The structure of Earth can be defined in two ways: by mechanical properties such as archeology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. The interior of Earth is divided into 5 important layers. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth are at the following depths below the surface.
A. Core: Earth’s core is the very hot center of our planet. The Earth has three major layers. All known life exists on the solid outer layer, called the crust. Beneath the crust is the mantle, a gooey, hot layer of magma and other semisolid rocks and minerals. Movement in the mantle leads to tectonic activity, such as volcanic eruptions and earthquakes, on the crust. The core, beneath the mantle, is the deepest and hottest layer of the Earth. It is made almost entirely of metal. The core is made of two layers: the outer core, which borders the mantle, and the inner core. The inner core is shaped like a ball. Scientists say the outer core is made mostly of iron and nickel. Iron and nickel are two important metals found everywhere on the planet. (On the surface of the Earth, these metals are found in solid form.) Iron and nickel in the outer core form an alloy, or a mixture of metallic elements. The outer core is approximately 2,300 kilometers (1,430 miles) thick. The alloy of the outer core is very hot, between 4,000 and 5,000 degrees Celsius (7,200 and 9,000 degrees Fahrenheit). The inner core is made mostly of iron. It is approximately 1,200 kilometers (750 miles) thick. Although the iron is extremely hot - between 5,000 and 7,000 degrees Celsius (9,000 and 13,000 degrees Fahrenheit)—the pressure from the rest of the planet is so great that the iron cannot melt. For this reason, the inner core is mostly solid.
Because the Earth has a ball of metal in the middle of it, the entire planet is magnetic. Scientists believe the liquid outer core is what controls the Earth’s magnetic field. The magnetic field acts almost like a bubble. It protects the planet from charged particles floating around in the solar system, such as those from the sun. The magnetic North and South Poles are opposing sides of Earth’s big magnet. The hard, metallic material in the core is balled up in the center of the Earth because it's the heaviest material on the planet. When Earth was formed about 4.5 billion years ago, all the heavier substances sank toward the middle. The lighter and less dense material, such as air and water, stayed closer to the crust.
Inside the core, the metals are constantly moving. The core of the Earth rotates regularly. Some scientists say the inner core actually rotates faster than the rest of theplanet! As the liquid outer core moves, it can change the location of the magnetic North and South Poles. The average density of Earth is 5,515 kg/m3. Since the average density of surface material is only around 3,000 kg/m3, we must conclude that denser materials exist within Earth's core. Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ~1,220 km and a liquid outer core extending beyond it to a radius of ~3,400 km.
The densities are between 9,900 and 12,200 kg/m3 in the outer core and 12,600–13,000 kg/m3 in the inner core. The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. It is not necessarily a solid, but, because it is able to deflect seismic waves, it must behave as a solid in some fashion. Experimental evidence has at times been critical of crystal models of the core. Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000K below those from shock laser (dynamic) studies. The laser studies create plasma, and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is plasma with the density of a solid. This is an area of active research.
In early stages of Earth's formation about four and a half billion (4.5×109) years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust. Some have argued that the inner core may be in the form of a single iron crystal.
Under laboratory conditions a sample of iron nickel alloy was subjected to the core like pressures by gripping it in a vise between 2 diamond tips, and then heating to approximately 4000 K. The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south.
The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.
Recent speculation suggests that the innermost part of the core is enriched in gold, platinum and other siderophile elements.
The matter that comprises Earth is connected in fundamental ways to matter of certain chondrite meteorites, and to matter of outer portion of the Sun. There is good reason to believe that Earth is, in the main, like a chondrite meteorite. Beginning as early as 1940, scientists, including Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrite, the most common type of meteorite observed impacting Earth, while totally ignoring another, albeit less abundant type, called enstatite chondrites. The principal difference between the two meteorite types is that enstatite chondrites formed under circumstances of extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.
Dynamo theory suggests that convection in the outer core, combined with the Coriolis Effect, gives rise to Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field strength in Earth's outer core is estimated to be 25 Gauss (2.5 mT), 50 times stronger than the magnetic field at the surface.
Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet; however, more recent studies in 2011 found this hypothesis to be inconclusive. Options remain for the core which may be oscillatory in nature or a chaotic system. In August 2005 a team of geophysicists announced in the journal Science that, according to their estimates, Earth's inner core rotates approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface.
The current scientific explanation for Earth's temperature gradient is a combination of heat left over from the planet's initial formation, decay of radioactive elements, and freezing of the inner core.
1. Inner core: Although we have never physically been to the inner core, we can use indirect methods to determine its properties. When an earthquake hits, seismic waves are released and travel through the earth. There are two types of seismic waves, primary and secondary. Both types travel out radially away from the epicenter. Of the seismic waves, only primary waves have the ability to travel through all layers of the Earth. It’s this property of primary waves that has given us a peek into the Earth's center. Throughout the 20th century, seismograms, an instrument designed to record seismic waves, were positioned around the globe to better understand seismic waves. Earthquakes occurred, and the seismograms worked as they were supposed to, recording the primary waves. To the surprise of scientists however, there were two distinct areas on the Earth that wouldn't record primary waves. Primary waves were recorded on seismograms until 110 degrees away from the epicenter and remained absent until 150 degrees, when they would reappear. This is known as a seismic shadow zone and it provides geologists insight into the properties of the inner core.
The inner core of the Earth has a number of surprising properties
Size: The Earth's inner core is surprisingly large, measuring 2,440 km (1,516 miles) across. It makes up 19 percent of the Earth's total volume, which makes it just 30 percent smaller than the moon.
Temperature: The temperature of the inner core is estimated to be between 3,000 and 5,000 Kelvin (4,940 to 8,540 degrees Fahrenheit). The high temperature comes from three main sources. There is residual heat left from the Earth's formation, and heat is generated by gravitational forces from the sun and moon as they tug and pull on the inner core. Finally, the radioactive decay of elements deep within the Earth also produces heat.
Phase and Composition: Scientists believe that the Earth's inner core is a solid and is mainly composed of iron. The scorching hot iron inner core is able to remain solid because of the extremely high pressures at the center of the Earth.
Spin: Experiments reported in July 1997 suggest that the inner core spins at a slightly faster speed than the Earth itself. The research conducted at Columbia University suggests that the inner core rotates in the same direction as the rest of the planet. However, the research shows that it makes one complete revolution two-thirds of a second faster than the rest of the planet.
Magnetic Field: Because the Earth's inner core is a solid lump of iron, you may think that it is the source of the Earth's magnetic field. But this is not the case. The Earth's outer core, which consists of molten iron and nickel, flows around the inner core, and this motion produces the magnetic field.
2. Outer core: Deep within the Earth, thousands of kilometers below your feet are the core of the Earth. Once thought to be a single ball of iron, scientists now know that the Earth’s core contains a solid inner core, surrounded by a liquid outer core. Let’s take a look at the outer core of Earth. The discovery that the core of the Earth contains a solid inner core surrounded by a liquid outer core was made by seismologist Inge Lehmann, who was studying how seismic waves bounce off the interior of the Earth. Instead of bouncing off a solid core, Lehmann observed that the liquid outer core caused the waves to reflect differently from how they bounced off the inner core.
Further studies have refined the size of the outer core. The inner core is thought to be 2,440 km across, and when you include the liquid outer core of the Earth, the entire core measures 6,800 km across; about twice as big as the Moon. It’s believed that the core of the Earth formed early on in our planet’s history, when the entire planet was made of molten rock and metal. Since it was a liquid, the heaviest elements, like iron, nickel, gold and platinum sunk down into the center, leaving the less dense elements on top. Without the outer core, life on Earth would be very different. Scientists believe that convection of liquid metals in the outer core create the Earth’s magnetic field. This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the Sun’s solar wind. Without this field, the solar wind would have blasted away our atmosphere, and Earth would be dead and lifeless like Mars.
The inner core is also known to rotate, turning approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface. In other words, the inner core makes an extra rotation every 700-1000 years compared to the surface.
