Geology is the scientific study of planet Earth and its history, through 4,600 million years to the pres­ent. This natural science is traditionally divided into two branches: physical geology and historical geology. Physical geology focuses on physical structure, materials, and geological processes of the earth. Historical geology examines the origin of our planet and life, and all the climatic, geo­graphic, oceanographic, and biological events that have taken place across geological time. This dual division is rather arbitrary, therefore both points of view (physical and historical) are found currently integrated within the framework of plate tectonics, the current paradigm of geological science.

Physical Geology

Physical geology includes such disciplines as: geo­physics (applies principles of physics to the study of the earth); geochemistry (the study of the chemical characteristics of minerals and rocks); mineralogy and petrology (the study of the origin, properties, structure, and classification of miner­als and rocks, respectively); hydrogeology (the study of the origin, occurrence, and movement of water masses); structural geology (the study of the deformational history of rocks and regions and of the forces responsible); geomorphology (the study of the origin and modification of land­forms); volcanology (the study of volcanoes and magma formation processes); sedimentology (the study of sedimentary rocks and the processes by which they were formed); and engineering geol­ogy (the study of the interactions of the earth’s crust with human-made structures such as tunnels and mines).

Some areas of specialization for professional geologists related to physical geology include exploration and extraction of natural resources (mineral deposits, coal, oil, etc.), prediction and evaluation of geological hazards (landslides, earth­quakes, volcanic eruptions, or meteoritic impacts), evaluation of the stability of construction sites, the search for supplies of clean water, and analysis of environmental problems such as soil and coastal erosion.

Historical Geology

A consubstantial part of geology is the study of how Earth’s materials and continents, surface environments, processes, and organisms have changed over geological time. Processes, fossils, and geological events are recorded in rocks. Thus the main objective of historical geology is the analysis of the geological record in order to reconstitute and understand the earth’s history. Historical geology is based on paleontology (the study of life in the past from the fossil record, including evolutionary relationships, and its applications in environmental recon­structions and in the relative dating of rocks); stratigraphy (the study of stratified rocks in terms of mode of origin, original succession, relative dating, and geologic history), and paleogeography (the reconstruction of the ancient geography of the earth’s surface). Other important disciplines closely related to histori­cal geology are: paleoclimatology (the applica­tion of geological science to determine past climatic conditions) and paleoceanography (the reconstruction of the history of the oceans, with regard to circulation, chemistry, or pat­terns of sedimentation).

When geologists come to interpret the earth’s history, they rely on two complementary types of dating of rocks: relative dating and absolute dating. Relative dating places historical events in their cor­rect temporal order, and absolute dating provides a numerical age for a rock and establishes how many years ago a geological event took place.

Relative Dating

The relative ages of rocks and events in geologic sequences can be established by interpreting the fos­sil record and utilizing several basic principles in stratigraphy. The four most important are: the prin­ciple of original horizontality (sedimentary rocks are formed in essentially horizontal beds named strata; nonhorizontal strata have been disturbed after lithification), the principle of superposition (in any undisturbed stratigraphic sequence, older strata are buried beneath younger strata), the principle of intersection (when a fault or igneous intrusion cuts across a formation of sedimentary rocks, the fault or the intrusion is younger than these strata), and the principle of inclusions (the inclusion of a rocky body in a sequence of strata is older than the sedi­mentary rocks that contain it). Since the 19th cen­tury, the application of these rules to establish the relative ages of rocks contributed to developing the standard geological column in geological sites undisturbed or minimally disturbed, such as the Zumaya stratigraphic section in Spain.

Geologists soon understood that to develop a global geological timescale, a comparison of rocks of similar age located in different regions or continents was required. This process is known in stratigraphy as “correlation.” Correlation involves matching up rock layers of similar age that are in different regions. When conditions of exposure are good, the litho-correlation across short dis­tances can be done by applying the principle of lateral continuity (sediments are deposited form­ing strata over a large area in a continuous sheet). When correlation involves a long distance (even between two different continents), geologists depend on the fossil remains of ancient organisms. In this case the bio-correlation is carried out using the principle of faunal succession (in a strati­graphic sequence, fossil species succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content).

