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 geologic time. Show all posts
Showing posts with label geologic time. Show all posts


What is anthropocene?

The anthropocene Epoch is atop of the table.
Almost everyone will ask, what is Anthropocene? What is the Anthropocene age or Anthropocene epoch or Anthropocene era? So, here let's start with the definition and learn the ages of man effects on Earth.
The Anthropocene definition, Earth's most recent geologic time period as being human-influenced, or anthropogenic, based on overwhelming global evidence that atmospheric, geologic, hydrologic, biospheric and other earth system processes are now altered by humans.
The word combines the root "anthropo", meaning "human" with the root "-cene", the standard suffix for "epoch" in geologic time.
The Anthropocene is distinguished as a new period either after or within the Holocene epoch, the current epoch, which began approximately 10,000 years ago (about 8000 BC) with the end of the last glacial period.

Epoch definition and Epoch meaning

  1. an event or a time marked by an event that begins a new period or developmentb
  2. memorable event or date
  3. an extended period of time usually characterized by a distinctive development or by a memorable series of eventsb 
  4. division of geologic time less than a period and greater than an age
  5. an instant of time or a date selected as a point of reference (as in astronomy)

From when Anthropocene Epoch Start?

The Anthropocene Epoch should begin about 1950, the experts said, and was likely to be defined by the radioactive elements dispersed across the planet by nuclear bomb tests, although an array of other signals, including plastic pollution, soot from power stations, concrete, and even the bones left by the global proliferation of the domestic chicken were now under consideration.
The current Holocene epoch, is the 12,000 years of stable climate since the last ice age during which all human civilisation developed. But the striking acceleration since Anthropocene timeline, the mid-20th century of carbon dioxide emissions and sea level rise, the global mass extinction of species, and the transformation of land by deforestation and development mark the end of that slice of geological time, the experts argue. The Earth is so profoundly changed that the Holocene must give way to the Anthropocene.

The Anthropocene Review or Anthropocene theory (Debate on current epoch)

The Anthropocene Review or Anthropocene theory, According to the International Union of Geological Sciences (IUGS), the professional organization in charge of defining Earth’s time scale, we are officially in the Holocene epoch (entirely recent), which began 11,700 years ago after the last major ice age.
Holocene and Anthropocene both are geological epoch however, Anthropocene era has become an environmental buzzword ever since the atmospheric chemist and Nobel laureate Paul Crutzen popularised it in 2000. This year, the word has picked up velocity in elite science circles: It appeared in nearly 200 peer-reviewed articles, the publisher Elsevier has launched a new aca­demic journal titled Anthropocene and the IUGS convened a group of scholars to decide by 2016 whether to officially declare that the Holocene is over and the Anthropocene age has begun.
Many stratigraphers (scientists who study rock layers) criticise the idea, saying clear-cut evidence for a new epoch simply isn’t there. “When you start naming geologic-time terms, you need to define what exactly the boundary is, where it appears in the rock strata,” says Whitney Autin, a stratigrapher at the SUNY College of Brockport, who suggests Anthropocene is more about pop culture than hard science. The crucial question, he says, is specifying exactly when human beings began to leave their mark on the planet: The atomic era, for instance, has left traces of radiation in soils around the globe, while deeper down in the rock strata, agriculture’s signature in Europe can be detected as far back as A.D. 900. The Anthropocene, Autin says, “provides eye-catching jargon, but from the geologic side, I need the bare bones facts that fit the code.”

