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Plate Tectonics & Earth History

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Christopher Ervin

on 13 May 2012

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Transcript of Plate Tectonics & Earth History

Plate Tectonics &
Earth History
Earth Science
C. Ervin
Understanding plate tectonics
A Journey through Earth's History
Earth has distinct layers, that include its crust, the mantle and the core.
Layers and Composition of the Earth
The crust is the outer most layer of Earth, it ranges from 5 to 100km thick.
There are 2 main types of Earth's crust.
Continental Crust and Oceanic Crust
Continental Crust has a composition similar to the rock called granite and has an average thickness of about 30km.
Oceanic Crust has a composition similar to basalt and has an average thickness of about 5 to 8km.
Oceanic crustal material is made up of a more dense rock, basalt than continental crust. Therefore oceanic crust is more dense than continental crust.
Another way to talk about the Earth and its structures is to describe its physical properties.
The Earth can be divided into 5 main physical layers - the lithosphere, asthenosphere, mesophere, outer core, and the inner core.
The Structure of the Earth
Starting on the inside and working our way out to the crust, we start with the inner core. The outer core, mesosphere, asthenosphere, and finally the lithosphere
The Mantle, is the layer of the Earth between the crust and the core. In contrast to the Earth's crust, the mantle is extremely thick and contains most of the Earth's mass. Scientists infer what the composition of the mantle is based on observations they have made on Earth's surface. As in the figure you see here, underwater volcanoes are the "window" into the mantle.
Scientists have learned that the composition of the mantle is similar to the mineral olivine, which has large amounts of iron and magnesium.
Layers and Composition of the Earth
The core extends from the bottom of the mantle to the center of the Earth. By studying other layers of the Earth geologists can get an idea of which elements each is made of. Scientists agree that the composition of the core is mostly made of iron, with smaller amounts of nickel and possibly some sulfer and oxygen.
Layers and Composition of the Earth
Inner Core - The inner core is the solid, dense center of our planet and extends from the bottom of the outer core to the center of the Earth about 6,378 km beneath the Earth's surface.
Outer Core - The outer core is the liquid layer of the core that lies beneath the mantle and surrounds the inner core.
Mesosphere - Beneath the asthenosphere is the strong, lower part of the mantle called the mesosphere. It extends from the bottom of the asthenosphere down to the Earth's core.
Asthenosphere - The asthenosphere ("weak sphere") is a soft layer of the mantle on which pieces of the lithosphere move. It is made of solid rock that, like putty, flows very slowly - at about the same rate your fingernails grow.
Lithosphere - The outer most, rigid layer of the Earth is called the lithosphere ("rock sphere") The lithosphere is made up of 2 parts - the crust and the rigid upper part of the mantle. The lithosphere is divided into pieces called tectonic plates.
The Structure of the Earth
Tectonic plates are pieces of the lithosphere that move around on the asthenosphere. But what exactly does a tectonic plate look like? How big are tectonic plates? How and why do they move around? To answer these questions, you should start thinking of the lithosphere as a giant jigsaw puzzle.
Tectonic Plates
Look at this world map. Notice that each tectonic plate fits the other tectonic plates that surround it. As you can see from the picture, tectonic plates are all different. They come in different sizes, shapes and, compositions.
Wegener's Theory of Continental Drift
Restless Continents
Alfred Wegener, early in the 1900's, wrote about his theory of continental drift. Continental drift is the theory that continents can drift apart from one another and have done so in the past. This theory explains a lot of the puzzling observations that scientists have made about Earth's surface.
Continental drift also explains why fossils of the same plant or animal species can be found on both sides of the Atlantic Ocean.
Many scientists needed proof that supported Wegener's theory. That evidence was not available at the time Wegener revealed his theory. Many years later the evidence that would support his theory came when mid-ocean ridges were discovered. Mid-ocean ridges are places where sea-floor spreading takes place. Sea-floor spreading is the process by which new oceanic lithosphere is created as older materials are pulled away.
Sea-Floor Spreading
The proof of sea-floor spreading supported Wegener's original idea that the continents move. But, because both oceanic and continental crust appear to move, a new theory was needed to explain both continental drift and sea-floor spreading.
Plate Tectonics is the theory that Earth's lithosphere is divided into tectonic plates that move around on top of the Asthenosphere.