Fossil species appear and disappear throughout the fossil record as a consequence of the evolution (speciation and extinction) of species. As each fos­sil species lived during a specific interval of geo­logical time, its presence (or sometimes absence) may be used to provide a relative age for the strata in which it is found. Each fossil species also lived at the same time in a more or less extensive geograph­ical region. For this reason, paleontologists can use fossils to establish a bio-correlation of strata among different localities, where fossils of one spe­cies or of a species assemblage were present.

Correlation based on fossils is the focus of bio­stratigraphy, a discipline that deals with the distri­bution of fossils in the stratigraphic record, and the organization of strata into correlatable units (bio­zones) on the basis of the fossils they contain. Most species lived for tens of millions of years before they became extinct or evolved into new species. Nevertheless, some species lived for only hundreds of thousands of years or a few million years. If they were limited to a short period of time, had a wide geographic distribution, and are abundant in the fossil record, these species are considered “index fossils.” Index fossils provide a precise means for estimating the relative age of sedimentary rock, and for correlating biozones. For example, many species of trilobites, ammonites, or foraminifera are ideal for biostratigraphy in marine series, and micro-mammals, grains of pollen, and spores are good index fossils for continental deposits.

Absolute Dating

The discovery of radioactivity and the develop­ment of the mass spectrometer in the 20th century permitted many radioactive elements to be used as geologic clocks. These techniques are based on the natural decay of radioactive elements (unstable isotopes) that cause the radioactive parent ele­ments to decay to stable daughter elements.

Through the radioactive decay of isotopes in rock, exotic daughter elements are introduced over time. By measuring the concentration of the stable end products of decay, coupled with knowledge of the half-life and initial concentration of the decaying elements, the age of a rock can be calculated. This technique is known as radiometric dating, a power­ful tool for reconstructing the earth’s history.

Potassium-40, for example, decays into argon-40 after a half-life of 1.25 billion years, so that after 1.25 billion years half of the potassium-40 in a rock will have become argon-40. This means that if a rock sample contained equal amounts of potassium-40 and argon-40, it would be 1.25 billion years old. Other isotopes used in radio­metric dating are: uranium-235, uranium-238, thorium-232, and rubidium-87.

Relative Versus Absolute Dating

Relative dating provides a relative timescale formed by bio- and chronostratigraphical units. Absolute dating provides an absolute timescale composed of geochronological units. But how are both scales correlated and integrated in the geo­logical timescale if fossils occur in sedimentary rocks, and most minerals that contain radioactive isotopes are in igneous rocks?

The simplest method to correlate both scales is the radiometric dating of volcanic ash or of oce­anic basaltic layers that are interbedded between sedimentary rocks. The age of the volcanic rock is younger than the underlying sedimentary rocks and older than the overlying sedimentary rocks. If these sedimentary rocks contain fossils, they are very relevant since they can be correlated with the biozones defined in this area. Thus, rocks whose ages have been determined by absolute dating can be incorporated into a succession of strata deter­mined by relative dating. Then geologists can use correlations to infer the ages of rocks and fossils that cannot be directly dated. In fact, the com­bined absolute/relative timescale is always being revised in order to produce an even more precise picture of Earth’s history.

A Brief History of the Earth

From the modern methods of radiometric age dat­ing, we know that the earth is around 4.6 billion years old. Historical geologists divide all this geo­logical time into three major divisions (eons): Archaean, Proterozoic, and Phanerozoic, although a fourth pregeologic eon is considered: Hadean. All of them are made up of eras that usually ended with profound changes in the disposition of the earth’s continents and oceans, and are character­ized by the emergence of new forms of life or by the disappearance of ancient ones.

Hadean Eon (4550-3900 Million Years Ago)

The Hadean eon is the geologic time extending from the birth of the solar system and the earth’s formation 4,600-4,550 million years ago, to the formation of the oldest rocks 3,900-3,800 million years ago (mya). The Hadean is the first eon in the earth’s History, but very little geological record was preserved because the earth’s surface was molten.