Nature of human effects


The human impact on biodiversity forms one of the primary attributes of the Anthropoceno. Humankind has entered what is sometimes called the Earth's sixth major extinction. Most experts agree that human activities have accelerated the rate of species extinction. The exact rate remains controversial perhaps 100 to 1000 times the normal background rate of extinction. A 2010 study found that "marine phytoplankton the vast range of tiny algae species accounting for roughly half of Earth's total photosynthetic biomass – had declined substantially in the world's oceans over the past century. From 1950 alone, algal biomass decreased by around 40%, probably in response to ocean warming and that the decline had gathered pace in recent years. Some authors have postulated that without human impacts the biodiversity of the planet would continue to grow at an exponential rate. 
Increases in global rates of extinction have been elevated above background rates since at least 1500, and appear to have accelerated in the 19th century and further since. A 13 July 2012 New York Times op-ed by ecologist Roger Bradbury predicted the end of biodiversity for the oceans, labelling coral reefs doomed: "Coral reefs will be the first, but certainly not the last, major ecosystem to succumb to the Anthropocene." This op-ed quickly generated much discussion among conservationists; The Nature Conservancy rebutted Bradbury on its website, defending its position of protecting coral reefs despite continued human impacts causing reef declines.
In a pair of studies published in 2015, extrapolation from observed extinction of Hawaiian snails led to the conclusion that "the biodiversity crisis is real", and that 7% of all species on Earth may have disappeared already. Human predation was noted as being unique in the history of life on Earth as being a globally distributed 'superpredator', with predation of the adults of other apex predators and with widespread impacts on food webs worldwide.


Permanent changes in the distribution of organisms from human influence will be identifiable in the geologic record. Many species have been documented moving into regions that were once too cold for them, often at rates faster than initially expected. This has occurred in part as a result of evolving climate, but also in response to farming and fishing, and the accidental introduction of non-native species to new areas by global travel. The ecosystem of the entire Black Sea may have changed during the last 2000 years as a result of nutrient and silica input from eroding deforested lands along the Danube River.


One geological symptom resulting from human activity is increasing atmospheric carbon dioxide (CO2) content. During the glacial–interglacial cycles of the past million years, natural processes have varied CO2 by approximately 100 ppm (from 180 ppm to 280 ppm). As of 2013, anthropogenic net emissions of CO2 increased atmospheric concentration by a comparable amount from 280 ppm (Holocene or pre-industrial "equilibrium") to approximately 400 ppm, with 2015–16 monthly monitoring data of CO2 displaying a rising trend above 400 ppm. This signal in the Earth's climate system is especially significant because it is occurring much faster, and to a greater extent, than previous, similar changes. Most of this increase is due to the combustion of fossil fuels such as coal, oil, and gas, although smaller fractions are the result of cement production and land-use changes (e.g. deforestation).


Changes in drainage patterns traceable to human activity will persist over geologic time in large parts of the continents where the geologic regime is erosional. This includes the paths of roads and highways defined by their grading and drainage control. Direct changes to the form of the Earth's surface by human activities (e.g., quarrying, landscaping) also record human impacts.


Sedimentological record

Human activities like deforestation and road construction are believed to have elevated average total sediment fluxes across the Earth's surface. However, construction of dams on many rivers around the world means the rates of sediment deposition in any given place do not always appear to increase in the Anthropocene. For instance, many river deltas around the world are actually currently starved of sediment by such dams, and are subsiding and failing to keep up with sea level rise, rather than growing.

Fossil record

Increases in erosion due to farming and other operations will be reflected by changes in sediment composition and increases in deposition rates elsewhere. In land areas with a depositional regime, engineered structures will tend to be buried and preserved, along with litter and debris. Litter and debris thrown from boats or carried by rivers and creeks will accumulate in the marine environment, particularly in coastal areas. Such manmade artefacts preserved in stratigraphy are known as "techno-fossils".
Changes in biodiversity will also be reflected in the fossil record, as will species introductions. An example cited is the domestic chicken, originally the red junglefowl Gallus gallus, native to south-east Asia but has since become the world's most common bird through human breeding and consumption, with over 60 billion consumed a year and whose bones would become fossilised in landfill sites.

Trace elements

In terms of trace elements, there are distinct signatures left by modern societies. For example, in the Upper Fremont Glacier in Wyoming, there is a layer of chlorine present in ice cores from 1960s atomic weapon testing programs, as well as a layer of mercury associated with coal plants in the 1980s. From 1945 to 1951, nuclear fallout is found locally around atomic device test sites, whereas from 1952 to 1980, tests of thermonuclear devices have left a clear, global signal of excess 14C, 239Pu, and other artificial radionuclides. The highest concentration of radionuclides was in 1964, one of the dates which has been proposed as a possible benchmark for the start of the formally defined age of Anthropocene.
Human burning of fossil fuels has also left distinctly elevated concentrations of black carbon, inorganic ash, and spherical carbonaceous particles in recent sediments across the world. Concentrations of these components increases markedly and almost simultaneously around the world beginning around 1950.

