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 erosion. Show all posts
Showing posts with label erosion. Show all posts

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.

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.
However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary: Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions. 

Banded-iron formation (BIF)
is 
consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.



                     Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John



In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that
the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in ocean environments today, where sediments and silica-shelled microorganisms accumulate gradually on the seafloor and eventually turn into rock. But iron is less common in younger oceanic sedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.


Banded iron-formation (10 cm), Northern Cape, South Africa.
Specimen and photograph: A. Fraser
Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas, it becomes insoluble (it cannot be dissolved in water) and forms a solidparticle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).
Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:
4Fe3 + 2O2 → 2Fe2O3
6Fe2 + 4O2 → 2Fe3O4
The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygen gas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.
Banded Iron Formation
Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extreme glaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with the atmosphere, limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate.

References:

1. Misra, K. (1999). Understanding Mineral Deposits Springer.
2. 
Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope. Science, 160(3829), 729–736.
3. 
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.

Siccar Point - the world's most important geological site and the birthplace of modern geology


Siccar Point is world-famous as the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.

James Hutton unconformity with annotations - Siccar Point 



In 1788, James Hutton first discovered Siccar Point, and understood its significance. It is by far the most spectacular of several unconformities that he discovered in Scotland, and very important in helping Hutton to explain his ideas about the processes of the Earth.At Siccar Point, gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.
Standing on the angular unconformity at Siccar Point (click to enlarge). Photo: Chris Rowan, 2009
As above, with annotations. Photo: Chris Rowan, 2009





Hutton used Siccar Point to demonstrate the cycle of deposition, folding, erosion and further deposition that the unconformity represents. He understood the implication of unconformities in the evidence that they provided for the enormity of geological time and the antiquity of planet Earth, in contrast to the biblical teaching of the creation of the Earth. 

   
How the unconformity at Siccar Point formed.



At this range, it is easy to spot that the contact between the two units is sharp, but it is not completely flat. Furthermore, the lowest part of the overlying Old Red Sandstone contains fragments of rock that are considerably larger than sand; some are at least as large as your fist, and many of the fragments in this basal conglomerate are bits of the underlying Silurian greywacke. These are all signs that the greywackes were exposed at the surface, being eroded, for a considerable period of time before the Old Red Sandstone was laid down on top of them.
The irregular topography and basal conglomerate show that this is an erosional contact. Photo: Chris Rowan, 2009

The Siccar Point which is a rocky promontory in the county of Berwickshire on the east coast of Scotland.

Weathering and Erosional Processes in Deserts

Weathering and Erosional Processes in Deserts 

Without the protection of foliage to catch rainfall and slow the wind, and without roots to hold regolith in place, rain and wind can attack and erode the land surface of deserts and soil tends to be sparse. The result, as we have noted, is that hill slopes are typically bare, and plains can be covered with stony debris or drifting sand. 

Arid Weathering and Desert Soil Formation 

In the desert, as in temperate climates, physical weathering happens primarily when joints (natural fractures) split rock into pieces. Joint-bounded blocks eventually break free of bedrock and tumble down slopes, fragmenting into smaller pieces as they fall. In temperate climates, thick soil develops and covers bedrock. In deserts, however, bedrock commonly remains exposed, forming rugged, rocky escarpments.
Chemical weathering happens more slowly in deserts than in temperate or tropical climates, because less water is available to react with rock. Still, rain or dew provides enough moisture for some weathering to occur. This water seeps into rock and leaches (dissolves and carries away) calcite, quartz, and various salts. Leaching effectively rots the rock by transforming it into a poorly cemented aggregate. Over time, the rock will crumble and form a pile of unconsolidated sediment, susceptible to transport by water or wind. 
Although enough rain falls in deserts to leach chemicals out of sediment and rock, there is not enough rain to carry the chemicals away entirely. So they precipitate to form calcite and other minerals in regolith beneath the surface. The new minerals may bind clasts together to form a rock-like material called calcrete.

