Final The Interior of the Earth and Introductory to Plate Tectonics

plate tectonics theory

A theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.

plate

A large, rigid slab of lithosphere.

arc

A line of volcanoes that forms parallel to the trench

trench

A long depression or trench in the seafloor where the oceanic plate begins to sink into the mantle. This includes the Mariana's trench.

Alfred Wegener

(Born November 1, 1880, Berlin, Germany—died November 1930, Greenland), German meteorologist and geophysicist who formulated the first complete statement of the continental drift hypothesis. The son of an orphanage director, Wegener earned a Ph.D. degree in astronomy from the University of Berlin in 1905. He had meanwhile become interested in paleoclimatology, and in 1906-08 he took part in an expedition to Greenland to study polar air circulation. He made three more expeditions to Greenland, in 1912-13, 1929, and 1930. He taught meteorology at Marburg and Hamburg and was a professor of meteorology and geophysics at the University of Graz from 1924 to 1930. He died during his last expedition to Greenland in 1930. Like certain other scientists before him, Wegener became impressed with the similarity in the coastlines of eastern South America and western Africa and speculated that those lands had once been joined together. In about 1910 he began toying with the idea that in the Late Paleozoic era (about 250 million years ago) all the present-day continents had formed a single large mass, or supercontinent, which had subsequently broken apart. Wegener called this ancient continent Pangaea. Other scientists had proposed such a continent but had explained the separation of the modern world's continents as having resulted from the subsidence, or sinking, of large portions of the supercontinent to form the Atlantic and Indian oceans. Wegener, by contrast, proposed that Pangaea's constituent portions had slowly moved thousands of miles apart over long periods of geologic time. His term for this movement was die Verschiebung der Kontinente ("continental displacement"), which gave rise to the term continental drift. Wegener first presented his theory in lectures in 1912 and published it in full in 1915 in his most important work, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans). He searched the scientific literature for geological and paleontological evidence that would buttress his theory, and he was able to point to many closely related fossil organisms and similar rock strata that occurred on widely separated continents, particularly those found in both the Americas and in Africa. Wegener's theory of continental drift won some adherents in the ensuing decade, but his postulations of the driving forces behind the continents' movement seemed implausible. By 1930 his theory had been rejected by most geologists, and it sank into obscurity for the next few decades, only to be resurrected as part of the theory of plate tectonics during the 1960s.

how do we know about the interior of the Earth?

1) By studying deflections the paths of and changes in the velocity of seismic waves as they pass through the Earth, we know about the structure of the interior of the Earth (dimensions, composition, etc.) Scientists concluded that Earth must be composed of distinct shells having varying compositions and/or mechanical properties. 2) Mines deepest mine: South African gold mine (about 3.6 kilometers) deepest hole: 12 kilometers in Kola peninsula, Russia 3)Pieces of the Earth's interior occasionally show up in lavas. However, these only come from depths of less than 150 kilometers. xenoliths (lith = rock, xeno = foreign) are located in the mantle, and are orange (because they're so hot) but cools and takes on an olivy green color.

what are the most important sources of the Earth's internal heat?

1) Radioactivity 2) Earth's primordial heat -- even after 4.6 billion years, the Earth is still shedding heat from impacts (kinetic energy of impacts is converted to thermal energy) 3) Pressure (with depth)

how we know about the chemistry of the Earth

1) the chemistry of the sun (the sun is made out of the same stuff planets are (came from same planetary nebula). If you remove hydrogen and helium and study the relative proportion of the heavier elements in the sun, you can get a sense of the Earth's composition). 2) the chemistry of chrondrite meteorites (formed from the clumping together of dust particles from the nebula that eventually formed the sun and planets. Analyzing asteroid planet bodies - families of meteorites may represent the crust, mantle, core of the meteorite planet body that had been annihilated by impacts. Studying meteorites that came from core of a parent body could give us information about our own core.

when was the layered structure of the Earth established?