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)
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.


1. Misra, K. (1999). Understanding Mineral Deposits Springer.
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.
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.

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:

3. A unique Normal Fault

4. The Rock Cycle
 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

The Cenozoic Era: The Modern World Comes to Be

The Cenozoic Era

The Cenozoic Era is focused which we know today of the world, the modern world.


 The two main active continental orogenic systems on the Earth today. The Alpine-Himalayan system formed when Africa, India, and Australia collided with Asia (inset). The Cordilleran and Andean systems reflect the consequences of convergent-boundary tectonism along the eastern Pacific Ocean.
During the last 65 million years, the map of the Earth has continued to change, gradually producing the configuration of continents and plate boundaries we see today. The final stages of the Pangaea breakup separated Australia from Antarctica and Greenland from North America, and formed the North Sea between Britain and continental Europe. The Atlantic Ocean continued to grow because of sea-floor spreading on the Mid- Atlantic Ridge, and thus the Americas have moved westward, away from Europe and Africa. Meanwhile, the continents that once constituted Gondwana drifted northward as the intervening Tethys Ocean was consumed by subduction. Collisions of the former Gondwana continents with the southern margins of Europe and Asia resulted in the formation of the largest orogenic belt on Earth today, the Alpine-Himalayan chain (figure above). India and a series of intervening volcanic island arcs and micro-continents  collided with Asia to form the Himalayas and the Tibetan Plateau to the north, while Africa along with some volcanic island arcs and micro-continents collided with Europe to  produce the Alps. 
As the Americas moved westward, convergent plate boundaries evolved along their western margins. In South America, convergent-boundary activity built the Andes, which remains an active orogen to the present day. In North America, convergent-boundary activity continued without interruption until about 40 Ma (the Eocene Epoch), yielding, as we have seen, the Laramide orogeny. Then, because of the rearrangement of plates off the western shore of North America, a transform boundary replaced the convergent boundary in the western part of the continent by 25 Ma. When this happened, volcanism and compression ceased in western North America, the San Andreas Fault system formed along the coast of the United States, and the Queen Charlotte Fault system developed off the coast of Canada. Along the San Andreas and Queen Charlotte faults today, the Pacific Plate moves northward with respect to North America at a rate of about 6 cm per year. In the western United States, convergent-boundary tectonics continues only in Washington, Oregon, and northern California, where subduction of the Juan de Fuca Plate generates the volcanism of the Cascade volcanic chain.

The Basin and Range Province is a rift. The inset shows a cross section along the red line.
As convergent tectonics ceased in the western United States south of the Cascades, the region began to undergo rifting (extension) in roughly an east-west direction. The result was the formation of the Basin and Range Province, a broad continental rift whose development has stretched the region to twice its original width (figure above). The Basin and Range gained its name from its topography the province contains long, narrow mountain ranges separated from each other by flat, sediment filled basins. This topography formed when the crust of the region was broken up by normal faults. Blocks of crust above these faults slipped down and tilted. Crests of the tilted blocks form the ranges, and the depressions between them, which rapidly filled with sediment eroded from the ranges, became basins. 
The Basin and Range Province terminates just north of the Snake River Plain, the track of the hot spot that now lies beneath Yellowstone National Park. As North America drifts westward, volcanic calderas formed along the Snake River Plain; Yellowstone National Park straddles the most recent caldera.
Recall that in the Cretaceous Period, the world was relatively warm and sea level rose so that extensive areas of continents were submerged. During the Cenozoic Era, however, the global climate rapidly became cooler, and by the early Oligocene Epoch (34 Ma), Antarctic glaciers reappeared for the first time since the Triassic. The climate continued to grow colder through the Late Miocene Epoch, leading to the formation of grasslands in temperate climates. About 2.5 Ma, the Isthmus of Panama formed, separating the Atlantic completely from the Pacific, changing the configuration of oceanic currents, perhaps leading the Arctic Ocean to freeze over. 