The Theory of Plate Tectonics
Ridge Push
Slab Pull
All tectonic plates have boundaries with other tectonic plates. These boundaries are divided into 3 main types depending on how the tectonic plates move relative to each other.
Tectonic Plate Boundaries
Tectonic plates can collide, seperate, or slide past each other.
Convergent Boundary
- When two tectonic plates push into one another, the boundary where they meet is a convergent boundary.
There are three types of convergent boundaries; Continental/continental, continental/oceanic, and oceanic/oceanic.
Divergent Boundaries
- When two tectonic plates move away from one another, it is known as a divergent boundary.
The midocean ridges that mark the spreading centers are the most common type of divergent boundary.
Transform Boundaries
- When two tectonic plates slide past each other horizontally the boundary is known as a transform boundary
Stress is the amount of force per unit area that is put on a given material. This principle also applies to rocks in Earth's crust.
The conditions under which a rock is stressed determine its behavior.
Deforming Earth's Crust
Rocks get Stressed
When a rock changes its shape due to stress, this reaction is called deformation.
The type of stress that occurs on an object when an object is squeezed, as when two tectonic plates collide, is called compression. Compression can have some spectacular results. The Rocky Mountains and the Cascade Range are 2 examples of compression at a convergent plate boundary.
Another form of stress is tension. Tension is a stress that occurs when force acts to stretch an object. Tension occurs at divergent plate boundaries, when two tectonic plates pull away from each other.
The law of Original Horizontality - This principle states that layers of sediment are originally deposited horizontally under the action of gravity. Meaning the oldest layers are on the bottom and the youngest layers are on the top.
Folding occurs when rock layers bend due to stress in the Earth's crust.
Depending on how the rock layers deform, different types of folds are made. This figure shows the 2 most common types of folds, anticlines and synclines.
While some rock layers bend when stress is applied, other rock layers break. The surface along which rocks break and slide past each other is called a fault. The blocks of crust on each side of the fault are called fault blocks.
There are 2 main types of faults. Normal faults and Reverse faults.
Normal Faults - A normal fault is shown in the diagram. The movement of a normal fault causes the hanging wall to move down relative to the footwall. Normal faults usually occur when tectonic forces cause tension that pulls rock apart.
Reverse Faults - A reverse fault is also shown in the diagram. The movement of the reverse fault causes the hanging wall to move up relative to the footwall, the "reverse" of a normal fault. Reverse faults usually happen when tectonic forces cause compression that pushes rocks together.
Strike-slip faults - A third type of fault is this strike-slip fault. These faults occur when forces cause the Earth's crust to break and move horizontally past each other.
When tectonic plates undergo compression or tension, they can form mountains in several ways. The three most common types of mountains are ; folded mountains, volcanic mountains and fault-block mountains.
Plate Tectonics and Mountain Building
Folded Mountains - These form when rock layers are squeezed together and pushed upward. (pile of paper).
Volcanic Mountains - most of the world's major volcanic mountains are located at convergent boundaries. Volcanic mountains form when molten rock erupts onto the Earth's surface. Most volcanic mountains tend to form over the type of convergent boundaries that include subduction zones. There are so many volcanic mountains around the rim of the Pacific Ocean that early exploreers named it the ring of fire.
Fault-block Mountains - When tectonic forces put enough tension on Earth's crust, a large number of normal faults can result. Fault-block mountains form when this faulting causes large blocks of Earth's crust to drop down relative to each other.
Few discussions in geology can occur without reference to geologic time. Geologic time is often discussed in two forms;
Earth's History
Geologists attempt to unravel events and materials that may have occurred or formed millions to billions of years ago. The immensity of geologic time is very difficult to appreciate from our human perspective, but appreciation is necessary to understand the history of the Earth. There are two basic ways to try an make sense of geologic time:
Relative Dating - Placing geologic events in sequential order as determined by their position in the geologic record. This method is responsible for producing the Geologic Time Scale.
Absolute Dating - Provides specific dates for events and materials expressed in years before the present. Radiometric dating (radioactivity was discovered in the 1800's) is the most common method used to obtain absolute dates. This method provided more specific dates to the Geologic Time Scale.