The Evolution of Drainage

Drainage Evolution

Beveling Topography 

Fluvial landscapes evolve over time.
Imagine a place where continental collision uplifts a region; the landscape will evolve (figure above a). At first, rivers have steep gradients, flow over many rapids and waterfalls, and cut deep valleys. But with time, rugged mountains become low, rounded hills; once-deep, narrow valleys broaden into wide floodplains with more gradual gradients. As more time passes, even the low hills are beveled down, becoming small mounds or even disappearing altogether. (Some geologists have referred to the resulting landscape as a peneplain, from the Latin paene, which means almost; it lies at an elevation close to that of a stream’s base level.) 
Though the above model makes intuitive sense, it is an oversimplification. Plate tectonics can uplift the land again, and/or global sea-level rise or fall can change the base level, so in reality peneplains rarely develop before downcutting begins again. Stream rejuvenation occurs when a stream starts to downcut into a land surface whose elevation lies near the stream’s base level. Rejuvenation can be triggered by several phenomena, such as: a drop in base level, as happens when sea level falls; an uplift event that causes the land to rise relative to the base level; or an increase in stream discharge that makes the stream more able to erode and transport sediment. As we've seen, rejuvenation can lead to formation of stream terraces in alluvium. In cases where rejuvenation causes a stream to erode deeply into the land, a new canyon or valley will develop. If the rejuvenated stream had a meandering course, downcutting produces incised meanders (figure above b). 

Stream Piracy and Drainage Reversal 

The concept of stream capture or “piracy.”
Stream piracy sounds like pretty violent stuff. In reality, it’s just a natural process that happens when headward erosion by one stream causes the stream to intersect the course of another stream. When this happens, the pirate stream “captures” the water of the stream that it has intersected, so that the water of the captured stream starts to flow down the pirate stream. Because of piracy, the channel of the captured stream, downstream of the point of capture, dries up (figure above a, b). In some cases, stream capture changes a “water gap” (a stream carved notch through a ridge) into a dry “wind gap.” In 1775, Daniel Boone, the legendary pioneer, led settlers through the Cumberland Gap, a wind gap in the Appalachians, to new homesteads in western Kentucky. 
The pattern of stream flow in an area can also be altered, over time, on a continental scale. For example, when South America and Africa were adjacent to each other in Pangaea, a highland existed along the boundary between the two continents, and the main drainage network of northern South America flowed westward. Later, when South America rifted away from Africa, a convergent plate boundary developed along the western margin of South America, causing the Andes Mountains to rise. The uplift of the Andes caused a drainage reversal, in that the overall slope direction of the drainage network became the opposite of what it once had been. As a consequence, westward flow became impossible, and the eastward-flowing Amazon drainage network developed.

Superposed and Antecedent Streams 

The structure and topography of the landscape do not always appear to control the path, or course, of a stream. For example, imagine a stream that carves a deep canyon straight across a strong mountain ridge why didn't the stream find a way around the ridge? We distinguish two types of streams that cut across resistant topographic highs: 

 Formation of superposed drainage
  • Superposed streams: Imagine a region in which streams start to flow over horizontal beds of strata that unconformably overlie folded strata. When the streams eventually erode down through the unconformity and start to downcut into the folded strata, they maintain their earlier course, ignoring the structure of the folded strata. Geologists call such streams superposed streams, because their pre-existing geometry has been laid down on underlying rock structure (figure above a, b). 
  • Antecedent streams: In some cases, tectonic activity (such as subduction or collision) causes a mountain range to rise up beneath an already established stream. If the stream downcuts as fast as the range rises, it can maintain its course and will cut right across the range. Geologists call such streams antecedent streams (from the Greek ante, meaning before) to emphasize that they existed before the range uplifted. Note that if the range rises faster than the stream downcuts, the new highlands divert (change) the stream’s course so that it flows parallel to the range face (figure below a–c).
 Development of antecedent and diverted streams.
Credits: Stephen Marshak (Essentials of Geology)

deposition in science (river)

Deposition in science (Streams and Their Deposits in the Landscape)

Deposition by river are in different places where slumps, alluvial fans, valleys, canyons, delta, waterfalls, floodplains. 

