomite contains dissolved calcium (Ca2 ) and magnesium (Mg2 ) ions; this water, called hard water, can be a problem because carbonate minerals precipitate from it to form “scale” that clogs pipes. Also, washing with hard water can be difficult because soap won’t develop a lather. Groundwater that has passed through iron-bearing rocks may contain dissolved iron oxide that precipitates to form rusty stains. Some groundwater contains dissolved hydrogen sulphide, which comes out of solution when the groundwater rises to the surface; hydrogen sulphide is a poisonous gas that has a rotten-egg smell. In recent years, concern has grown about arsenic, a highly toxic chemical that enters groundwater when arsenic-bearing minerals dissolve in groundwater. 

Human-Caused Groundwater Contamination 

Contamination plumes in groundwater.
As we’ve noted, some contaminants in groundwater occur naturally. But in recent decades, contaminants have increasingly been introduced into aquifers because of human activity (figure above a). These contaminants include agricultural waste (pesticides, fertilizers, and animal sewage), industrial waste (dangerous organic and inorganic chemicals), effluent from “sanitary” landfills and septic tanks (including bacteria and viruses), petroleum products and other chemicals that do not  dissolve in water, radioactive waste (from weapons manufacture, power plants, and hospitals), and acids leached from sulfide minerals in coal and metal mines. The cloud of contaminated groundwater that moves away from the source of contamination is called a contaminant plume (figure above b).
The best way to avoid such groundwater contamination is to prevent contaminants from entering groundwater in the first place. This can be done by placing contaminants in sealed containers or on impermeable bedrock so that they are isolated from aquifers. If such a site is not available, the storage area should be lined with plastic or with a thick layer of clay, for the clay not only acts as an aquitard, but it can bond to contaminants. Fortunately, in some cases, natural processes can clean up groundwater contamination. Chemicals may be absorbed by clay, oxygen in the water may oxidize the chemicals, and bacteria in the water may metabolize the chemicals, thereby turning them into harmless substances. 
Where contaminants do make it into an aquifer, environmental engineers drill test wells to determine which way and how fast the contaminant plume is flowing; once they know the flow path, they can close wells in the path to prevent consumption of contaminated water. Engineers may attempt to clean the groundwater by drilling a series of extraction wells to pump it out of the ground. If the contaminated water does not rise fast enough, engineers drill injection wells to force clean water or steam into the ground beneath the contaminant plume (figure above c). The injected fluids then push the contaminated water up into the extraction wells. 
More recently, environmental engineers have begun exploring techniques of bioremediation: injecting oxygen and nutrients into a contaminated aquifer to foster growth of bacteria that can react with and break down molecules of contaminants. Needless to say, cleaning techniques are expensive and generally only partially effective.

Unwanted Effects of Rising Water Tables 

We’ve seen the negative consequences of sinking water tables, but what happens when the water table rises? Is that necessarily good? Sometimes, but not always. If the water table rises above the level of a house’s basement, water seeps through the foundation and floods the basement floor. Catastrophic damage occurs when a rising water table weakens the base of a hillslope or a failure surface underground triggers landslides and slumps. 
Figures credited to Stephen Marshak.