Relative Dating Principles
Earth History
The chronological sequence of rock units can be determined by six basic principles:
Principle (law) of Superposition - In an undeformed sequence of sedimentary rocks, the youngest beds are at the top and the oldest beds are on the bottom (also applies to volcanic rocks). Unfortunately, there is not one single place on Earth where the entire history of sedimentation is preserved in tact.
Principle of Original Horizontality - The observation that sediment particles deposited from water under the influence of gravity form essentially horizontal layers. If you encounter non-horizontal rocks, they have been disturbed after deposition and lithification.
Principle of Lateral Continuity - Sediment extends laterally in all directions until it thins, or pinches out, or terminates against the edge of the depositional basin.
Relative Dating Principles (Continued)
Earth History
Law of Cross-Cutting Relationships - An intrusion or fault that cuts through another rock is younger than the rock it cuts.
Principle of Inclusion - Any inclusion inside a rock is older than the rock it is found in.
Principle of Faunal Succession - Fossil organisms succeed one another in a definite and determinable order, so, any time period can be recognized by its fossil content. **General evolution pattern is from simple to complex organisms.
The geologic history of the Earth began 4.567 (4.6) billion years ago when the planets of the solar system were formed out of the solar nebula, a disk shaped mass of dust and gas left over from the formation of the Sun.
The geologic history of the Earth can be broadly classified into two Eons; the Precambrian Super Eon and the Phanerozoic Eon.
Earth History
Earth's geologic history is broken down into eons, eras, periods, and epochs. Each one having its own name and meaning.
Earth's history has 2 eons, 3 eras and numerous other time divisions.
Geologic Time Periods
The beginning of Earth's history starts in the Precambrian Eon.
The Precambrian includes approximately 90% of all of Earth's geologic history. It starts 4.6 billion years ago and ends at the beginning of the Cambrian period (about 570 million years ago). It includes 3 eons; the Hadean, Archean, and Proterozoic.
Hadean Eon
During the Hadean eon (4.6 - 3.8 Gya), the solar system was forming, probably within a large cloud of gas and dust around the sun, called a protoplanetary disk. The Hadean eon is not formally recognized, but it essentially marks the era before there were any rocks. The oldest zircon found in any rock date from about 4400 Mya - very close to the hypothesized time of Earth's formation.
Geologic Time Periods
Archean Eon
The Earth of the early Archean (3.8 - 2.5 Gya) may have had a different tectonic style. During this time, the Earth's crust cooled enough that rocks and continental plates began to form. Most scientists think because the Earth was hotter, that the plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. Others argue that the lack of Archean rocks is a function of erosion and subsequent tectonic activity.
Geologic Time Periods
Archean rocks are often heavily metamorphosed deep water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations, consisting of alternating high and low-grade metamorphic rocks.
Life Has a History
Explorations Through time
Proterozoic Eon
The geologic record of the Proterozoic (2.5 - .57 Gya) is much better than that of the preceding Archean. In contrast to the deep water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas. These rocks are less metamorphosed than the Archean-age ones, and a large number of them are unaltered. Study of these rocks show that the eon featured massive, rapid continental accretion, super continent cycles, and wholly-modern orogenic activity.
Phanerozoic Eon
The Phanerozoic eon is the current eon in the geologic timescale.
This eon roughly covers 545 million years. During the period, continents drifted about, eventually collected into a single landmass known as Pangea, and then split up into the current continental landmasses. The Phanerozoic is divided into three eras - the Paleozoic, the Mesozoic, and the Cenozoic.
Geologic Time Periods
Geologic Eras
Paleozoic Era - Meaning, "Old life"
The Paleozoic spanned from roughly 545 Mya to roughly 251 Mya, and is subdivided into 6 geologic periods; from oldest to youngest they are: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
Geologically, the Paleozoic starts shortly after the break-up of a super continent called Pannotia, and at the end of a global ice age. Throughout the early Paleozoic, the Earth's landmasses were broken into a substantial number of relatively small continents. Toward the end of the era, the continents came together into a supercontinent called Pangea, which included most of Earth's land area.
Plaeozoic Continued
Geologic Time Periods
Cambrian Period
The Cambrian is a major division of the geologic timescale that begins about 545 Mya. Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic super continent called Pannotia.