Valleys and Canyons

About 17 million years ago, a large block of crust, the region now known as the Colorado Plateau (located in Arizona, Utah, Colorado, and New Mexico), began to rise. Before the rise, the Colorado River had been flowing over a plain not far above sea level, causing little erosion. But as the land uplifted, the river began to downcut. Eventually, its channel lay as much as 1.6 km below the surface of the plateau at the base of a steep walled gash now known as the Grand Canyon. The formation of the Grand Canyon illustrates a general phenomenon. In regions where the land surface lies well above the base level, a stream can carve a deep trough, much deeper than the channel itself. If the walls of the trough slope gently, the landforms is a valley, but if they slope steeply, the landforms is a canyon. 

The shape of a canyon or valley depends on the resistance of its walls to erosion slumping.
Whether stream erosion produces a valley or a canyon depends on the rate at which down cutting occurs relative to the rate at which mass wasting causes the walls on either side of the stream to collapse. In places where a stream downcuts through its substrate faster than the walls of the stream collapse, erosion creates a slot (steep-walled) canyon. Such canyons typically form in hard rock, which can hold up steep cliffs for a long time (figure above a). In places where the walls collapse as fast as the stream downcuts, landslides and slumps gradually cause the slope of the walls to approach the angle of repose. When this happens, the stream channel lies at the floor of a valley whose cross-sectional shape resembles the letter V (figure above b); this landforms is called a V-shaped valley. Where the walls of the stream consist of alternating layers of hard and soft rock, the walls develop a stair-step shape such as that of the Grand Canyon (figure above c).

 The evolution of alluvium-filled stream valleys and the development of terraces.
In places where active down cutting occurs, the valley floor remains relatively clear of sediment, for the stream especially when it floods carries away sediment that has fallen or slumped into the channel from the stream walls. But if the stream’s base level rises, its discharge decreases, or its sediment load increases, the valley floor fills with sediment, creating an alluvium-filled valley (figure above a). The surface of the alluvium becomes a broad floodplain. If the stream’s base level later drops again and/or the discharge increases, the stream will start to cut down into its own alluvium, a process that generates stream terraces bordering the present floodplain (figure above b).

Rapids and Waterfalls 

Examples of rapids and waterfalls.
When Lewis and Clark forged a path up the Missouri River, they came to reaches that could not be navigated by boat because of rapids, particularly turbulent water with a rough surface (figure above a). Rapids form where water flows over steps or large clasts in the channel floor, where the channel abruptly narrows, or where its gradient abruptly changes. The turbulence in rapids produces eddies, waves, and whirlpools that roil and churn the water surface, in the process creating white water, a mixture of bubbles and water. Modern-day white water rafters thrill to the unpredictable movement of rapids. 
A waterfall forms where the gradient of a stream becomes so steep that some or all of the water literally free falls above the stream bed (figure above b). The energy of falling water may scour a depression, called a plunge pool, at the base of the waterfall. Though a waterfall may appear to be a permanent feature of the landscape, all waterfalls eventually disappear because headward erosion slowly eats back the resistant ledge. We can see a classic example of headward erosion at Niagara Falls. As water flows from Lake Erie to Lake Ontario, it drops over a 55-m-high ledge of resistant Silurian dolostone, which overlies a weak shale. Erosion of the shale leads to undercutting of the dolostone. Gradually, the overhang of dolostone becomes unstable and collapses, with the result that the waterfall migrates upstream. Before the industrial age, the edge of Niagara Falls cut upstream at an average rate of 1 m per year; but since then, the diversion of water from the Niagara River into a hydroelectric power station has decreased the rate of headward erosion to half that (figure below a, b).

The formation of Niagara Falls, at the border between Ontario, Canada, and New York State. The falls tumble over the Lockport Dolomite, a relatively strong rock layer.

Alluvial Fans and Braided Streams 

 Examples of depositional landforms produced from stream sediment.
Where a fast-moving stream abruptly emerges from a mountain canyon into an open plain at the range front, the water that was once confined to a narrow channel spreads out over a broad surface. As a consequence, the water slows and drops its sedimentary load, forming a sloping apron of sediment (sand, gravel, and cobbles) called an alluvial fan (figure above a). The stream then divides into a series of small channels that spread out over the fan. During particularly strong floods, the water contains so much sediment that it becomes a debris flow that spreads over and smooths out the fan’s surface. 
In some localities, streams carry abundant coarse sediment during floods but cannot carry this sediment during normal flow. Thus, during normal flow, the sediment settles out and chokes the channel. As a consequence, the stream divides into numerous strands weaving back and forth between elongate bars of gravel and sand. The result is a braided stream the name emphasizes that the streams entwine like strands of hair in a braid (figure above b).