The solar system’s planets, including the earth, were formed by accretion from a cloud of gas and dust known as solar nebula. Grains in orbit around the central primitive sun (protoplanetary disk) began to join, collecting in bodies called planetesimals. In a few million years, several large planets grew through low-velocity collisions between nearby planetesimals. The planets and satellites formed in the inner solar system—Earth, Mars, and Venus—were composed mainly of materials with high melting points, such as sili­cates and metals (iron and nickel).

Since the Hadean lacks any official status, this eon has been arbitrarily subdivided in three infor­mal periods, taking into account the primitive geo­logical history of the moon: Cryptic (4,550-4,500 mya), Ryderian (4,500-4,100 mya), and Nectarian (4,100-3,900 mya). The first period includes the accretion time of Earth from the solar disk, the big whack event, and the formation of the moon. The big whack is a hypothetical event that occurred roughly 4,530 mya in which a Mars-sized body, usually called Theia, impacted the proto-Earth at an oblique angle. The giant impact destroyed Theia, ejecting into space most of its mass together with a significant portion of the earth’s silicate mantle. A ring of debris began to orbit near the earth’s equator, coalescing into the moon about 100 years after the huge impact. The Ryderian era includes the progressive cooling of the earth (and the moon) and the process of differentiation of the earth’s core, mantle, and protocrust, when the earth acquired a primitive inner structure.

A great number of asteroiDalí and cometary objects remained among the newborn planets, starting a well-known period called the Late Heavy Bombardment (LHB) during the Nectarian era. The LHB happened about 4,100-3,800 mya, resulting in a large number of impact craters on the earth as well as on the rest of the planets and satellites in the solar system. This cataclysm period created an enormous amount of heat, completely melting the earth and allowing its materials to separate definitively into three main layers: iron core, silicate mantle, and thin outer crust. This intensive bombardment probably destroyed the primitive earth’s protocrust, retain­ing only individual zircon crystals that were rede­posited in most modern sediments. The oldest known zircons were radiometrically dated to about 4,400 million years and are found in the Acasta Gneiss in western Canada. The study of some of these zircons suggests that there was liq­uid water at that period, indicating that primitive atmosphere and oceans must have existed then. The rock vapor might condense around the young Earth, resulting in a dense atmosphere of carbon dioxide, water, methane, nitrogen, and hydrogen.

Archean Eon (3,900-2,500 Million Years Ago)

The Archean eon is the geologic time that extends from the formation of the oldest rocks 3,900-3,800 mya to 2,500 mya. It is formally subdivided into four eras: Eoarchaean (3,900­3,600 mya), Paleoarchean (3,600-3,200 mya), Mesoarchean (3,200-2,800 ma) and Neoarchean (2,800-2,500 mya). A significant event occurred at the beginning of the Archaean eon: the origin of life, in a scenario with an atmosphere composed mostly of carbon dioxide.

The oldest known rocks on Earth are found in the Issua Greenstone Belt (southwestern Greenland) and include well-preserved volcanic, metamorphic, and sedimentary rocks dated at 3,800-3,700 mil­lion years. Archean volcanic activity was probably considerably more intensive than it is today, and plate tectonics were surely very active because the inner earth was much hotter at that time. A greater rate of recycling of crustal material should have occurred then, preventing the formation of conti­nents. For this reason, only some Archean rocks survive, including metamorphized igneous rocks, such as granites, peridotites, and unusual ultrama­fic mantle-derived volcanic rocks called komatiites. Archean rocks also include stromatolites (the old­est fossil traces of prokaryotic organisms), and hard metamorphized deepwater sediments.

Only when the mantle cooled and convection slowed down could the tectonic activity slow down. Small protocontinents (cratons) were the norm during the Archean eon. Several Archean cratons have been identified, including a hypo­thetical first continent now called Vaalbara. According to radiometric dating, the Vaalbara continent existed at least 3,300 mya during the Paleoarchean era. This continent collected the two only known Eoarchean cratons: Kaapvaal craton (South Africa) and Pilbara craton (Western Australia). Other continents were probably formed 3,000 mya in the Mesoarchean era, grouping small cratons of present western Australia, eastern India, eastern Antarctica, and southeastern Africa. It is now called the Ur continent.