The waters of the Cambrian period appear to have been widespread and shallow. Continental drift rates may have been anomalously high. Gondwana started to drift toward the South Pole. Minor oceans included the proto-Tethys Ocean, Iapetus Ocean, and Khanty Ocean.
During this time in Earth's history life on Earth experienced a rapid increase. This rapid increase in life is known as the Cambrian Explosion of Life.
Paleozoic Continued
Geologic Time Periods
Ordovician Period
The Ordovician period started at a major extinction event called the Cambrian Ordovician extinction. The Cambrian-Ordovician extinction event occurred around 488 Mya. During the Ordovician, the southern continents were collected into a single continent called Gondwana.
Gondwana started the period in the equitorial latitudes and, as the period progressed, drifted toward the South Pole. By the end of the period, Gondwana had neared or approached the pole and was largely glaciated.
Life in the Ordovician
Geologic Time Periods
Ordovician strata are characterized by numerous and diverse life, things like trilobites and conodonts (phosphatic fossils with a tooth-like appearance) are found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastiods, bryozoans, corals, crinids, as well as many kinds of brachiopods, snails, clams and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments.
Geologic Time Periods
Remains of Ostraderms (a jaw less, armored fish) from the Ordovician rocks comprise some of the oldest vertebrate fossils known.
The Ordovician comes to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct. The only larger one was the Permian-Triassic extinction event
Oldest to most recent
-Ordovician–Silurian extinction (440-450 Mya)
-Late Devonian extinction (360-375 Mya)
-Permian–Triassic extinction (end of the Permian 251 Mya)
-Triassic–Jurassic extinction (end of the Triassic 205 Mya)
-Cretaceous–Tertiary extinction (end of the Cretaceous 65 Mya)
Geologic Time Periods
The Silurian time period started about 443 Mya. During the Silurian, Gondwana continues its slow southward drift, to high latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation. The melting of the icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity.
During this time, other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.
Paleozoic Era
Silurian Period, Continued
Geologic Time Periods
This time period also marks the wide and rapid spread of jawless fish, along with the important appearances of both the first known freshwater fish and the appearance of jawed fish. Other marine fossils commonly found throughout the Silurian record include trilobites, graptolites, conodonts, stromatoporids, and mollusks.
Paleozoic Era
Devonian Period
Geologic Time Periods
The Devonian is known as "The Age of Fishes".
Fish and land plants become abundant and diverse. First tetrapods appear toward the end of the period. The first amphibians make their appearance. First sharks, bony fish, ammonoids. many coral reefs, brachiopods, and crinoids. New insects, like springtails, appeared. Mass extinction (345 mya) wiped out 30% of all animal families probably due to glaciation or meteorite impact.
Paleozoic Era
Carboniferous Period
Wide spread coal swamps abound during the carboniferous period. Other additions to the Earth during the Carboniferous Period include foraminiferans, corals, bryozoans, brachiopods, blastoids, seed ferns, lycopsids, and other plants. Amphibians also become more common also.
Pictures of the Carboniferous Period
Geologic Time Periods
Paleozoic Era
Carboniferous Continued
Geologic Time Periods
There are 2 sub periods associated with the Carboniferous, the Mississippian and Pennsylvanian.
The Mississippian is a sub period in the geologic timescale or a subsystem of the geologic record. It is the earliest/lowermost of the 2 sub periods of the Carboniferous period lasting from roughly 360 to 320 mya. As with most other geochronologic units, the rock beds that define the Mississippian are well identified, but the exact start dates are uncertain by a few million years. The Mississippian is so named because the rocks with this age are exposed in the Mississippi River Valley.
The Pennsylvanian is, the younger of two subperiods of the Carboniferous Period. It lasted from roughly 320 to 300 mya. As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few million years. The Pennsylvanian is named after the American state of Pennsylvania, where rocks with this age are widespread.
Paleozoic, Continued
Permian Period
Geologic Time Periods
The Permian is a geologic period and system which extends from 300 to 250 Mya. The Permian follows the Carboniferous and precedes the Triassic, and is characterized among land vertebrates by the diversification of the early amniotes into the ancestral groups of the mammals, and turtles. The world at the time was very hot and dry, and was dominated by a single supercontinent known as Pangaea. The extensive rainforests of the Carboniferous had disappeared, leaving behind vast swathes of desert. The Permian Period (along with the Paleozoic Era) ended with the largest mass extinction in Earth's history, the Permo-Triassic extinction event, in which nearly 90% of marine species and 70% of terrestrial species died out.