Hot Springs and Geysers

Hot Springs and Geysers

 Geothermal waters and examples of their manifestation in the landscape.
Hot springs, springs that emit water ranging in temperature from about 30° to 104°C, are found in two geologic settings. First, they occur where very deep groundwater, heated in warm bedrock at depth, flows up to the ground surface. This water brings heat with it as it rises. Such hot springs form in places where faults or fractures provide a high-permeability conduit for deep water, or where the water emitted in a discharge region followed a trajectory that first carried it deep into the crust. Second, hot springs develop in geothermal regions, places where volcanism currently takes place or has occurred recently, so that magma and/or very hot rock resides close to the Earth’s surface (figure above a). Hot groundwater dissolves minerals from rock that it passes through because water becomes a more effective solvent when hot, so people use the water emitted at hot springs as relaxing mineral baths (figure above b). Natural pools of geothermal water may become brightly coloured the gaudy greens, blues, and oranges of these pools come from thermophyllic (heat-loving) bacteria and archaea that thrive in hot water and metabolize the sulphur containing minerals dissolved in the groundwater (figure above c). 
Numerous distinctive geologic features form in geothermal regions as a result of the eruption of hot water. In places where the hot water rises into soils rich in volcanic ash and clay, a viscous slurry forms and fills bubbling mud pots. Bubbles of steam rising through the slurry cause it to splatter about in goopy drops. Where geothermal waters spill out of natural springs and then cool, dissolved minerals in the water precipitate, forming colourful mounds or terraces of travertine and other chemical sedimentary rocks (figure above d).
Under special circumstances, geothermal water emerges from the ground in a geyser (from the Icelandic spring, Geysir, and the word for gush), a fountain of steam and hot water that erupts episodically from a vent in the ground (figure above e). To understand why a geyser erupts, we first need a picture of its underground plumbing. Beneath a geyser lies a network of irregular fractures in very hot rock; groundwater sinks and fills these fractures. Heat transfers from the rock to the groundwater and makes the water’s temperature rise. Since the boiling point of water (the temperature at which water vaporizes) increases with increasing pressure, hot groundwater at depth can remain in liquid form even if its temperature has become greater than the boiling point of water at the Earth’s surface. When such “superheated” groundwater begins to rise through a conduit toward the surface, pressure in it decreases until eventually some of the water transforms into steam. The resulting expansion causes water higher up to spill out of the conduit at the ground surface. When this spill happens, pressure in the conduit, from the weight of overlying water, suddenly decreases. A sudden drop in pressure causes the super-hot water at depth to turn into steam instantly, and this steam quickly rises, ejecting all the water and steam above it out of the conduit in a geyser eruption. Once the conduit empties, the eruption ceases, and the conduit fills once again with water that gradually heats up, starting the eruptive cycle all over again. 
Figures credited to Stephen Marshak.

Tapping Groundwater Supplies

Tapping Groundwater Supplies 

We can obtain groundwater at wells or springs. Wells are holes that people dig or drill to obtain water. Springs are natural outlets from which groundwater flows. Wells and springs provide welcome sources of water but must be treated with care if they are to last.

Wells 

Pumping groundwater at a normal well affects the water table.
In an ordinary well, the base of the well penetrates an aquifer below the water table (figure above a). Water from the pore space in the aquifer seeps into the well and fills it to the level of the water table. Drilling into an aquitard, or into rock that lies above the water table, will not supply water, and thus yields a dry well. Some ordinary wells are seasonal and function only during the rainy season, when the water table rises. During the dry season, the water table lies below the base of the well, so the well is dry.
To obtain water from an ordinary well, you either pull water up in a bucket or pump the water out. As long as the rate at which groundwater fills the well exceeds the rate at which water is removed, the level of the water table near the well remains about the same. However, if users pump water out of the well too fast, then the water table sinks down around the well, in a process called drawdown, so that the water table becomes a downward-pointing, cone-shaped surface called a cone of depression (figure above b, c). Drawdown by a deep well may cause shallower wells that have been drilled nearby to run dry. 

Artesian wells, where water rises from the aquifer without pumping.
An artesian well, named for the province of Artois in France, penetrates confined aquifers in which water is under enough pressure to rise on its own to a level above the surface of the aquifer. If this level lies below the ground surface, the well is a nonflowing artesian well. But if the level lies above the ground surface, the well is a flowing artesian well, and water actively fountains out of the ground (figure above a). Artesian wells occur in special situations where a confined aquifer lies beneath a sloping aquitard. 
We can understand why artesian wells exist if we look first at the configuration of a city water supply (figure above b). Water companies pump water into a high tank that has a significant hydraulic head relative to the surrounding areas. If the water were connected by a water main to a series of vertical pipes, pressure caused by the elevation of the water in the high tank would make the water rise in the pipes until it reached an imaginary surface, called a potentiometric surface, that lies above the ground. This pressure drives water through water mains to household water systems without requiring pumps. In an artesian system, water enters a tilted, confined aquifer that intersects the ground in the hills of a high-elevation recharge area (figure above c). The confined groundwater flows down to the adjacent plains, which lie at a lower elevation. The potentiometric surface to which the water would rise, were it not confined, lies above this aquifer. Pressure in the confined aquifer pushes water up a well.