B. Mantle: The mantle is one of the three main layers of the Earth. It lies between the innermost layer, the core, and the thin outermost layer, the crust. The mantle consists of hot, dense, semisolid rock and is about 2,900 kilometers (1,802 miles) thick.
The mantle is divided into several layers
Layers of the mantle
Lithosphere: The thin outermost shell of the upper mantle is similar to the crust, though cooler and more rigid. Together with the crust, this layer is called the Earth’s lithosphere.
The lithosphere is the solid, outer part of the Earth. The Earth consists of three main layers: the core, or the inner layer; the mantle, in the middle; and the crust, which includes the continents and ocean floor. The lithosphere, which is about 100 kilometers (60 miles) deep in most places, includes the brittle upper portion of the mantle and the crust.
The lithosphere is always moving, but very slowly. It is broken into huge sections called tectonic plates. The extreme heat from the mantle part of the lithosphere makes it easier for the plates to move; this is similar to how iron is bendable once it's heated. The movement of the lithosphere, called plate tectonics, is the reason behind a lot of Earth's most dramatic geologic events. When one plate moves beneath another, or when two plates rub together, they can create earthquakes and volcanoes.
The U.S. state of Hawaii was formed on a tectonic plate called the Pacific plate. Its islands are in a chain because the plate was constantly moving above the mantle.
Plate tectonics may explain why we have continents. Scientists believe the continents used to be one major landmass called Pangaea, which separated because the lithosphere broke apart. There is evidence to support this theory. For example, the eastern coast of South America and the western coast of Africa look like they would fit together. And even though they are separated by an ocean, similar animals and plants are found in both regions.
Asthenosphere: The lithosphere is actually broken up into several large pieces, or plates. They “float” on a softer mantle layer called the asthenosphere. Their very slow motion is the cause of plate tectonics, a process associated with continental drift, earthquakes, volcanoes, and the formation of mountains. The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The lithosphere-asthenosphere boundary is conventionally taken at the 1300°C isotherm, above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion. Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same. This decreasing in seismic wave’s velocity from lithosphere to asthenosphere could be caused by the presence of small percentage of melt in the asthenosphere. The lower boundary of the LVZ lies at a depth of 180– 220 km, whereas the base of the asthenosphere lies at a depth of about 700 km. This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic wave’s decreases with decreasing rigidity.
In the old oceanic mantle the transition from the lithosphere to the asthenosphere, the so-called lithosphere-asthenosphere boundary (LAB) is shallow (about 60 km in some regions) with a sharp and large velocity drop (5-10%). At the mid-ocean ridges the LAB rises to within a few kilometers of the ocean floor.
The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, creating the movement of tectonic plates.
Upper mantle: Below the asthenosphere lies another layer, stronger and more solid than the asthenosphere. All layers below the crust down to a depth of about 670 kilometers (416 miles) are known as the upper mantle.
Lower mantle: The rest of the mantle between the upper mantle and the core is known as the lower mantle. It is denser and hotter than the upper mantle.
Exploring the Deep Earth
Most of the Earth’s interior is much too deep for us to explore directly. Instead, scientists tell the mantle apart from the crust and core by measuring the spread of shock waves from earthquakes, called seismic waves. Two types of seismic waves pass through the Earth’s interior: P-waves, which represent vertical motion, and S-waves, which represent horizontal motion. Instruments placed around the world measure these waves as they arrive at different points on the Earth’s surface after an earthquake.
Seismic waves travel at different speeds and strengths through different material. For example, surface waves from a powerful earthquake near Northridge, California, in 1994 took 30 minutes to reach a point about 6,700 kilometers (4,163 miles) away, but it took P-waves only 10 minutes and S-waves just under 20 minutes to travel the same distance. Near the surface, P-waves travel about 6 kilometers per second through the ground. Below a certain point—an average depth of about 35 kilometers—they travel about 8 km/s, indicating that the waves have reached denser material at that point.