About 50 million years after the formation of the solar system (the time it would take for iron to coalesce to form the core), when the Earth was still molten. Compositional layering occurred early in Earth's history as meteorite impacts and decay of radioactive elements caused the temperature of our planets to steadily increase. Eventually Earth became hot enough so that at least some melting occurred. During this period of partial melting, heavier elements, principally iron and nickel, sank as the lighter rocky components floated upward. This segregation of material is still occurring, but at a much reduced rate. Because of this chemical differentiation, Earth's interior is not homogeneous. Rather, it consists of three major zones, each defined by its chemical composition - the crust, mantle, and core.

lithosphere

Based on physical properties, Earth's outermost layer consists of the crust and uppermost mantle and forms a relatively cool, rigid shell. Although this layer is composed of materials with markedly different chemical compositions, it tends to act as a unit that exhibits rigid behavior - mainly because it is cool and thus strong. The lithosphere averages about 100 kilometers in thickness but may be 250 kilometers thick or more below the older portions of continents. Within ocean basins, the lithosphere is only a few kilometers thick along the oceanic ridges but increases to perhaps 100 kilometers in regions of older and cooler oceanic crust.

asthenosphere

Beneath the lithosphere in the uppermost mantle lies a soft, comparatively weak layer known as the asthenosphere. The top portion of the asthenosphere has a temperature/pressure regime that results in a small amount of melting. Within this very weak zone, the lithosphere is mechanically detached from the layer below. The result is that the lithosphere is able to move independently of the asthenosphere.

Edmond Halley (1656-1742)

Born on November 8, 1656 in Haggerston, Shoreditch -- died January 14, 1742 in Greenwich. Halley was an English astronomer and mathematician, and the first to calculate the orbit of a comet later named after him. He is also noted for his role in the publication of Isaac Newton's Philosophae Naturalis Principia Mathematica.

Henry Cavendish (1731-1810)

British natural philosopher, the greatest experimental and theoretical English chemist and physicist of his age. He was distinguished for great accuracy and precision in researches into the composition of the atmospheric air, the properties of different gases, the synthesis of water, the law governing electrical attraction and repulsion, a mechanical theory of heat, and calculations of density (and hence the weight of) the Earth. His experiment to weigh the Earth has come to be known as the Cavendish experiment. Cavendish experiment (1797-1798) was the first experiment to measure the force of gravity between masses in the laboratory and first to yield accurate calculations of the gravitational constant. In the Cavendish experiment, he calculated that the average density of the Earth was about 5.5 g/cm^3 but rocks on the surface to be about 2.5-3.5 g/cm^3. This was the first indication of an iron core.

Pickering Van Allen

Credited for the discover of the Van Allen belts.

which layers are solid?

Crust, Mantle, Inner core.

coldest later of the Earth?

Crust.

thinnest layer of the Earth?

Crust.

Christopher Wren (1632-1723)

Designer, astronomer, geometrician, and the greatest English architect of his time. Wren designed 53 London churches, as well as many secular buildings of note. He was a founder of the Royal Society and his scientific work was highly regarded by Isaac Newton and Blaise Pascal. Was knighted in 1673.

what are the three types of plate boundaries?

Divergent (plates moving away from one another): At ocean ridges, new oceanic lithosphere is created by upwelling mantle that melts, resulting in basaltic magmas that ascend, intrude, and erupt at the surface to create new oceanic crust. The age of the crust becomes progressively older as in both directions away from the ridge. Very little sediment accumulations on the ridges. Sediment thickness increases in both directions of the ridge (thickest where the oceanic crust is the oldest). Sea floor topography is controlled by age of oceanic lithosphere and rate of spreading. (If spreading rate is high (relative velocity of plates), magma must be rapidly rising and lithosphere is relatively hot beneath. Thus, for fast spreading centers, the ridge stands at higher elevations than for slow spreading centers). Convergent: 2 colliding plates (includes subduction zones, the Wadatii - Benioff Zone, which is a planar zone of earthquakes along the subduction zone that occurs within the subducting plate as it descends into the mantle) As an oceanic plate subducts, it begins to heat up. The release of water into the overlying mantle asthenosphere. The water reduces the melting point and results in the production of magmas. Magmas rise to the surface and create volcanic (oceanic under continental) or island arcs (oceanic under oceanic). Transform: Lithospheric plates slide past each other in a horizontal manner. Earthquakes along transform faults are shallow focus earthquakes. Most transform faults occur where oceanic ridges are offset on the sea floor. They occur because the spreading takes place on the spherical surface of the Earth, and some parts of a plate must therefore be moving at higher relative velocities than others.