The maximum advance of the Pleistocene ice sheet in North America.
During the overall cold climate of the past 2 million years, continental glaciers have expanded and retreated across northern continents at least 20 times, resulting in the  Pleistocene Ice Age (figure above). Each time the glaciers grew, sea level fell so much that the continental shelf became exposed to air. At times, a land bridge formed across the Bering Strait, west of Alaska, providing migration routes for animals and people from Asia into North America. A partial land bridge also formed from southeast Asia to Australia, making human migration to Australia easier. Erosion and deposition by the glaciers created much of the landscape we see today in northern temperate regions. About 11,000 years ago, the climate warmed, and we entered the interglacial time interval we are still experiencing today.

The present-day Bahamas serve as an example of what the interior of the United States might have looked like during intervals of the Paleozoic. Shallow land areas were submerged and became the site of shallow-marine sedimentation.

Life evolution

When the skies finally cleared in the wake of the K-T boundary catastrophe, plant life recovered, and soon forests of both angiosperms and gymnosperms grew. The grasses, which first appeared in the Cretaceous, spread across the plains in temperate and subtropical climates by the middle of the Cenozoic Era, transforming them into vast grasslands. The dinosaurs, except for their descendants the birds, were gone for good. Mammals rapidly diversified into a variety of forms to take their place. In fact, most of the modern groups of mammals that exist today originated at the beginning of the Cenozoic Era, giving this time the nickname Age of Mammals. During the latter part of the era, huge mammals appeared (such as mammoths, giant beavers, giant bears, and giant sloths), but these became extinct during the past 10,000 years, probably because of hunting by humans.
It was during the Cenozoic that our own ancestors first appeared. Ape-like primates diversified in the Miocene Epoch (about 20 Ma), and the first human-like primate appeared at about 4 Ma, followed by the first members of the human genus, Homo, at about 2.4 Ma. Fossil evidence, primarily from Africa, indicates that Homo erectus, capable of making stone axes, appeared about 1.6 Ma, and the line leading to Homo sapiens (our species) diverged from Homo neanderthalensis (Neanderthal man) about 500,000 years ago. According to the fossil record, modern people appeared about 200,000 years ago, and initially shared the planet with two other species of the genus Homo, the Neanderthals and the Denisovans. The last Neanderthals and Denisovans died off over 25,000 years ago, leaving Homo sapiens as the only human species on Earth. 
Earth’s history reflects the complex consequences of plate  interactions, sea-level changes,  atmospheric changes, life evolution, and even meteorite impact. In the past few millennia, humans have had a huge effect on the planet, causing changes significant enough to be obvious in the geologic record of the future.
Credits: Stephen Marshak (Essentials of Geology)

The Mesozoic Era: When Dinosaurs Ruled

The Mesozoic Era

The Mesozoic Era where Dinosaurs were dominant spanned from 251-65 Ma.

The Early and Middle Mesozoic Era  (Triassic–Jurassic Periods, 251–145 Ma) 


Pangaea began to break up in the Triassic, and by Jurassic time, a narrow North Atlantic Ocean existed.
Pangaea, the super-continent formed at the end of the Paleozoic Era, existed for about 100 million years, until rifting commenced during the Late Triassic and Early Jurassic Periods and the super-continent began to break up. By the end of the Jurassic Period, rifting had succeeded in splitting North America from Europe and Africa. The Mid-Atlantic Ridge formed, and the North Atlantic Ocean started to grow (figure above). According to the record of sedimentary rocks, Earth overall had a warm climate during the Triassic and Early Jurassic. But during the Late Jurassic and Early Cretaceous, the climate cooled. Pangaea’s interior remained a non-marine environment in which red sandstone and shale, now exposed in the spectacular cliffs of Zion National Park, were deposited. By the Middle Jurassic Period, sea level began to rise, and a shallow sea submerged much of the Rocky Mountain region. On the western margin of North America, convergent margin tectonics became the order of the day. Beginning with Late Permian and continuing through Mesozoic time, subduction generated volcanic island arcs and caused them, along with micro-continents and hot-spot volcanoes, to collide with North America. Thus, North America grew in land area by the accretion of crustal fragments on its western margin. Because these fragments consist of crust that formed elsewhere, not originally on or adjacent to the continent, geologists call them exotic terranes. From the end of the Jurassic through the Cretaceous Period, a major continental volcanic arc, the Sierran arc, grew on the western margin of North America itself; you’ll learn more about this arc later.