During the Neoarchean era, 2,700 mya, the ear­liest known supercontinent, called Kenorland, was born. It formed as result of a series of events of accretion, comprising several cratons: Laurentia (including Canada and Greenland), Baltica (including Scandinavia and the Baltic area), Yilgarn (Western Australia), and Kalahari (Botswana, South Africa, and Namibia).

Proterozoic Eon (2,500-542 Million Years Ago)

The Proterozoic eon began 2,500 mya, and ended with the disappearance of the complex Ediacaran biota 542 mya. The geological record of the Proterozoic is much better known than that of the Archean, since Proterozoic rocks are less metamorphized and are more abundant. Many Proterozoic sedimentary rocks were deposited in extensive shallow epicontinental seas, and their study suggests that Proterozoic plate tectonics were both massive and rapid.

The Proterozoic eon has been formally subdi­vided into three eras: Paleoproterozoic (2,500­1,600 mya), Mesoproterozoic (1,600-1,000 mya), and Neoproterozoic (1,000-542 mya). Both the start and the end of the Proterozoic were marked by widespread glaciation: the Huronian and Varangian glaciations, respectively.

Paleoproterozoic Era (2,500-1,600 Million Years Ago)

The Huronian glaciation began 2,400 mya and lasted until 2,100 mya. It was one of the most severe ice ages, and it is possible that the earth’s surface was entirely covered by ice. It is perhaps related to the decrease of atmospheric carbon dioxide, consumed and captured by photosyn­thetic microorganisms, and the virtual disappear­ance of the greenhouse gas methane due to chemical oxidation.

The neoarchaic Kenorland supercontinent began to break up at the start of the Siderian period (2,500-2,300 mya) when two Baltic fragments, the Kola and Karelia cratons (today in Russia), began to drift apart. During the Rhyacian (2,300­2,050 mya), several continents were created out of Kenorland when it broke up: Arctica (including today’s Canada and Siberia), Atlantica (including today’s eastern South America and western Africa), and Baltica (northern Europe). Cyanobacteria developed in the Siderian and Rhyacian seas, pro­ducing a great quantity of photosynthetic oxygen that resulted in a large increase of this gas in the atmosphere. The combination of oxygen with the iron dissolved in the oceans formed a distinctive Archaic-Paleoproterozoic type of rock called banded iron formations (BIFs) that are composed of iron oxides like magnetite and hematite.

The atmosphere became oxygen rich during the Orosirian period (2,050-1,800 mya) with a pro­gressive decrease in iron dissolved in the ocean. The formation of BIFs ceased, but the deposition of the so-called red beds began. These rocks are tinged with hematite, indicating an increase in atmospheric oxygen. The progressive accumula­tion of oxygen in the atmosphere might have caused the extinction of numerous groups of anaerobic bacteria, the only type of life that existed up to that time. This event has sometimes been called the oxygen catastrophe. From the point of view of plate tectonics, collisions between cratons were common during this period, forming multiple orogens worldwide that were the prelude to the formation of the supercontinent Columbia.

The last period of the Paleoproterozoic, the Statherian (1,800-1,600 mya), was characterized by its tectonic stability. At the beginning of the Statherian, the supercontinent Columbia was formed and lasted from approximately 1,800 to 1,500 mya. It consisted of almost all of Earth’s continents: Laurentia (proto-North America), Nena (Arctica, Baltica, and eastern Antarctica), Atlantica (Amazonia and western Africa), Ur (Australia, India, and northern Antarctica), and possibly northern China and Kalahari (southern Africa). At the beginning of the Statherian a new type of cell appeared, eukaryotic cells, that expe­rienced great evolutionary exits in later periods of mass extinction.

Mesoproterozoic Era (1,600-1,000 Million Years Ago)

The Mesoproterozoic era began with the breakup of Columbia and ended with the forma­tion of a new supercontinent: Rodinia. By about the Mesoproterozoic era, 80% of the earth’s conti­nental crust had been formed. Columbia began to fragment at the start of the Calymnian period (1,600-1,400 mya), forming several continents that drifted apart, including Laurentia, Nena, Australia- Antarctica, Amazonia, Sahara (northern Africa), Congo (west-central Africa), Kalahari (southern Africa), and Sao Francisco (southeastern South America).