Springs 

Many towns were founded next to springs, places where groundwater naturally flows or seeps onto the Earth’s surface, for springs can provide fresh, clear water for drinking or irrigation, without the expense of drilling or digging. Some springs spill water onto dry land. Others bubble up through the bed of a stream or lake. Springs form under a variety of conditions: 

Geological settings in which springs form.
  • Where the ground surface intersects the water table in a discharge area (figure above a); such springs typically occur in valley floors, where they may add water to lakes or streams. 
  • Where flowing groundwater collides with a steep, impermeable barrier, and pressure pushes it up to the ground along the barrier (figure above b). 
  • Where a perched water table intersects the surface of a hill (figure above c).
  • Where downward-percolating water runs into a relatively impermeable layer and migrates along the top surface of the layer to a hillslope (figure above d). 
  • Where a network of interconnected fractures channels groundwater to the surface of a hill (figure above e). 
  • Where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure is sufficient to drive the water to the surface; such an occurrence is an artesian spring. 
Springs can provide water in regions that would otherwise be uninhabitable. For example, oases in deserts may develop around a spring. An oasis is a wet area, where plants can grow, in an otherwise bone-dry region.
Figures credited to Stephen Marshak.

Groundwater Flow

Groundwater Flow

What happens to groundwater over time? Does it just sit, unmoving, like the water in a stagnant puddle, or does it flow and eventually find its way back to the surface? Countless measurements confirm that groundwater enjoys the latter fate groundwater indeed flows, and in some cases it moves great distances underground. Let’s examine factors that drive groundwater flow. 
In the unsaturated zone the region between the ground surface and the water table water percolates straight down, like the water passing through a drip coffee maker, for this water moves only in response to the downward pull of gravity. But in the zone of saturation the region below the water table water flow is more complex, for in addition to the downward pull of gravity, water responds to differences in pressure. Pressure can cause groundwater to flow sideways, or even upward. (If you've ever watched water spray from a fountain, you've seen pressure pushing water upward.) Thus, to understand the nature of groundwater flow, we must first understand the origin of pressure in groundwater. For simplicity, we’ll consider only the case of groundwater in an  unconfined aquifer. 

The shape of water table beneath hilly topography.
Pressure in groundwater at a specific point underground is caused by the weight of all the overlying water from that point up to the water table. (The weight of overlying rock does not contribute to the pressure exerted on groundwater, for the contact points between mineral grains bear the rock’s weight.) Thus, a point at a greater depth below the water table feels more pressure than does a point at lesser depth. If the water table is horizontal, the pressure acting on an imaginary horizontal reference plane at a specified depth below the water table is the same everywhere. But if the water table is not horizontal, as shown in above, the pressure at points on a horizontal reference plane at depth changes with location. For example, the pressure acting at point p1, which lies below the hill in figure above, is greater than the pressure acting at point p2, which lies below the valley, even though both p1 and p2 are at the same elevation. 
Both the elevation of a volume of groundwater and the pressure within the water provide energy that, if given the chance, will cause the water to flow. Physicists refer to such stored energy as potential energy. The potential energy available to drive the flow of a given volume of groundwater at a location is called the hydraulic head. To measure the hydraulic head at a point in an aquifer, hydrogeologists drill a vertical hole down to the point and then insert a pipe in the hole. The height above a reference elevation (for example, sea level) to which water rises in the pipe represents the hydraulic head water rises higher in the pipe where the head is higher. As a rule, groundwater flows from regions where it has higher hydraulic head to regions where it has lower hydraulic head. This statement generally implies that groundwater regionally flows from locations where the water table is higher to locations where the water table is lower. 