Meandering Streams  and Their Floodplains 

The character and evolution of meandering streams and floodplains.
A riverboat cruising along the lower reaches of the Mississippi River cannot sail in a straight line, for the river channel winds back and forth in a series of snake-like curves called meanders (figure above a). In fact, the boat has to go 500 km along the river channel to travel 100 km as the crow flies.
How do meanders evolve? Even if a stream starts out with a straight channel, natural variations in the water depth and associated friction cause the fastest-moving current to swing back and forth. The water erodes the side of the stream more effectively where it flows faster, so it begins to cut away faster on the outer arc of the curve. Thus, each curve begins to migrate sideways and grow more pronounced until it becomes a meander (figure above b). On the outside edge of a meander, erosion continues to eat away at the channel wall, forming a cut bank. On the inside edge, water slows down so that its competence decreases and sediment accumulates, forming a wedge-shaped deposit called a point bar, as noted earlier. 
With continued erosion, a meander may curve through more than 180 degree, so that the cut bank at the meander’s entrance approaches the cut bank at its end, leaving a meander neck, a narrow isthmus of land separating the portions of the meander. When erosion eats through a meander neck, a straight reach called a cut off develops. The meander that has been cut off is called an oxbow lake if it remains filled with water, or an abandoned meander if it dries out (figure above c). Streams that develop many meanders are known, not surprisingly, as meandering streams. The course of a meandering stream naturally changes over time, on a time scale of years to centuries, as new meanders grow and old ones are cut off and abandoned.
Most meandering stream channels cover only a relatively small portion of a broad, gently sloping floodplain (figure above d). Floodplains, as we noted earlier, are so named because during a flood, water over-tops the edge of the stream channel and spreads out over the floodplain. In many cases, a floodplain terminates at its sides along a bluff, or escarpment; large floods may cover the entire floodplain from bluff to bluff. As the water rises above the channel walls and starts to spread out, over the floodplain, friction slows down the flow. This slowdown decreases the competence of the running water, so sediment settles out along the edge of the channel. Over time, the accumulation of this sediment creates a pair of low ridges, called natural levees, on either side of the stream. Natural levees may grow so large that the floor of the channel may become higher than the surface of the floodplain.

Deltas: Deposition at the Mouth of a Stream 

Along most of its length, only a narrow floodplain covered by green, irrigated farm fields borders the Nile River in Egypt. But at its mouth, the trunk stream of the Nile divides into a fan of smaller streams, called distributaries, and the area of green agricultural lands broadens into a triangular patch. The Greek historian Herodotus noted that this triangular patch resembles the shape of the Greek letter delta, and so the region became known as the Nile Delta. Deltas develop where the running water of a stream enters standing water, the current slows, the stream loses competence, and sediment settles out. This can happen in either a lake or the sea.

Delta shape varies depending on current activity, waves, and vegetation.
Geologists refer to any wedge of sediment formed at a river mouth as a delta, even though relatively few have the triangular shape of the Nile Delta (figure above a–c). Some deltas curve smoothly outward, whereas others consist of many elongate lobes formed at different times. Bird’s-foot deltas, so-named because they resemble the scrawny toes of a bird, develop where several distributaries extend far out into relatively calm water; the end of the Mississippi’s active channel ends in a bird’s-foot delta  (figure above a–c). 