Rodinia began forming in the Ectasian period (1,400-1,200 mya) and finished assembling dur­ing the Stenian period (1200-1000 mya). The Laurentian continent was the core of Rodinia, and was sandwiched between two large blocks: East Gondwana (the original continent of Ur) and West Gondwana (the original continent of Atlantica). Moreover, Baltica and Siberia were nearby, to the southeast and northeast of Laurentia, respectively. Rodinia was surrounded at that time by a great superocean called Mirovia, in which the first cells with sexual reproduction appeared.

Neoproterozoic Era (1,000-542 Million Years Ago)

The Neoproterozoic era was a time of complex continental drift that followed the breakup of Rodinia. The breakup of this supercontinent started in the Tonian period (1,000-850 mya) and continued through the Cryogenian period (850­630 mya). In the first phase, 800 mya, Rodinia split into two large segments. The lands of East Gondwana and Congo (the Proto-Gondwana con­tinent) moved north while rotating counterclock­wise, beginning the formation of the Panthalassic superocean. The great block composed of Laurentia, Siberia, Baltica, South China, and West Gondwana (the Proto-Laurasia continent) drifted southward, rotating clockwise. The Panafrican Ocean formed between these macrocontinents.

Recent studies indicate that at least seven inde­pendent continents must have existed around 750 mya: Laurentia-Baltica-West Gondwana, North China, Siberia, Australia-East Antarctica, South China,Malani(India,Iran, Arabia, and Madagascar), and Congo-Kalahari. During the Ediacaran period (630-542 mya), most of the earth’s large conti­nents came together again in the southern hemi­sphere. They formed the Pannotia supercontinent, which was short lived since it lasted only about 60 million years.

The Cryogenian period is characterized by the massive Varanginian glaciations that are repre­sented by worldwide tillite deposits, suggesting that the earth suffered the most severe ice age of its history with glaciers extending as far as the equator. It is believed that all the planetary oceans were deeply frozen, a phenomenon known as Snowball Earth. Finally, the last Neoproterozoic period, the Ediacaran, is unusual because it con­tains strange soft bodied fossils known as Ediacaran biota. The severe Neoproterozoic glaciations caused profound changes in oxygen levels and ocean chemistry, which could explain why life developed intensively during the Ediacaran and later during the Cambrian.

Phanerozoic Eon (542 Million Years Ago to the Present)

The Phanerozoic eon left a rich fossil record, starting with the Cambrian explosion about 540 mya. It is formally subdivided into three eras: Paleozoic (542-251 mya), Mesozoic (251-65 mya), and Cenozoic (65 mya to the present).

Earth seems to have gone through alternating icehouse (with ice caps) and greenhouse (without ice caps) phases during the Phanerozoic, perhaps partly controlled by how the continents and oceans were distributed. At least three icehouse phases are known: Late Ordovician-Silurian (about 460-416 mya), Late Carboniferous-Permian (about 318-251 mya), and the Oligocene-Neogene period (34 mya to the present). Throughout the Phanerozoic new continents and oceans appear and disappear, assem­bling and separating until reaching their present locations. This is a complex history, and the reader is invited to visit the Paleomap project’s Web page where several full-color paleogeographic maps are available that show the changing distribution of lands and seas, as well as the various continental and oceanic plates that developed in the earth’s recent history.

Paleozoic Era (542-251 Million Years Ago)

The Paleozoic era covers the geological time from the first occurrence of an abundant fossil record (Cambrian Explosion) to the greatest mass extinction event in the earth’s history: the Permian- Triassic boundary event. The Cambrian Explosion was the greatest evolutionary radiation in the earth’s history, bringing forth nearly all the major groups or phyla of animals, including the trilo- bites. The Paleozoic began with the breakup of the Pannotia supercontinent and ended with the for­mation the last known supercontinent, Pangea.

Pannotia started to break up at the beginning of the Cambrian period (542-488 mya) and formed four continents: Laurentia (North America), Baltica (Northern Europe), Siberia, and Gondwana. The three first drifted toward the north and Gondwana drifted toward the south, with most of the land staying in equatorial lati­tudes. The Panthalassic superocean covered most the northern hemisphere, and two new minor oceans began to form: Proto-Tethys and Iapetus. Proto-Tethys formed between the proto-Laurasian continents (Laurentia, Baltica, and Siberia) and Gondwana. Iapetus formed between Baltica and Laurentia. The Cambrian oceans seem to have been broad and shallow, causing a climate signifi­cantly warmer than that of the preceding periods dominated by ice ages.