The flow of groundwater.
Hydrogeologists have calculated how hydraulic head changes with location underground, by taking into account both the effect of gravity and the effect of pressure. These calculations reveal that groundwater flows along concave-up curved paths, as illustrated in cross section (figure above a, b). These curved paths eventually take groundwater from regions where the water table is high (under a hill) to regions where the water table is low (below a valley), but because of flow-path shape, 
some groundwater may flow deep down into the crust along the first part of its path and then may flow back up, toward the ground surface, along the final part of its path. The location where water enters the ground (where the flow direction has a downward trajectory) is called the recharge area, and the location where groundwater flows back up to the surface is called the discharge area (see figure above a). 
Flowing water in an ocean current moves at up to 3 km per hour, and water in a steep river channel can reach speeds of up to 30 km per hour. In contrast, groundwater moves at less than a snail’s pace, between 0.01 and 1.4 m per day (about 4 to 500 m per year). Groundwater moves much more slowly than surface water, for two reasons. First, groundwater moves by percolating through a complex, crooked network of tiny conduits, so it must travel a much greater distance than it would if it could follow a straight path. Second, friction between groundwater and conduit walls slows down the water flow. 
Simplistically, the velocity of groundwater flow depends on the slope of the water table and the permeability of the material through which the groundwater is flowing. Thus, groundwater flows faster through high-permeability rocks than it does through low-permeability rocks, and it flows faster in regions where the water table has a steep slope than it does in regions where the water table has a gentle slope. For example, groundwater flows relatively slowly (2 m per year) through a low-permeability aquifer under the Great Plains, but flows relatively quickly (30 m per year) through a high-permeability aquifer under a steep hillslope. In detail, hydrogeologists use Darcy’s Law to determine flow rates at a location.

Darcy’s Law for Groundwater Flow 

 The level to which water rises in a drill hole is the hydraulic head (h). The hydraulic gradient (HG) is the difference in head divided by the length of the flow path.
The rate at which groundwater flows at a given location depends on the permeability of the material containing the groundwater; groundwater flows faster in a more permeable material than it does in a less permeable material. The rate also depends on the hydraulic gradient, the change in hydraulic head per unit of distance between two locations, as measured along the flow path. 
To calculate the hydraulic gradient, we divide the difference in hydraulic head between two points by the distance between the two points as measured along the flow path. This can be written as a formula:
hydraulic gradient = h1 - h2/j
where h1 - h2 is the difference in head (given in meters or feet, because head can be represented as an elevation) between two points along the water table, and j is the distance between the two points as measured along the flow path. A hydraulic gradient exists anywhere that the water table has a slope. Typically, the slope of the water table is so small that the path length is almost the same as the horizontal distance between two points. So, in general, the hydraulic gradient is roughly equivalent to the slope of the water table. 
In 1856, a French engineer named Henry Darcy carried out a series of experiments designed to characterize factors that control the velocity at which groundwater flows between two locations (1 and 2),  each of which has a different hydraulic head (h1 and h2). Darcy represented the velocity of flow by a quantity called the discharge (Q), meaning the volume of water passing through an imaginary vertical plane perpendicular to the groundwater’s flow path in a given time. He found that the discharge depends on the the hydraulic head (h1- h2); the area (A) of the imaginary plane through which the groundwater is passing; and a number called the hydraulic conductivity  (K). The hydraulic conductivity represents the ease with which a fluid can flow through a material. This, in turn, depends on many factors (such as the viscosity and density of the fluid), but mostly it reflects the permeability of the material. The relationship that Darcy discovered, now known as Darcy’s law, can be written in the form of an equation as:
Q = KA(h1 - h2)/j 
The equation states that if the hydraulic gradient increases, discharge increases, and that as conductivity increases, discharge increases. Put in simpler terms, the flow rate of groundwater increases as the permeability increases and as the slope of the water table gets steeper.
Figures credited to Stephen Marshak.

Where Does Groundwater Reside?

Where does groundwater reside?

Groundwater as we know the drinking water which is pulled out of the ground, where does it comes from?

The Underground Reservoir 

Water moves among various reservoirs during the hydrologic cycle. Of the water that falls on land, some evaporates directly back into the atmosphere, some gets trapped in glaciers, and some becomes runoff that enters a network of streams and lakes that drains to the sea. The remainder sinks or percolates downward, by a process called infiltration, into the ground. In effect, the upper part of the crust behaves like a giant sponge that can soak up water.
Of the water that does infiltrate, some descends only into the soil and wets the surfaces of grains and organic material making up the soil. This water, called soil moisture, later evaporates back into the atmosphere or gets sucked up by the roots of plants and transpires back into the atmosphere. But some water sinks deeper into sediment or rock, and along with water trapped in rock at the time the rock formed, makes up groundwater. Groundwater slowly flows underground for anywhere from a few months to tens of thousands of years before returning to the surface to pass once again into other reservoirs of the hydrologic cycle. 