Life evolution

During the Jurassic, giant dinosaurs roamed the land. This painting shows several species.
During the early Mesozoic Era, a variety of new plant and animal species appeared, filling the ecological niches left vacant by the Late Permian mass extinction. Reptiles swam in the oceans, and new kinds of corals became the predominant reef builders. On land, gymnosperms and reptiles diversified, and the Earth saw its first turtles and flying reptiles. And at the end of the Triassic Period, the first true dinosaurs evolved. Dinosaurs differed from other reptiles in that their legs were positioned under their bodies rather than off to the sides, and they were possibly warm blooded. By the end of the Jurassic Period, gigantic sauropod dinosaurs (weighing up to 100 tons), along with other familiar examples such as stegosaurus, thundered across the landscape, and the first feathered birds, such as Archaepteryx, took to the skies (figure above). The earliest ancestors of mammals appeared at the end of the Triassic Period, in the form of small, rat-like creatures.

The Late Mesozoic Era  (Cretaceous Period, 145–65 Ma) 


During the Late Cretaceous, a continental volcanic arc formed. A fold thrust belt formed to the east, as did
a transcontinental seaway.
During the Cretaceous Period, the Earth’s climate continued to shift to warmer conditions, and sea level rose significantly, reaching levels that had not been attained  for the previous 200 million years. Great seaways flooded most of the continents (figure above). In fact, during the latter part of the Cretaceous Period, a shark could have swum from the Gulf of Mexico to the Arctic Ocean, or across much of western Europe. 

Paleogoegraphy is Late Cretaceous through Eocene time.
The breakup of Pangaea continued through the Cretaceous Period, with the opening of the South Atlantic Ocean and the separation of South America and Africa from Antarctica and Australia. India broke away from Gondwana and headed rapidly northward toward Asia (figure above a, b). Along the continental margins of the newly formed Mesozoic oceans, passive-margin basins developed that filled with great thicknesses of sediments. In western North America, the Sierran arc, a large continental volcanic arc that initiated at the end of the Jurassic Period, continued to be active. This arc resembled the present day Andean arc of western South America. Though the volcanoes of the Sierran arc have long since eroded away, we can see their roots in the form of the plutons that now constitute the granitic batholith of the Sierra Nevada Mountains. Compressional stresses along the western North American convergent boundary activated large thrust faults east of the arc, an event geologists refer to as the Sevier orogeny. This orogeny produced a fold-thrust belt whose remnants you can see today in the Canadian Rockies and in western Wyoming (figure above c). At the end of the Cretaceous Period, continued compression along the convergent boundary of western North America caused slip on large faults in the region of Wyoming, Colorado, eastern Utah, and northern Arizona. In contrast to the faults of fold-thrust belts, these faults penetrated deep into the Precambrian rocks of the continent, and thus movement on them generated basement uplifts (figure above d). Overlying layers of Paleozoic strata warped into large monoclines, folds whose shape resembles the drape of a carpet over a step. This event, which geologists call the Laramide orogeny, formed the structure of the present Rocky Mountains in the United States. Geologists have determined that sea-floor spreading rates may have been as much as three times faster during the Cretaceous than they are today. As a result, more of the oceanic crust was younger and warmer than it is today, and since young sea floor lies at a shallower depth than does older sea floor (due to isostasy; see Interlude D), Cretaceous mid-ocean ridges occupied more volume than they do today. The extra volume of the ridges displaced sea water, causing sea level to rise. Also during the Cretaceous, huge submarine plateaus formed from basalts erupted at hot-spot volcanoes. The existence of these plateaus implies that particularly active mantle plumes, or super plumes, reached the base of the lithosphere. Growth of submarine plateaus displaced sea water and thus also contributed to sea-level rise. Volcanism associated with extra-rapid sea-floor spreading, as well as with submarine plateau growth, likely released CO2 into the atmosphere. Geologists hypothesise that this increased atmospheric CO2 concentration led to a global rise in atmospheric temperature. Rising temperatures would cause sea water to expand and polar ice sheets to melt, both phenomena that would make sea level go up even more. Considering all the phenomena that caused sea level to rise during the Cretaceous, it’s no surprise that the continents flooded and that large epicontinental seas formed during this era.