convergent margins

Earthquakes? Yes! Shallow ones, intermediate ones, and deep ones. Earthquakes occur within the upper part of the lithosphere as it descends into the mantle. Below a depth of 700 kilometers, the sinking lithosphere moves as a plastic and there are no earthquakes. Volcanic Activity? Yes! This happens for a very different reason than at ridges (divergent margins). As the oceanic lithosphere sinks into the mantle, it heats up, forcing water to leave it and enter the mantle above the plate. The product of this melting of the mantle is a line of volcanoes (an arc) over the subducted lithosphere. Examples: Mt. St. Helena, Cascades, Mt. Fujiyama, Krakatoa. Volcanoes that form at convergent margins are often more explosive and dangerous than volcanoes that form at divergent boundaries because they contain more gases such as water vapor and carbon dioxide. Structures: trench: a long depression or trench in the seafloor where the oceanic plate begins to sink into the mantle. This includes the Mariana's trench. subduction zone: where cold, dense, oceanic lithosphere sinks into the mantle. arc: a line of volcanoes that forms parallel to the trench.

rigid

Elastic. When under stress, rigid materials will break. When stress is removed, rigid materials will return to their original shape (like an elastic).

divergent margins

Example: mid-Atlantic ridge Earthquakes? Yes! shallow ones (usually less than 100 km in depth) Volcanic activity? Yes! Here, the mantle melts due to decompression melting. The resulting magma ascends, erupts, and solidifies to create a volcanic rock called basalt. This basalt represents new oceanic crust. When this crust forms, older crust is pushed farther and farther from the ridge on either side. As a result, the youngest oceanic crust in the ocean basins is at mid-ocean ridges and the oldest crust is the farthest from ridges, usually disappearing down subduction zones close to continents.

What were the main pieces of evidence that Wegener used to construct his theory of continental drift? (geological, paleontological, paleoclimatological?)

Geological: similarity between the coastlines of Africa and South America Paleontological correlations between Africa and South America of species that could not have themselves have crossed an abyssal ocean. Distribution of fossils. Rock strata, mountain chains. Ancient glaciations

in what hemisphere are the majority of earthquakes and volcanoes located? why?

Northern Hemisphere because the majority of plate boundaries happen to be in the Northern Hemisphere.

Robert Hooke (1635-1703)

Hooke was an English natural philosopher, architect, polymath, and physicist who discovered the law of elasticity, known as Hooke's Law, and who did research in a remarkable variety of fields. In 1655, he was employed by Robert Boyle to construct the Boylean air pump. In 1660, he discovered the Law of Elasticity and applied these studies in his designs for balance springs and watches. In 1662, he was appointed curator of experiments to the Royal Society of London, and was elected a fellow the following year. Hooke was one of the first men to build a Gregorian reflecting telescope, he discovered the 5th star in Trape Zium, an asterism in Orion. He was the first to suggest Jupiter rotates on it axis. His sketches of Mars were used in the 19th century to determine planet's rate of rotation, and he became a professor of geometry in 1665. Micrographia - included studies and illustrations of the crystal structure of snowflakes, microscopic fossils, silkworms, and was the first to use cell to describe honeycomb cavities in cork. Suggested that the force of gravity could be measured utilizing the motion of a pendulum and attempted to show the elliptical path of Earth and the moon around the Sun. Offered a wave theory of light (diffraction). Stated the inverse square law to describe planetary motion, a law Newton later used in modified form. Hooke complained that he was not given enough credit for this discovery and became involved in a bitter controversy with Newton. He was the first to sate in general that all matter expands when heated and air is made of particles separated from each other by relatively large distances.

densest layer of the Earth?

Inner core.

hottest layer of the Earth?

Inner core.

of what is the core composed?

Iron and nickel.

what are the three types of convergent boundaries?