Porosity: Open Space in Rock and Regolith 

Contrary to popular belief, only a small proportion of underground water occurs in caves. Most groundwater resides in  relatively small open spaces between grains of sediment or between grains of seemingly solid rock, or within cracks of various sizes. The term pore refers to any open space within a volume of sediment, or within a body of rock, and the term porosity refers to the total amount of open space within a material, specified as a percentage. For example, if we say that a block of rock has 30% porosity, then 30% of the block consists of pores. Geologists distinguish between two basic kinds of porosity primary and secondary.

Porosity is the open space in rock or sediment, whereas permeability is the degree to which the pores are connected.
Primary porosity develops during sediment deposition and during rock formation (figure above a,b). It includes the pores between clastic grains that exist because the grains don’t fit together tightly during deposition. Secondary porosity refers to new pore space produced in rocks some time after the rock first formed. For example, when rocks fracture, the opposing walls of the fracture do not fit together tightly, so narrow spaces remain in between. Thus, joints and faults may provide secondary porosity for water (figure above c). As groundwater passes through rock, it may dissolve and remove some minerals, creating solution cavities that also provide secondary porosity.

Permeability: The Ease of Flow 

If solid rock completely surrounds a pore, the water in the pore cannot flow to another location. For groundwater to flow, pores must be linked by conduits (openings). The ability of a material to allow fluids to pass through an interconnected network of pores is a characteristic known as permeability. Groundwater flows easily through a material, such as loose gravel, that has high permeability. In gravel, the water is able to pass quickly from pore to pore, so if you pour water into a gravel-filled jar, it will trickle down to the bottom of the jar, where it displaces air and fills the pores (figure above d). In tightly packed sediments or in rock, the water flows more slowly because it follows a tortuous path through tiny conduits. Water flows slowly or not at all through an impermeable material. Put another way, an impermeable material has low permeability or even no permeability. The permeability of a material depends on several factors:
  • Number of available conduits: As the number of conduits increases, permeability increases. 
  • Size of the conduits: More fluids can travel through wider conduits than through narrower ones. 
  • Straightness of the conduits: Water flows more rapidly through straight conduits than it does through crooked ones. 
Note that the factors that control permeability in rock or sediment resemble those that control the ease with which traffic moves through a city. Traffic can flow quickly through cities with many straight, multilane boulevards, whereas it flows slowly through cities with only a few narrow, crooked streets. Porosity and permeability are not the same feature. A material whose pores are isolated from each other can have high porosity but low permeability. 

Aquifers and Aquitards 

Water in the ground-aquifers, aquitards and the water table.
With the concept of permeability in mind, hydrogeologists distinguish between an aquifer, sediment or rock with high permeability and porosity, and an aquitard, sediment or rock that does not transmit water easily and therefore retards the motion of water. An aquifer that is not overlain by an aquitard is an unconfined aquifer. Water can infiltrate down into an unconfined aquifer from the Earth’s surface, and groundwater can rise to reach the Earth’s surface from an unconfined aquifer. An aquifer that is overlain by an aquitard is a confined aquifer its water is isolated from the ground surface (figure above a).

The Water Table 

Infiltrating water can enter permeable sediment and bedrock by percolating along cracks and through conduits connecting pores. Nearer the ground surface, water only partially fills pores, leaving some space that remains filled with air (figure above b). The region of the subsurface in which water only partially fills pores is called the unsaturated zone. Deeper down, water completely fills, or saturates, the pores. This region is the saturated zone. In a strict sense, geologists use the term “groundwater” specifically for subsurface water in the saturated zone, where water completely fills pores. 
The term water table refers to the horizon that separates the unsaturated zone above from the saturated zone below. Typically, surface tension, the electrostatic attraction of water molecules to each other and to mineral surfaces, causes water to seep up from the water table (just as water rises in a thin straw), filling pores in the capillary fringe, a thin layer at the base of the unsaturated zone. Note that the water table forms the top boundary of groundwater in an unconfined aquifer. 
The depth of the water table in the subsurface varies greatly with location. In some places, the water table defines the surface of a permanent stream, lake, or marsh, and thus effectively lies above the ground level (figure above c). Elsewhere, the water table lies hidden below the ground surface. In humid regions, it typically lies within a few meters of the surface, whereas in arid regions, it may lie hundreds of meters below the surface. Rainfall rates affect the water table depth in a given locality  (figure above d) the water table drops during the dry season and rises during the wet season. Streams or ponds that hold water during the wet season may, therefore, dry up during the dry season because their water infiltrates into the ground below.