Life evolution

In the seas of the late Mesozoic world, modern fish appeared and became dominant. In contrast with earlier fish, the new fish had short jaws, rounded scales, symmetrical tails, and specialised fins. Huge swimming reptiles and gigantic turtles (with shells up to 4 m across) preyed on the fish. On land, cycads largely vanished, and angiosperms (flowering plants), including hardwood trees, began to compete successfully with conifers for dominance of the forest. Dinosaurs reached their peak of success at this time, inhabiting almost all environments on Earth. Social herds of grazing dinosaurs roamed the plains, preyed on by the fearsome Tyrannosaurus rex (a Cretaceous, not a Jurassic, dinosaur, despite what Hollywood says). Pterosaurs, with wingspans of up to 11 m, soared overhead, and birds began to diversify. Mammals also diversified and developed larger brains and more specialised teeth, but for the most part, they remained small and rat-like.

The “K-T boundary event” 

The Cretaceous tertiary impact. The aftermath probably caused extinction of the dinosaurs and other species.
Geologists first recognised the K-T boundary (K stands for Cretaceous and T for Tertiary) from 18th-century studies that identified an abrupt global change in fossil assemblages. Until the 1980s, most geologists assumed the faunal turnover took millions of years. But modern dating techniques indicate that this change happened almost instantaneously and that it represents a sudden mass extinction of most species on Earth. The dinosaurs, which had ruled the planet for over 150 million years, simply vanished, along with 90% of plankton species in the ocean and up to 75% of plant  species. What kind of catastrophe could cause such a sudden and extensive mass extinction? The cause of the K-T mass extinction remained a mystery until the late 1970s, when Walter Alvarez, an American geologist, and his colleagues examined a shale layer deposited exactly at the K-T boundary. They found that this shale contained relatively high concentrations of iridium, an element that comes primarily from meteorites. Further study showed that the clays of this age contained other unusual materials, such as tiny glass spheres formed when a spray of molten rock freezes, grains of coesite (a mineral that forms when intense shock waves pass through quartz), and even carbon from burned vegetation. All these features pointed to the occurrence of a huge meteorite impact at the time of the K-T boundary. Subsequently, geologists found a 100-km-diameter and 16-km-deep meteorite crater buried beneath younger strata of the Yucatan  Peninsula in Mexico (figure above a, b). Isotopic dating indicates that formation of the crater occurred at 65 o 0.4 Ma, the time of the K-T boundary event. Because of its age and size, this crater, known as the Chicxulub crater, may be the grave of the deadly object whose impact with Earth eliminated so much life.
The impact caused so much destruction because it not only formed a crater, blasting huge quantities of debris into the sky, but probably also generated 2-km-high tsunamis that inundated the shores of continents and generated a blast of hot air that set forests on fire. The blast and the blaze together could have ejected so much debris into the atmosphere that for months there would have been perpetual night and winter like cold. In addition, chemicals ejected into the air could have combined with water to produce acid rain. These conditions would cause photosynthesis to all but cease, and thus would break the food chain and trigger extinctions.
Credits: Stephen Marshak (Essentials of Geology) 

The Paleozoic Era: Continents Reassemble and Life Gets Complex

The Paleozoic Era

The Paleozoic Era comprises of Cambrian to Ordovician Periods which time span is 542-251 Ma.