This abrupt divide between slower and faster speeds marks the boundary between the crust and the mantle.
It is called the Mohorovicic discontinuity, or simply the Moho. At the base of the mantle, 2,900 kilometers (1,802 miles) below the surface, S-waves, which can’t continue in liquid, suddenly disappear, and P-waves are strongly refracted, or bent. This point, called the Gutenberg discontinuity, marks the end of the mantle and the beginning of the Earth’s liquid core.
C. CRUST: The Earth's crust is an extremely thin layer of rock, like the skin of an apple in relative terms. It amounts to less than half of 1 percent of the planet. But the crust is exceptionally important, and not just because we live on it.
The crust can be thicker than 80 kilometers in some spots, less than one kilometer in others. Underneath it is the mantle, a layer of rock some 2700 kilometers thick that accounts for the bulk of the Earth. The crust is primarily made of granite and basalt while the mantle beneath is made of peridotite.
How We Know the Earth Has a Crust
Just a century ago, we didn't know the Earth has a crust. Until the 1900s all we knew was that our planet wobbles in relation to the sky as if it had a large, dense core. Astronomical observations told us so. Then along came seismology, which brought us a new type of evidence from below: seismic velocity, or the speed of sound in rock as measured using seismic waves from earthquakes. In 1909 a paper by the seismologist Andrija Mohorovicic established that about 50 kilometers deep in the Earth there is a sudden change in seismic velocity—a discontinuity of some sort. Seismic waves bounce off it (reflect) and bend (refract) as they go through it, the same way that light behaves at the discontinuity between water and air. That discontinuity, named the Mohorovicic discontinuity or "Moho," is the accepted boundary between the crust and mantle.
Crusts and Plates
The crust is not the same thing as the plates of plate tectonics. Plates are thicker than the crust and consist of the crust and the shallow mantle just beneath it; the two-layered combination is stiff and brittle and is called the lithosphere ("stony layer" in scientific Latin). The lithospheric plates lie on a layer of softer, more plastic mantle rock (the asthenosphere or "weak layer") that allows the plates to move slowly over it like a raft in thick mud.
We know that the Earth's outer layer is made of two grand categories of rocks: basaltic and granitic. Basaltic rocks underlie the seafloors and granitic rocks make up the continents. We know that the seismic velocities of these rock types, as measured in the lab, match those seen in the crust down as far as the Moho, so we're pretty sure that the Moho marks a real change in rock chemistry. The Moho isn't a perfect boundary, because some crustal rocks and mantle rocks can masquerade as the other, but even so everyone who talks about the crust, whether in seismological or petro logical terms, fortunately means the same thing. In general, then, there are two kinds of crust, oceanic crust (basaltic) and continental crust (granitic).
Oceanic Crust
Oceanic crust covers about 60 percent of the Earth's surface. Oceanic crust is thin and young—no more than about 20 km thick and no older than about 180 million years. Everything older has been pulled underneath the continents by subduction. Oceanic crust is born at the midocean ridges, where plates are pulled apart. As that happens, the pressure upon the underlying mantle is released and the peridotite there responds by starting to melt. The fraction that melts becomes basaltic lava, which rises and erupts while the remaining peridotite becomes depleted.
The midocean ridges migrate over the Earth like Roombas, extracting this basaltic component from the peridotite of the mantle as they go. This works like a chemical refining process. Basaltic rocks contain more silicon and aluminum than the peridotite left behind, which has more iron and magnesium. Basaltic rocks are also less dense. In terms of minerals, basalt has more feldspar and amphibole, less olivine and pyroxene, than peridotite. In geologist's shorthand, oceanic crust is mafic while oceanic mantle is ultramafic.
Oceanic crust, being so thin, is a very small fraction of the Earth—about 0.1 percent—but its life cycle serves to separate the stuff of the upper mantle into a heavy residue and a lighter set of basaltic rocks. It also extracts the so-called incompatible elements, which don't fit into mantle minerals and move into the liquid melt. These in turn move into the continental crust as plate tectonics proceeds. Meanwhile, the oceanic crust reacts with seawater and carries some of it down into the mantle.