Topography of the Water Table 

Factors that influence the position of the groundwater.
In hilly regions, if the subsurface has relatively low permeability, the water table is not a planar surface. Rather, its shape mimics, in a subdued way, the shape of the overlying topography (figure above a). This means that the water table lies at a higher elevation beneath hills than it does beneath valleys. But the relief (the vertical distance between the highest and lowest elevations) of the water table is not as great as that of the overlying land, so the surface of the water table tends to be smoother than that of the landscape. 
At first thought, it may seem surprising that the elevation of the water table varies as a consequence of ground-surface topography. After all, when you pour a bucket of water into a pond, the surface of the pond immediately adjusts to remain horizontal. The elevation of the water table varies because groundwater moves so slowly through rock and sediment that it cannot quickly assume a horizontal surface. When rain falls on a hill and water infiltrates down to the water table, the water table rises a little. When it doesn't rain, the water table sinks slowly, but so slowly that when rain falls again, the water table rises before it has had time to sink very far. 
k (such as shale) may lie within a thick aquifer. A mound of groundwater accumulates above such aquitard lenses. The result is a perched water table, a groundwater top surface that lies above the regional water table because the underlying lens of impermeable rock or sediment prevents the groundwater from sinking down to the regional water table (figure above b).
Figures credited to Stephen Marshak.

Running Water

The Work of Running Water

How Do Streams Erode? 

The energy that makes running water move comes from gravity. As water flows downslope from a higher to a lower elevation, the gravitational potential energy stored in water transforms into kinetic energy. About 3% of this energy goes into the work of eroding the walls and beds of stream channels. Running water causes erosion in four ways:
  • Scouring: Running water can remove loose fragments of sediment, a process called scouring. 
  • Breaking and lifting: In some cases, the push of flowing water can break chunks of solid rock off the channel floor or walls. In addition, the flow of a current over a clast can cause the clast to rise, or lift off the substrate. 
  • Abrasion: Clean water has little erosive effect, but sedimentladen water acts like sandpaper and grinds or rasps away at the channel floor and walls, a process called abrasion. In places where turbulence produces long-lived whirlpools, abrasion by sand or gravel carves a bowl-shaped depression, called a pothole, into the floor of the stream (figure below a, b). 
  • Dissolution: Running water dissolves soluble minerals as it passes, and carries the minerals away in solution. 

Erosion and transportation in streams.
The efficiency of erosion depends on the velocity and volume of water and on its sediment content. A large volume of fast moving, turbulent, sandy water causes more erosion than does a trickle of quiet, clear water. Thus, most erosion takes place during floods, when a stream carries a large volume of fast moving, sediment-laden water.

How Do Streams Transport Sediment?

The Mississippi River received the nickname “Big Muddy” for a reason its water can become chocolate brown because of all the clay and silt it carries. Geologists refer to the total volume of sediment carried by a stream as its sediment load. The sediment load consists of three components (figure above c): 
  • Dissolved load: Running water dissolves soluble minerals in the sediment or rock that it flows over, and groundwater seeping into a stream brings dissolved minerals with it. The ions of these dissolved minerals constitute a stream’s dissolved load. 
  • Suspended load: The suspended load of a stream usually consists of tiny solid grains (silt or clay size) that swirl along with the water without settling to the floor of the channel. 
  • Bed load: The bed load of a stream consists of large particles (such as sand, pebbles, or cobbles) that bounce or roll along the stream floor. Bed-load movement commonly involves saltation. During saltation, a multitude of grains bounce along in the direction of flow, within a zone that extends up from the surface of the stream bed for a distance of several centimetres to several tens of centimetres. Each saltating grain in this zone follows a curved trajectory up through the water and then back down to the bed. When it strikes the bed, it knocks other grains upward, and thus supplies grains to the saltation zone.
When describing a stream’s ability to carry sediment, geologists specify its competence and capacity. The competence of a stream refers to the maximum particle size it carries; a stream with high competence can carry large particles, whereas one with low competence can carry only small particles. Competence depends on water velocity. Thus, a fast-moving, turbulent stream has greater competence (it can carry bigger particles) than a slow-moving stream, and a stream in flood has greater competence than a stream with normal flow. In fact, the huge boulders that litter the bed of a mountain creek move only during floods. The capacity of a stream refers to the total quantity of sediment it can carry. A stream’s capacity depends on its competence and discharge. So a large river has more capacity than a small creek.