The Early Paleozoic Era  (Cambrian–Ordovician Periods, 542–444 Ma) 


Land and sea in the early Paleozoic Era.
At the beginning of the Paleozoic Era, Pannotia broke up, yielding smaller continents including Laurentia (composed of North America and Greenland), Gondwana (South America, Africa, Antarctica, India, and Australia), Baltica (Europe), and Siberia (figure above a). New passivemargin basins formed along the edges of these new continents. In addition, sea level rose, so that vast areas of continental interiors were flooded with shallow seas called epicontinental seas (figure above b). These regions are now cratonic platforms. In many places, water depths in epicontinental seas reached only a few meters, creating a well-lit marine environment in which life abounded. Deposition in these seas, therefore, yielded layers of fossiliferous sediment. Sea level, however, did not stay high for the entire early Paleozoic Era; regressions and transgressions took place, the former marked by unconformities and the latter by accumulations of sediment. The layer cake of strata in the Grand Canyon is rock formed from such sediment. The geologically peaceful world of the early Paleozoic Era in Laurentia abruptly came to a close in the Middle Ordovician Period, for at this time its eastern margin rammed into a volcanic island arc and other crustal fragments. The resulting collision, called the Taconic orogeny, deformed and metamorphosed strata of the continent’s margin and produced a mountain range in what is now the eastern part of the Appalachians (figure above c). 

Life evolution

A museum diorama illustrates what early Paleozoic marine organisms may have looked like.
The fossil record indicates that soon after the Cambrian began, life underwent remarkable diversification. This event, which paleontologists refer to as the Cambrian explosion, took several million years. What caused this event? No one can say for sure, but considering that it occurred roughly at the time a supercontinent broke up, it may have had something to do with the production of new ecological niches and the isolation of populations that resulted when small continents formed and drifted apart. The first animals to appear in the Cambrian Period had simple tube- or cone-shaped shells, but soon thereafter, the shells became more complex. Shells on other organisms may have evolved as a means of protection against predation by organisms such as conodonts, small, eel-like organisms with hard parts that resemble teeth. By the end of the Cambrian, 
trilobites were grazing the sea floor. Trilobites shared the environment with mollusks, brachiopods, nautiloids, gastropods, graptolites, and echinoderms (figure above). Thus, a complex food chain arose, which included plankton, bottom feeders, and at the top, predators. Many of the organisms crawled over or swam around reefs composed of mounds of sponges with mineral skeletons. The Ordovician Period saw the first crinoids and the first vertebrate animals, jawless fish. At the end of the Ordovician, mass extinction took place, perhaps because of the brief glaciation and associated sea-level lowering of the time.

The Middle Paleozoic Era  (Silurian–Devonian Periods, 444–359 Ma) 


Paleogeography and fossils of Silurian and Devonian time .
As the world entered the Silurian Period, global climate warmed, sea level rose, and the continents flooded once again. In some places, where water in the epicontinental seas was clear and could exchange with water from  the oceans, huge reef complexes grew, forming a layer of  fossiliferous limestone on the continents. Also, distinct orogenies took place, yielding new mountain belts during the middle Paleozoic Era. For example, collisions on the eastern side of  Laurentia during Silurian and Devonian time produced the Caledonian orogen (affecting eastern Greenland, western Scandinavia, and Scotland) and the Acadian orogen in the region that is now the Appalachians (figure above a). Throughout much of the middle Paleozoic, the western margin of North America continued to be a passive-margin basin. Finally, in the Late Devonian, the quiet environment of the west-coast passive margin ceased, possibly because of a collision with an island arc. This event, known as the Antler orogeny, was the first of many orogenies to affect the western margin of the continent. The Caledonian, Acadian, and Antler orogenies all shed deltas of sediment onto the continents; these deposits formed thick successions of red beds, such as those visible today in the Catskill Mountains (figure above b). 