 A map showing ancient lobes of the Mississippi Delta. A major flood could divert water from the Mississippi into the channel of the Atchafalaya.
The existence of several overlapping deltas indicates that the main course of the river in the delta has shifted on several occasions. These shifts occur when a toe builds so far out into the sea that the slope of the stream becomes too gentle to allow the river to flow. At this point, the river overflows a natural levee upstream and begins to flow in a new direction, an event called an avulsion. The distinct lobes of the Mississippi Delta suggest that avulsions have happened several times during the past 9,000 years (figure above). New Orleans, built along one of the Mississippi’s distributaries, may eventually lose its riverfront, for a break in a levee upstream of the city could divert the Mississippi into the Atchafalaya River channel. 
The shape of a delta depends on many factors. Deltas that form where the strength of the river current exceeds that of ocean currents have a bird’s-foot shape, since the sediment can be carried far offshore. In contrast, deltas that form where the ocean currents are strong have a $ shape, for the ocean currents redistribute sediment in bars running parallel to the shore. And in places where waves and currents are strong enough to remove sediment as fast as it arrives, a river has no delta at all.
With time, the sediment of a larger delta compacts, and the weight of the delta pushes down the crust below. As a consequence, the surface of a delta slowly sinks. Distributaries can provide sediment that fills the resulting space so that the delta’s surface remains at or just above sea level, forming a broad, flat area called a delta plain. But if people build up artificial levees to constrain the river to its channel, sediment gets carried directly to the seaward edge of the delta and the delta’s interior “starves” (does not receive sediment). When this happens, the delta’s surface drops below sea level. Because of this process, much of New Orleans lies below sea level.
Figures credited to Stephen Marshak.

Soil

What is Soil?

What is soil?. If you've ever had the chance to dig in a garden, you've seen first hand that the material in which flowers grow looks and feels different from beach sand or potter’s clay. We call the material in a garden “dirt” or, more technically, soil. Soil consists of rock or sediment that has been modified by physical and chemical interaction with organic material, rainwater, and organisms over time. Soil is one of our planet’s most valuable resources, for without it there could be no agriculture, forestry, ranching, or home gardening.

How Does Soil Form?


How does soil form?. Three processes taking place at or just below the surface of the Earth contribute to soil formation. First, chemical and physical weathering produces loose debris, new minerals (such as clay), and ions in solution. Second, rainwater percolates through the debris and carries dissolved ions and clay flakes downward. The region in which this downward transport takes place is called the zone of leaching, because leaching means extracting, absorbing, and removal. Farther down, new mineral crystals precipitate directly out of the water or form by reaction of the water with debris. Also, the water leaves behind its load of fine clay. The region in which new minerals and clay collect is the zone of accumulation. Third, microbes, fungi, plants, and animals interact with sediment by producing acids that weather grains, by absorbing nutrient atoms, and by leaving behind organic waste and remains. Plant roots and burrowing animals (insects, worms, and gophers) churn and break up the soil, and microbes metabolise minerals and organic matter and release chemicals. Because different soil-forming processes operate at different depths, soils typically develop distinct zones, known as soil horizons, arranged in a vertical sequence called a soil profile. Let’s look at an idealised soil profile, from top to bottom, using a soil formed in a temperate forest as our example. The highest horizon is the O-horizon (the prefix stands for organic), so called because it consists almost entirely of humus (plant debris) and contains barely any mineral matter. Below the O-horizon, we find the A-horizon, in which humus has decayed further and has mixed with mineral grains (clay, silt, and sand). Water percolating through the A-horizon causes chemical weathering reactions to occur and produces ions in solution and new clay minerals. Downward-moving water eventually carries soluble chemicals and fine clay deeper into the subsurface. The O- and  A-horizons constitute dark-gray to blackish-brown topsoil, the fertile portion of soil that farmers till for planting crops. (In some places, the A-horizon grades downward into the E-horizon, a soil level that has undergone substantial leaching but has not yet mixed with organic material.) Beneath the A-horizon (or the A- and E-horizons) lies the B- horizon. Ions and clay accumulate in the B-horizon, or subsoil. Note from our description that the O-, A-, and E-horizons  make up the zone of leaching, whereas the B-horizon makes up the zone of accumulation. Finally, at the base of a soil profile we find the C-horizon, which consists of material derived from the substrate that’s been chemically weathered and broken apart, but has not yet undergone leaching or accumulation. The C-horizon grades downward into unweathered bedrock, or into unweathered sediment. As farmers, foresters, and ranchers well know, the soil in one locality can differ greatly from the soil in another, in terms of composition, thickness, and texture. Indeed, crops that grow well in one type of soil may wither and die in another nearby. Such diversity exists because the make up of a soil depends on several soil-forming factors:
  • Climate: Large amounts of rainfall and warm temperatures accelerate chemical weathering and cause most of the soluble elements to be leached. Small amounts of rainfall and cooler temperatures result in slower rates of weathering and leaching, so soils take a long time to develop and can retain unweathered minerals and soluble components. Climate is the single most important factor in determining the nature of soils that develop. 
  • Substrate composition: Some soils form on basalt, some on granite, some on volcanic ash, and some on recently deposited quartz silt. These different substrates consist of different materials, so the soils formed on them end up with different chemical compositions. 
  • Slope steepness: A thick soil can accumulate under land that lies flat. But on a steep slope, weathered rock may wash away before it can evolve into a soil. Thus, all other factors being equal, soil thickness increases as the slope angle decreases. 
  • Wetness: Depending on the details of local topography and on the depth below the surface at which groundwater occurs, some soil is wetter than other soil in the same region. Wet soils tend to contain more organic material than do dry soils. 
  • Time: Because soil formation is an evolutionary process, a young soil tends to be thinner and less developed than an old soil. The rate of soil formation varies greatly with environment. 
  • Vegetation type: Different kinds of plants extract or add different nutrients and quantities of organic matter to a soil. Also, some plants have deeper root systems than others and help prevent soil from washing away.