Depositional Processes 

A raging torrent of water can carry coarse and fine sediment the finer clasts rush along with the water as suspended load, whereas the coarser clasts may bounce and tumble as bed load. If the flow velocity decreases, either because the slope of the stream bed becomes shallower or because the channel broadens out and friction between the bed and the water increases, then the competence of the stream decreases and sediment settles out. The size of the clasts that settle at a particular locality depends on the decrease in flow velocity at the locality. For example, if the stream slows by a small amount, only large clasts settle; if the stream slows by a greater amount, medium-sized clasts settle; and if the stream slows almost to a standstill, the fine grains settle. Because of this process of sediment sorting, stream deposits tend to be segregated by size gravel accumulates in one location and mud in another.

Sediments, carried and deposited by streams. The clast size depends upon the stream velocity.
Geologists refer to sediments transported by a stream as fluvial deposits (from the Latin fluvius, meaning river) or alluvium. Fluvial deposits may accumulate along the stream bed in elongate mounds, called bars (figure above a, b). In cases where the stream channel makes a broad curve, water slows along the inner edge of a curve, so a crescent-shaped point bar bordering the shoreline of the inner curve develops. During floods, a stream may over-top the banks of its channel and spread out over its floodplain, a broad flat area bordering the stream. Friction slows the water on the floodplain, so a sheet of silt and mud settles out to comprise floodplain deposits. Where a stream empties at its mouth into a standing body of water, the water slows and a wedge of sediment, called a delta, accumulates (figure above c).
Figures credited to Stephen Marshak.

Land drainage

Draining the Land 

Water that drains the land has a series of streams network which is filled from either the ground water or the water from the atmosphere, hydrologic cycle.

Forming Streams and Drainage Networks

Excess surface water (runoff) comes from rain, melting ice or snow, and ground water springs. On flat round, water accumulates in puddles ow swamps, but no slopes, it flows downslope in streams.
Where does the water in a stream come from? Recall that water enters the hydrologic cycle by evaporating from the Earth’s surface and rising into the atmosphere. After a relatively short residence time, atmospheric water condenses and falls back to the Earth’s surface as rain or snow that accumulates in various reservoirs. Some rain or snow remains on the land as surface water (in puddles, swamps, lakes, snowfields, and glaciers), some flows downslope as a thin film called sheetwash, and some sinks into the ground, where it either becomes trapped in soil (as soil moisture) or descends below the water table to become groundwater. (the water table is the level below which groundwater fills all the pores and cracks in subsurface rock or sediment. Above the water table, air partially or entirely fills the pores and cracks.) Streams can receive input of water from all of these reservoirs (figure above). Specifically, gravity pulls surface water (including meltwater) downhill into stream channels, the pressure exerted by the weight of new rainfall squeezes existing soil moisture back out of the ground, and groundwater seeps out of the channel walls into the channel, if the floor of the channel lies below the water table. 
Running water collects in stream channels, because a channel is lower than the surrounding area and gravity always moves material from higher to lower elevation. How does a stream channel form in the first place? The process of channel formation begins when sheetwash starts flowing downslope. Like any flowing fluid, sheetwash erodes its substrate (the material it flows over). The efficiency of such erosion depends on the velocity of the flow faster flows erode more rapidly. In nature, the ground is not perfectly planar, not all substrate has the same resistance to erosion, and the amount of vegetation that covers and protects the ground varies with location. Thus, the velocity of sheetwash also varies with location. Where the flow happens to be a bit faster, or the substrate is a little weaker, erosion scours (digs) a channel. Since the channel is lower than the surrounding ground, sheetwash in adjacent areas starts to head toward it. With time, the extra flow deepens the channel relative to its surroundings, a process called downcutting, and a stream forms. 