Triassic Period
Mesozoic Era
The Triassic is a geologic period and system that extends from about 250 to 200 Mya. As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic Period has recently been more accurately dated, but as with most older geologic periods, the rock beds that define the start and end are well identified.
The Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth's biosphere impoverished. The early half of the period witnessed life's slow recovery, while the latter half is characterized by the archosaurs' rise to dominance (dinosaurs did not actually become dominant until the Jurassic[4]).
Triassic Period, continued
Mesozoic Era
The first true mammals also evolved during the Triassic, as well as the first flying vertebrates, the pterosaurs. The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to gradually rift into two separate landmasses, Laurasia to the north and Gondwana to the south. The global climate in the Triassic was mostly hot and dry, but the Earth became cooler and wetter as Pangaea drifted apart.
Marine Life, because the Permian Extinction depopulated the world's oceans, the Triassic period was ripe for the rise of early marine reptiles like Placodus and Nothosaurus. The vast Panthalassan Ocean was soon restocked with new species of prehistoric fish, as well as simple animals like corals and cephalopods.
Jurassic Period
Mesozoic Era
The Jurassic is a geologic period and system that extends from about 200 Mya to 145 Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic is known as the age of reptiles. The Jurassic constitutes the middle period of the Mesozoic era.The start of the period is marked by the major Triassic–Jurassic extinction event. However, the end of the Jurassic period did not witness any major extinction event. The start and end of the period are defined by carefully selected locations.
By the beginning of the Jurassic, the supercontinent Pangaea had rifted into two landmasses, Laurasia to the north and Gondwana to the south. This created more coastlines and caused a change in global climate from hot and dry to warm and humid, and many of the arid deserts of the Triassic were replaced by lush rainforests. The dinosaurs continued to dominate the land, and reached their peak in this period as they diversified into a wide variety of groups, ranging from the carnivorous theropods to the massive, herbivorous sauropods. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Marine reptiles such as ichthyosaurs and plesiosaurs ruled the oceans, while the flying reptiles called pterosaurs continued to dominate the skies.
Mesozoic Era
Cretaceous Period
The Cretaceous, derived from the Latin "creta" (chalk), usually abbreviated K for its German translation Kreide (chalk), is a geologic period and system from circa 145 to 65 (Mya). In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the Cenozoic era. It is the youngest period of the Mesozoic era, and at 80 million years long, the longest period of the Phanerozoic Eon. The end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras. In many languages this period is known as "chalk period".
The Cretaceous was a period with a relatively warm climate and high eustatic sea level. The oceans and seas were populated with now extinct marine reptiles, ammonites and rudists; and the land by dinosaurs. At the same time, new groups of mammals and birds as well as flowering plants appeared. The Cretaceous ended with one of the largest mass extinctions in Earth history, the K–T extinction, when many species, including non-avian dinosaurs, pterosaurs, and large marine reptiles, disappeared.
The Cenozoic Era; meaning "new life", from Greek kainos "new", and zoe "life") is the current and most recent of the three Phanerozoic geological eras and covers the period from 65 mya to the present. The Cenozoic is also known as the Age of Mammals, because the extinction of the non-avian dinosaurs allowed them to greatly diversify and dominate the Earth. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs (as well as other terrestrial and marine flora and fauna) and the end of the Mesozoic Era. The era is ongoing.
Cenozoic Era
Geologically, the Cenozoic is the era when the continents moved into their current positions. Australia-New Guinea, having split from Gondwana during the early Cretaceous, drifted north and, eventually, collided with South-east Asia; Antarctica moved into its current position over the South Pole; the Atlantic Ocean widened and, later in the era, South America became attached to North America.
India collided with Asia between 55 and 45 mya; Arabia collided with Eurasia, closing the Tethys ocean, around 35 million years ago.
The 2 eons are the Precambrian and the Phanerozoic. The 3 eras are the Paleozoic, Mesozoic, and the Cenozoic. Paleo means "old" zoic means "life", combined they mean "old life." Meso means "middle" zoic is still life, so mesozoice translates to middle life. Ceno means recent or new, so cenozoic is recent life or new life.
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