Soil Classification 


Soil scientists worldwide have struggled mightily to develop a rational scheme for classifying soils. Not all schemes utilize the same criteria, and even today there is not worldwide agreement on which works best. In the United States, a country that includes many climates at mid-latitudes, many soil scientists use the U.S. Comprehensive Soil Classification System, which distinguishes among 12 orders of soil based on the physical characteristics and environment of soil formation. Canadians use a different scheme focusing only on soils that develop north of the 40th parallel. The Canadian  scheme works well for cooler, high-latitude climates. As we've noted, rainfall and vegetation play a key role in determining the type of soil that forms. For example, in deserts, where there is very little rainfall and sparse vegetation, an aridisol forms. (In older classifications, these were known as “pedocal” soils.) Aridisols have no O-horizon (because there is so little organic material), and the A-horizon is thin. Soluble minerals, specifically calcite, that would be washed away entirely if there were more rainfall, instead accumulate in the B-horizon. In fact, capillary action may bring calcite up from deeper down as water evaporates at the ground surface. The calcite locally cements clasts together in the B-horizon to form a rock-like mass called caliche or calcrete. In temperate environments, an alfisol forms this soil has an O-horizon, and because of moderate amounts of rainfall, materials leached from the A-horizon accumulate in the B-horizon. (In older classifications, these were known as “pedalfer” soils.) In a tropical climate, oxisols develop. Here, so much rainfall percolates down into the ground that all reactive minerals in the soil undergo chemical weathering, producing ions and clay that flush downward. This process leaves an A-horizon that contains substantial amounts of stable iron-oxide, aluminium-oxide, and aluminium-hydroxide residues. The resulting soil tends to be brick-red and is traditionally called laterite.
U.S. Department of Agriculture map of soil types around the world.

Soil Erosion 

As we have seen, soils take time to form, so soils capable of supporting crops or forests are a natural resource worthy of protection. However, agriculture, overgrazing, and clear cutting have led to the destruction of soil. Crops rapidly remove nutrients from soil, so if they are not replaced, the soil will not contain sufficient nutrients to maintain plant life. When the natural plant cover disappears, the surface of the soil becomes exposed to wind and water. Actions such as the impact of falling raindrops or the rasping of a plow break up the soil at the surface, with the result that it can wash away in water or blow away as dust. When this happens, soil erosion, the removal of soil by running water or by wind, takes place. In some localities, erosion carries away almost six tons of soil from an acre of land per year. Human activities can increase rates of soil erosion by 10 to 100 times, so that it far exceeds the rate of soil formation. Droughts exacerbate the situation. For example, during the 1930s a succession of droughts killed off so much vegetation in the American plains that wind stripped the land of soil and caused devastating dust storms. Large numbers of people were forced to migrate away from the Dust Bowl of Oklahoma and adjacent areas. The consequences of rainforest destruction have particularly profound effects on soil. In an established rain forest, lush growth provides sufficient organic debris so that trees can grow. But if the forest is logged, or cleared for a griculture, the humus rapidly  disappears, leaving laterite that contains few nutrients. Crop plants consume whatever nutrients remain so rapidly that the soil becomes infertile after only a year or two, useless for agriculture and unsuitable for regrowth of rainforest trees.