 An example of headward erosion. The main stream flows in a deep valley. Side streams are cutting into the bordering plateau.
As its flow increases, a stream channel begins to lengthen at its origin, a process called headward erosion (figure above). Headward erosion occurs for two reasons. First, it happens when the surface flow converging at the entrance to a channel has sufficient erosive power to downcut. Second, it happens at locations where groundwater seeps out of the ground and enters the entrance to the stream channel. Such seepage, called “groundwater sapping,” gradually weakens and undermines the soil or rock just upstream of the channel’s endpoint until the material collapses into the channel; the collapsed debris eventually washes away during a flood. Each increment of collapse makes the channel longer.
As downcutting deepens the main channel, the surrounding land surfaces start to slope toward the channel. Thus, new side channels, or tributaries, begin to form, and these flow into the main channel. Eventually, an array of linked streams evolves, with the smaller tributaries flowing into a trunk stream. The array of interconnecting streams together constitute the drainage network. Like transportation networks of roads, drainage networks of streams reach into all corners of a region, providing conduits for the removal of runoff. 

 Block diagrams illustrating five types of drainage networks.
The configuration of tributaries and trunk streams defines the map pattern of a drainage network. This pattern depends on the shape of the landscape and the composition of the substrate. Geologists recognize several types of networks on the basis of the network’s map pattern (figure above).
  • Dendritic: When rivers flow over a fairly uniform substrate with a fairly uniform initial slope, they develop a dendritic network, which looks like the pattern of branches connecting to the trunk of a deciduous tree. 
  • Radial: Drainage networks forming on the surface of a cone shaped mountain flow outward from the mountain peak, like spokes on a wheel. Such a pattern defines a radial network. 
  • Rectangular: In places where a rectangular grid of fractures (vertical joints) breaks up the ground, channels form along the preexisting fractures, and streams join each other at right angles, creating a rectangular network. 
  • Trellis: In places where a drainage network develops across a landscape of parallel valleys and ridges, major tributaries flow down a valley and join a trunk stream that cuts across the ridges. The resulting map pattern resembles a garden trellis, so the arrangement of streams constitutes a trellis network. 
  • Parallel: On a uniform slope, several streams with parallel courses develop simultaneously. The group comprises a parallel network.

Drainage Basins and Divides

Drainage divides and basins.
A drainage network collects water from a broad region, variously called a drainage basin, catchment, or watershed, and feeds it into the trunk stream, which carries the water away. The highland, or ridge, that separates one watershed from another is a drainage divide (figure above a, b). A continental divide separates drainage that flows into one ocean from drainage that flows into another. For example, if you straddle the continental divide where it runs along the crest of the Rocky Mountains in the western United States, and pour a cup of water out of each hand, the water in one hand flows to the Atlantic, and the water in the other flows to the Pacific. Three divides bound part of the Mississippi drainage basin, which drains the interior of the United States.

Streams That Last, Streams That Don’t

The contact between permanent and ephemeral streams.
Permanent streams flow all year long, whereas ephemeral streams flow only for part of the year. Some ephemeral streams flow only for tens of minutes to a few hours, following a heavy rain. Most permanent streams exist where the floor (or bed) of the stream channel lies below the water table (figure above a). In these streams, which occur in humid or temperate climates, water comes not only from upstream or from surface runoff, but also from springs through which groundwater seeps. If the bed of a stream lies above the water table, then the stream can be permanent only when the rate at which water  arrives from upstream exceeds the rate at which water infiltrates into the ground below. For example, the downstream portion of the Colorado River in the dry Sonoran Desert of Arizona flows all year, because enough water enters it from the river’s wet headwaters upstream in Colorado; hardly any water enters the stream from the desert itself. 
Streams that do not have a sufficient upstream source, and whose beds lie above the water table, are ephemeral, because the water that fills a channel due to a heavy rain or a spring thaw eventually sinks into the ground and/or evaporates, and the stream dries up (figure above b). Streams whose watersheds lie entirely within an arid region tend to be ephemeral. The dry bed of an ephemeral stream is variously called a dry wash, an arroyo, or a wadi.
Credits: Stephen Marshak (Essentials of Geology)