Life evolution

Life on Earth underwent radical changes in the middle Paleozoic Era. In the sea, new species of trilobites,  gastropods, crinoids, and bivalves replaced species that had disappeared during the mass extinction at the end of the Ordovician Period. On land, vascular plants with woody tissues, seeds, and veins (for transporting water and food) rooted for the first time. With the evolution of veins and wood, plants could grow much larger, and by the Late Devonian Period the land surface hosted swampy forests with tree-sized relatives of club mosses and ferns. Also at this time, spiders, scorpions, insects, and crustaceans began to exploit both dry-land and freshwater habitats, and jawed fish such as sharks and bony fish began to cruise the oceans. Finally, at the very end of the Devonian Period, the first amphibians crawled out onto land and inhaled air with lungs (figure above c).

The Late Paleozoic Era  (Carboniferous–Permian Periods, 359–251 Ma)


Paleogeography at the end of the Paleozoic Era.
The climate cooled significantly in the late Paleozoic. Seas gradually retreated from the continents, so that during the Carboniferous Period, regions that had hosted the limestone-forming reefs of epicontinental seas now became coastal areas and river deltas in which sand, shale, and organic debris accumulated. In fact, during the Carboniferous Period, Laurentia lay near the equator, so it enjoyed tropical and semitropical conditions that favored lush growth in swamps. This growth left thick piles of plant debris that transformed into coal after burial. Much of Gondwana and Siberia, in contrast, lay at high latitudes, and by the Permian Period became covered by ice sheets. The late Paleozoic Era also saw a succession of continental collisions, culminating in the formation of a single supercontinent, Pangaea (figure above a). The largest collision occurred during Carboniferous and Permian time, when Gondwana rammed into Laurentia and Baltica, causing the Alleghanian orogeny of North America (figure above b). 

 Features of the Appalachian Mountains in the eastern United States.
During this event, the final stage in the development of the Appalachians, eastern North America rammed against northwestern Africa, and what is now the Gulf Coast region of North America squashed against the northern margin of South America. A vast mountain belt grew, in which deformation generated huge faults and folds. We now see the eroded remnants of rocks deformed during this event in the Appalachian and Ouachita Mountains. Along the  continental side of the range, a wide band of deformation called the Appalachian fold-thrust belt formed (figure above). Movement on the faults displaced strata and resulted in the formation of large folds. At depth, the thrust faults merged with a near-horizontal sliding surface, called a detachment, just above the Precambrian basement. Stresses generated during the Alleghanian orogeny were so strong that preexisting faults in the continental crust clear across North America became active again. The movement produced uplifts and sediment-filled basins in the Midwest and in the region of the present-day Rocky Mountains. Geologists refer to the late Paleozoic uplifts of the Rocky Mountain region as the Ancestral Rockies.
The assembly of Pangaea involved a number of other collisions around the world as well. Notably, Africa collided with southern Europe to form the Hercynian orogen. Also, a rift or small ocean in Russia closed, leading to the uplift of the Ural Mountains, and parts of China along with other fragments of Asia attached to southern Siberia. 

Life evolution

A museum diorama of a Carboniferous coal swamps includes a giant dragonfly, with a wingspan of about 1 m.
The insect photo gives series of its size relative to human.
The fossil record indicates that during the late Paleozoic Era, plants and animals continued to evolve toward more familiar forms. In coal swamps, fixed-wing insects including huge dragonflies flew through a tangle of ferns, club mosses, and scouring rushes, and by the end of the Carboniferous Period insects such as the cockroach, with foldable wings, appeared (figure above). Forests containing gymnosperms (“naked seed” plants such as conifers) and cycads (trees with a palm-like stalk and fern-like fronds) became widespread in the Permian Period. Amphibians and, later, reptiles populated the land. The appearance of reptiles marked the evolution of a radically new component in animal reproduction: eggs with a protective covering. Such eggs permitted reptiles to reproduce without returning to the water. The late Paleozoic Era came to a close with a major mass extinction event, during which over 95% of marine species disappeared. Why this event occurred remains a subject of debate. According to one hypothesis, the terminal Permian mass extinction occurred as a result of an episode of extraordinary volcanic activity in the region that is now Siberia; basalt sheets extruded during the event are known as the “Siberian traps.” Eruptions could have clouded the atmosphere, acidified the oceans, and disrupted the food chain.
Credits: Stephen Marshak (Essentials of Geology)