At the start of the Ordovician period (488-443 mya), Gondwana began to drift toward the South Pole. The Ordovician climate was very warm, and Laurentia, Baltica, and Gondwana were widely covered by warm shallow seas, allowing the develop­ment of shelled organisms and the deposition of great amounts of biogenic limestone. While the Panthalassic superocean still covered most of the northern hemisphere, another ocean was born, the Paleo-Tethys, which was taking over territory at the expense of ancient Proto-Tethys. The new Rheic Ocean was the result of the formation of the Avalonia microcontinent (composed mainly of Newfoundland and England) that had broken off from Gondwana and drifted toward Baltica. That ocean was gaining territory at the expense of the old Iapetus Ocean. By the end of the Ordovician, Gondwana was approach­ing the South Pole and was largely glaciated. The extensive glaciation of Gondwana and the subse­quent fall of the sea level may have triggered the Late Ordovician mass extinction event between 447-444 mya, in which the 85% of species died off.

The icecaps of the Silurian period (443-416 mya) were less extensive than those of the Late Ordovician, and the melting of the Silurian ice­caps and glaciers contributed to a rise in the sea level. During this period, continents drifted near the equator and the earth’s climate entered a long greenhouse phase with warm shallow seas cover­ing much of the equatorial landmasses. Coral reefs expanded, and land plants began to colonize the barren continents. Baltica and Laurentia started to collide, completely closing off the Iapetus Ocean and forming the Euramerica macrocontinent. Such collisions folded the sediments deposited in the previous Iapetus basin, forming the Caledonian orogen (Appalachian Mountains, the Anti-Atlas in Morocco, and the Caledonian Mountains in Great Britain and Scandinavia). At that time, the Rheic and Paleo-Tethys oceans occupied the area between Euramerica and Gondwana, and between Siberia and Gondwana, respectively. During the Silurian a new ocean was formed, the Ural Ocean, located between Siberia and Baltica.

The Devonian period (416-359 mya) was a time of great tectonic activity as the continents drew closer together. The Ural Ocean disappeared toward the end of the Devonian when Siberia col­lided with the Baltica coast of Euramerica, form­ing the Ural Mountains and the Laurussia macrocontinent, also called the Old Red Continent. Sea levels were high worldwide during the Devonian, with broad areas of continents widely submerged under shallow seas where tropical reef organisms were plentiful. The deep waters of the giant Panthalassic superocean covered most of the earth, Paleo-Tethys continued spreading, and other minor oceans the Rheic Ocean began to close off. The Devonian ended with a major mass extinction event that affected up to 82% of all species, probably caused by several meteorite impacts like those forming the 120-kilometer- diameter Woodleigh crater (Australia).

The well-known Pangea supercontinent began to form in the Carboniferous period (359-299 mya). Laurussia and Gondwana collided during this period, definitively closing the Rheic Ocean. The collision occurred along the eastern coast of North America (where the Alleghenian orogen was formed) and the northwestern coast of South America, Africa, and South Europe (where the Hercinian orogen emerged). Finally, North China collided with Siberia at the end of the Carboniferous, closing off another old ocean, the Proto-Tethys. Most the continents were grouped together toward the end of the Carboniferous, although South China was still separated from Laurussia by the Paleo-Tethys Ocean. The Carboniferous was a period of active mountain-building. There was also a drop in south polar temperatures, glaciating the southern portion of Gondwana. Nevertheless, the tropical latitudes were warm and humid, allowing for the development of extensive forests and swamps.

Except for the South China continent, all of Earth’s major land masses were grouped together as Pangea in the Permian period (299-251 mya); Pangea reached from the equator toward both poles. This affected ocean currents in the Panthalassic superocean and the Paleo-Tethys, which was located between the Asian part of Laurussia and Gondwana. At the beginning of the Permian, a rift started to open from the north of Gondwana to form a new ocean: the Tethys Ocean. The Permian ended with the most exten­sive mass extinction event in geological history: the Permian-Triassic extinction event, affecting more than 90% of species. The Late Permian gla­ciations, Siberian Traps volcanism, severe drop in sea levels, and even several large meteorite impacts, among other causes, have been proposed to explain the massive extinction.