Continental Crust
Continental crust is thick and old—on average about 50 km thick and about 2 billion years old—and it covers about 40 percent of the planet. Whereas almost all of the oceanic crust is underwater, most of the continental crust is exposed to the air. he continents slowly grow over geologic time as oceanic crust and seafloor sediments are pulled beneath them by subduction. The descending basalts have the water and incompatible elements squeezed out of them, and this material rises to trigger more melting in the so-called subduction factory. The continental crust is made of granitic rocks, which have even more silicon and aluminum than the basaltic oceanic crust; they also have more oxygen thanks to the atmosphere. Granitic rocks are even less dense than basalt. In terms of minerals, granite has even more feldspar, less amphibole than basalt and almost no pyroxene or olivine, plus it has abundant quartz. In geologist's shorthand, continental crust is felsic.
Continental crust makes up less than 0.4 percent of the Earth, but it represents the product of a double refining process, first at midocean ridges and second at subduction zones. The total amount of continental crust is slowly growing. The incompatible elements that end up in the continents are important because they include the major radioactive elements uranium, thorium and potassium. These create heat, which makes the continents act like electric blankets on top of the mantle. The heat also softens thick places in the crust, like the Tibetan Plateau, and makes them spread sideways. Continental crust is too buoyant to return to the mantle. That's why it is, on average, so old. When continents collide, the crust can thicken to almost 100 km, but that is temporary because it soon spreads out again. The relatively thin skin of limestone’s and other sedimentary rocks tend to stay on the continents, or in the ocean, rather than return to the mantle. Even the sand and clay that is washed off into the sea returns to the continents on the conveyor belt of the oceanic crust. Continents are truly permanent, self-sustaining features of the Earth's surface.
What the Crust Means
The crust is a thin but important zone where dry, hot rock from the deep Earth reacts with the water and oxygen of the surface, making new kinds of minerals and rocks. It's also where plate-tectonic activity mixes and scrambles these new rocks and injects them with chemically active fluids. Finally, the crust is the home of life, which exerts strong effects on rock chemistry and has its own systems of mineral recycling. All the interesting and valuable variety in geology, from metal ores to thick beds of clay and stone, finds its home in the crust and nowhere else.
Earth's crust is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates. Tectonic plates actually slide around on the mantle, causing earthquakes, mountain formation, continental drift, volcanoes, and other geologic activity on the crust.
Billions of years ago, the Earth started out as a hot, gooey ball of rock. The heaviest material, mostly iron and nickel sank to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. These surface rocks became the crust.
The crust is divided into two types: oceanic crust and continental crust. Oceanic crust, found under the ocean floor, is made of dense rocks such as basalt. It is about 7 kilometers (4 miles) thick. Continental crust, found under land masses, is made of less dense rocks such as granite. Its thickness varies between 10 and 75 kilometers (6 to 47 miles).
Continental crust is almost always much older than oceanic crust. Some of the oldest rocks in the world can be found in the Nuvvuagittuq greenstone belt in Quebec, Canada. This continental crust formation has rocks that are about 4 billion years old. Unlike continental crust, oceanic crust is still being formed in places called mid-ocean ridges. Here, magma from the mantle erupts through cracks in the ocean floor, creating crust as it cools. Oceanic crust is heavier than continental crust. The heavy oceanic crust is constantly sinking, very slowly, underneath the lighter continental crust. This important process is called subduction. A chain of volcanoes formed at a subduction zone is called a volcanic arc. One such volcanic arc exists where the oceanic crust of the Australian plate subducts under the continental crust of the Eurasian plate. The Indonesian Island Arc, which includes the islands of Sumatra and Java in Indonesia, has some of the most powerful volcanoes in the world. Eventually, oceanic crust sinks low enough to enter the mantle. Once this happens, the crust melts, and then rises up again as magma in the mid-ocean ridges. In this way, the Earth enjoys a brand-new oceanic crust once every 200 million years or so.