Oceanic + Oceanic The older, colder, denser plate subducts Oceanic + Continental Oceanic plate subducts because it has a higher density Continental + Continental Neither subducts because the density of continental crust is too low. The crust will buckle to form a mountain chain.

coffee shop story

January 14, 1684 (a Wednesday) at a coffee house. Halley, Wren, and Hooke - 3 famous figures. Halley was about 20, and was already an established figure. These three were members of the Royal Society (of London), which was founded by King Charles II. Motto of Royal Society: Nullinus in Verba ("no words"). Figuratively, this means "don't take anybody's word for something... let it be demonstrated by experiment." Topic of discussion was Kepler's Law of Planetary motion. These had been made by Kepler's observations, but Wren and Hooke want to come up with a mathematical proof. Halley goes to London, goes to Cambridge to see Isaac Newton (known at this point in time for optics, and had designed the newtonium telescope) to ask for his thoughts on a proof for Kepler's Laws. Issac says, "Yes, I have the proof," but can't find it. So he says he'll mail it to Halley when he does. (He never actually ends up finding it, but ends up mailing his solution(s) to Halley). Francis Wiloughby - had a book of many beautiful drawings of sea creatures (inspired by Hooke's Micrographia and the success of it). Wiloughby goes to the Royal Society, asking them to publish it. But once it is published, no one bought it (huge failure). At this point, Halley was clerk of the Royal Society and wants to publish his and Newton's Principia but the Royal Society is out of money (blew all money on Francis's fish book). So Halley takes out his own money so that he can get Principia published. (Royal Society couldn't even pay Halley his own salary, so Halley received all the unsold fish books as payment). Newton's Law of Universal Gravitation: The force of gravity, F(g), is given by F(g) = G(m1, m2)/R^2 where, G = gravitational constant m1 = mass of the object 1 m2 = mass of object 2 R = distance between the objects The only thing missing was the value of the gravitational constant, which was later discovered by Henry Cavendish in his Cavendish experiment. Cavendish calculated that the average density of the Earth was about 5.5 g/cm^3. But rocks on Earth's surface were only about 2.5-3.5 g/cm^3. This was the first indication that there was some very dense material to compensate for light density rocks at the surface in Earth's interior - 1st indication of a dense core, and the only material dense enough to account for the difference in density was iron.

thickest layer of the Earth?

Mantle.

there are two types of plates: oceanic and continental. What are the primary differences between these two?

Oceanic plates are thinner and denser. Continental plates are thicker and less dense.

oceanic ridge

Oceanic ridges are created at divergent margins. These are long chains of mountains within ocean basins. In a few locations, so much volcanic activity has taken place at the ridge that if forms volcanic islands such as Iceland. Typically, a rift valley exists at the center of the ridge. The rift at the center of the ridge represents the boundary between two diverging plates.

which layers are liquid?

Outer core.

mantle

Over 82% of Earth's volume is contained in the mantle, a solid, rocky shell that extends to a depth of 2900 kilometers. The boundary between the crust and the mantle represents a marked change in chemical composition. The dominant rock in the uppermost mantle is peridotite, which has a density of 3.3 g/cm^3. At greater depths peridotite changes by assuming a more compact crystalline structure and hence a greater density.

what is a plate? of what is a plate composed?

Plates are composed of lithosphere, about 100 km thick, that "float" on the ductile asthenosphere. While continents do appear to drift, they only do so because they are part of larger plates that float and move horizontally on the upper mantle astheosphere. The plates behave has rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries between plates.

lithostatic pressure

Pressure exerted by the overlying lithosphere.

why is the outer core liquid while the hotter inner core is frozen?

Pressure is greater in the inner core, which raises its melting point. Pressure prevents it from volumetrically expanding (changing state from solid to liquid). As interior cools off, more and more will be under the melting point and inner core will continue to expand as it has been.

Pangea

Supercontinent proposed by Alfred Wegener that consisted of all the present-day continents and existed in the Late Paleozoic era.

convection

The asthenosphere is capable of convection, which is the result of unequal heating in the Earth's interior. Hotter, less dense rock rises as colder, more dense rock sinks.

from a tectonic point of view, where are the majority of earthquakes and volcanoes located?