Mesozoic Era (251-65 Million Years Ago)

The Mesozoic era spans geological time from the Permian-Triassic boundary event 251 mya to the Cretaceous-Tertiary boundary event 65 mya. It was an era of intensive tectonic, climatic, and evolutionary activity.

During the Triassic period (251-199 mya), almost all of the earth’s landmasses had collected into a single supercontinent, Pangea, that was surrounded by the Panthalassic superocean. The recently formed Tethys Ocean continued opening at the expense of the Paleo-Tethys. Since Pangea’s large size limited the moderating effect of the oceans, the Triassic climate became very hot and dry, forming typical red bed sandstones and gyp­sum. This kind of terrestrial climate was suitable for reptiles. The Triassic ended with another major mass extinction episode that affected mainly marine environments. The cause of this extinction is uncertain, but global cooling or even meteorite impacts have been proposed. These extinctions allowed the dinosaurs to expand, ini­tiating the Age of Dinosaurs that spanned the Jurassic (199-145 mya) and the Cretaceous (145-65 mya).

In the Early Jurassic, the oceanic crust of the Paleo-Tethys was completely subducted. This epi­sode resulted in the collision of South China with Laurussia, initiating the Cimmerian orogeny that created mountain ranges as high as today’s Himalayas. Pangea was shaped like a “C” at that time, curling around the Tethys Ocean with the concave area occupied by the Tethys Ocean. Both Pangea and Tethys were surrounded by the huge Panthalassic superocean. As in the Triassic, the Jurassic climate was warm; there is no evidence of glaciation since no continent was near either pole. During this period, Pangea began to break up into two major landmasses: Laurasia (North America and Eurasia) and Gondwana (South America, Africa, Australia, Antarctica, and India), which began the opening of the Central Atlantic Ocean. Toward the end of the Jurassic, the Panthalassic superocean converted into the current Pacific Ocean.

Pangea was definitively broken up during the Cretaceous into today’s continents, although the continents had positions substantially different from today’s. The progressive drift of the Laurasian and Gondwanan landmasses opened the western Tethys Ocean (today’s Mediterranean Sea) and separated North America and South America, forming a continuous ocean current around the equator. The North Atlantic Ocean opened, separating Iberia from Newfoundland and England- Scandinavia from Greenland. The separation of South America and Africa formed the South Atlantic Ocean. Finally, the drift of India northward formed the Indian Ocean, which was gaining territory at the expense of eastern Tethys. This active rifting during the opening of the Atlantic Ocean raised mountain ranges (including the giant American Cordillera: the Rocky Mountains, the Sierra Madre, and the Andes Mountains) around the entire coast­line of old Pangea. This intensive tectonic activity raised the sea level, forming broad shallow seas over North America (the Western Interior Seaway) and Europe. The Cretaceous climate was very warm, devoid of ice at the poles. The Cretaceous ended with the most recent major mass extinction event: the Cretaceous-Tertiary event 65 mya, affecting more than 75% of species. A meteorite impact that formed the approximately 180-kilome- ter-diameter Chicxulub crater (Yucatan, Mexico) appears to be its cause.

Cenozoic Era (65 Million Years Ago to the Present)

The Cenozoic era covers geological time from the Cretaceous-Tertiary mass extinction event to the present. During the Cenozoic, mammals evolved from a few small insectivores that had survived the Cretaceous-Tertiary extinction. The continents moved to their current positions, causing remark­able climatic and oceanographic changes.

During the Paleogene period (65-23 mya), the Laurasian and Gondwanan plates formed in the Cretaceous continued to split apart, with North America, Eurasia, South America, Africa, India, and Antarctica-Australia pulling away from each other. India continued its migration toward central Eurasia, and Africa also headed north toward western Eurasia, slowly closing the Tethys Ocean during the Paleocene epoch (65-56 mya). The rift­ing and splitting apart of Eurasia, Greenland, and North America increased hydrothermal and volca­nic activity in the North Atlantic. This tectonic episode combined with the movement of the Indian plate northward, restricted the Tethys oceanic cur­rent, and helped to trigger the Paleocene-Eocene Thermal Maximum event 56 mya. During the Eocene epoch (56-34 mya), Antarctica and Australia began to split, leaving Antarctica isolated in its current location at the South Pole. Moreover, the Indian microcontinent collided with central Eurasia, folding the Himalayas upward and closing off the eastern Tethys Ocean. The western Tethys was converting into the present Mediterranean Sea, which was being progressively narrowed by the drift of Africa (including Arabia) toward Europe. The Alps started to rise in Europe as the African continent continued to push north into the Eurasian plate. All these continental movements culminated in the Oligocene epoch (34-23 mya). At that time Antarctica was isolated definitively, forming the circumantarctic ocean current and allowing a per­manent ice cap to develop. A global cooling occurred during the Late Eocene and the Oligocene, causing a gradual major extinction episode.

The Neogene period covers the past 23 million years, during which modern birds and mammals, including humans, evolved. Continents continued to drift toward their current positions. During the Miocene epoch (23-5.3 mya), global mountain building continued to take place, raising the western American cordilleras, the Alps, and the Himalayas. Arabia, which became part of the Africa plate, col­lided with Eurasia, forming the Caucasus Mountains, separating the Indian Ocean and the Mediterranean Sea and closing off the remnants of the old Tethys. The rise of mountains in the western Mediterranean (the Betic Cordillera in southern Spain and the Rif Mountain in northern Morocco), combined with the global drop in sea level due to formation of Antarctica ice cap, caused what is known as the Messinian Salinity Crisis in the Mediterranean Sea approximately 6 mya. The Mediterranean Sea has dried up several times due to repeated closings of the old Straits of Gibraltar, thus forming enormous evaporative deposits throughout the Mediterranean. When the present Strait of Gibraltar eventually opened, the Atlantic would have poured a vast vol­ume of water into the dry Mediterranean basin in a gigantic waterfall much more than 1,000 meters high and far more powerful than Niagara Falls. During the Pliocene epoch (5.3-1.8 mya), South America and North America joined, creating the Isthmus of Panama. This tectonic episode had major consequences for global temperatures: warm equa­torial ocean currents were cut off and the climate became cooler and drier, resulting in the formation of the Arctic ice cap 2 mya. Both the Antarctic and the Arctic became much colder. During the Pleistocene epoch (1.8 million to 11,500 years ago), the modern continents were essentially at their present positions, initiating repeated glacial cycles. Four major glacial episodes have been identified (usually called Günz, Mindel, Riss, and Würm in the Alps; Nebraskan, Kansan, Illinoian, and Wisconsin in North America; Weichsel or Vistula, Saale, Elster, and Menapian in Northern Europe; and Devensian, Wolstonian, Anglian, and Beestonian in Britain), separated by interglacial episodes. The Holocene is the geologi­cal epoch that spans the last 11,500 years of the earth’s history (from 9,500 BCE to the present), starting with the retreat of the Pleistocene glaciers. It was preceded by the Younger Dryas cold period, the final part of the Pleistocene, and it is charac­terized by global warming. The optimum climate of the Holocene has favored the flourishing of human civilizations.

Jose Antonio Arz

See also Chronostratigraphy; Decay, Radioactive; Earth, Age of; Fossil Record; Geological Column; Geologic Timescale; Hutton, James; Lyell, Charles; Paleontology; Plate Tectonics; Steno, Nicolaus; Stratigraphy; Wegener, Alfred

Further Readings

Grotzinger, J., Jordan T., Press F., & Siever R. (2006). Understanding Earth (5th ed.). New York: Freeman.

Poort, J. M., & Carlson, R. J. (2004). Historical geology: Interpretations and applications (6th ed.). New York: Prentice Hall.

Stanley, S. M. (2004). Earth system history (2nd ed.). New York: Freeman.

Wicander, R., & Monroe J. S. (2003). Historical geology: Evolution of Earth and life through time (4th ed.). London: Brooks/Cole.

Web Sites

Paleomap Project: scotese.com/earth.htm