The boundaries between plates is where the vast majority of earthquakes occur. When you stress rocks in the lithosphere, it will break (or return to original shape because it is rigid).

core

The core is a sphere composed of an iron-nickel alloy having a radius of 3486 kilometers. At the extreme pressures found in the core, the iron-rich material has a an average density of 11 g/cm^3 and approaches nearly 14 times the density of water at Earth's center. Composed mostly of an iron-nickel alloy, and is divided into two regions that exhibit very different mechanical strengths.

crust

The crust, Earth's comparatively thin, rocky outer skin, is generally divided into oceanic and continental crust. The oceanic crust is roughly 7 kilometers thick and composed of the dark igneous rock called basalt. By contrast, the continental crust averages 35-40 kilometers but may exceed 70 kilometers in some montainous regions. Unlike oceanic crust, which has a relatively homogeneous chemical composition, the continental crust consists of many types of rocks. The upper crust has an average composition of a granitic rock called granodiorite, whereas the composition of the lowermost continental crust is more akin to basalt. Continental rocks have an average density of about 2.7 g/cm^3, and some have been discovered that are 4 billion years old. The rocks of the oceanic crust are younger (about 180 million years or less) and more dense (about 3 g/cm^3) than continental rocks.

increase in temperature as depth increases

The gradual increase in temperature and density with depth affects the physical properties and hence the mechanical behavior of Earth's materials. When a substance is heated, its chemical bonds weaken and its mechanical strength (resistance to deformation) is reduced. If the temperature exceeds the melting point of an Earth material, the material's chemical bonds break and melting ensues. Pressure also increases with depth and tends to increase rock strength. Furthermore, because melting is accompanied by an increase in volume, it occurs at higher temperatures at depth because of greater confining pressure. Thus, depending on the physical environment (temperature and pressure), a particular Earth material may behave like a brittle solid, deform in a putty-like manner, or even melt and become liquid. Earth can be divided into 5 main layers based on physical properties and hence mechanical strength: the lithosphere, the asthenosphere, mesosphere (lower mantle), outer core, and inner core.

inner core

The inner core is a sphere having a radius of 1216 kilometers. Despite its higher temperature, greater confining pressure here (than in the solid) causes the inner core to behave like a solid.

which layer of the Earth is under the most pressure? why?

The inner core, due to the weight of the overlying layers.

what is the difference between the asthenosphere and the lithosphere in terms of temperature, pressure, and rigidity?

The lithosphere is the outermost, rigid (elastic) shell of the earth, consisting of the crust and the uppermost mantle. The lithosphere is divided into large and small plates. The asthenosphere is basically the rest of the mantle, and is plastic (which means that when you stress the rock, it deforms, and when you remove the stress it remains deformed). The asthenosphere is capable of flowing, like peanut butter, which means that it is capable of motion and convection (because of temperature and pressure even though the rock is solid). Convection in the asthenosphere is the primary way of transferring internal heat to the surface.

outer core

The outer core is a liquid layer 2270 kilometers thick. It is the convective flow of metallic iron within this zone that generates Earth's magnetic field.

radioactivity

The primary source of Earth's internal heat.

magnetosphere

The region surrounding the earth or another astronomical body in which its magnetic field is the predominant effective magnetic field.

what causes the Earth's magnetic field?

The rotation of the liquid inner core which generates electrical currents which thereafter produce a magnetic field.

why was Wegener's theory not accepted for several decades?

The weakness of his theory (and why it was not readily accepted) was that he proposed that the continents slid over the ocean floor. Geophysicists disagreed, stating the ocean floor did not have enough strength to hold continents and too much frictional resistence would be encountered.

transform fault

Where two plates slide past each other. Earthquakes? Yes (usually shallow ones 0-100 km) Volcanic Activity: No. Most transform faults occur in the middle of oceans and connect segments of oceanic ridges (that is, segments of divergent margins). The most famous example is the San Andreas fault in California.

is the Earth cooling off or heating up?

cooling (?)