The Earth's Interior
Just as a child may shake an unopened present in an attempt to discover the contents of a gift, so man must listen to the ring and vibration of the package of our Earth in an attempt to discover its content. This is accomplished through seismology which has become the principle method used in studying Earth's interior. Seismos is a Greek word meaning shock; akin to earthquake, shake, or violently moved. Seismology on Earth deals with the study of vibrations which are produced by earthquakes, the impact of meteorites, or artificial means such as an explosion. On these occasions a seismograph may be used to measure and record the actual movements and vibrations within the Earth and of the ground.

Types of seismic waves
Seismic movements have been categorized into four types of diagnostic waves which travel at speeds ranging from 3 to 15 km per second. Two of the waves travel around the surface of the Earth in rolling swells. The other two, Primary (P) or compression waves and Secondary (S) or shear waves penetrate the interior of the Earth. Primary waves compress and dilate the matter they travel through (either rock or liquid) similar to sound waves. They also have the ability to move twice as fast as S waves. Secondary waves propagate through rock but are not able to travel through liquid. Both P and S waves refract or reflect at points where layers of differing physical
properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities.

Divisions in the Earth's Interior
Seismic discontinuities aid in distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region,upper mantle, and crust (oceanic and continental). Lateral discontinuities have also been distinguished and mapped through seismic tomography.

Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 km. The inner core is solid and unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids when temperature decreases or pressure increases.

Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 km.
The outer core is a hot electrically conducting liquid within which convective motion occurs. This combined with the Earth as a rotating body creates a dynamo effect which maintains the system of electrical currents known as Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron which indicates the presence of lighter elements. Scientists suspect about 10% of sulfur and/or oxygen because of their abundance in the cosmos and due to the fact that they would dissolve readily in molten iron.

D": 3% of Earth's mass; depth of 2,700-2,890.
This layer is 200-300 km thick and represents about 4% of the mantle-crust mass. Although it is often identified as part of the lower mantle, seismic discontinuities suggest the D" layer may differ chemically from the lower mantle lying above it.
Theories suggest the material either dissolved in the core at some point or because of its density, was able to sink through the mantle but not into the core.

Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 km.
The lower mantle contains 72.9% of the mantle-crust mass and by deduction contains mainly silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminum. These deductions are made by assuming the Earth has a similar abundance of cosmic elements as found in the Sun and primitive meteorites (including by inference other planets) and according to the proportions found thereon. It is amazing what scientists can learn through deduction, inference, elimination and assumption.

Transition region: 7.5% of Earth's mass; depth of 400-650 km.
The transition region or mesosphere (for middle mantle), sometimes called the fertile layer, contains 11.1% of the mantle-crust mass and is the source of basaltic magmas. It also contains calcium, aluminum, and garnet, which is a complex aluminum-bearing silicate mineral. This layer is dense when cold because of the garnet and buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.

Upper mantle: 10.3% of Earth's mass; depth of 10-400 km.
The upper mantle contains 15.3% of the mantle-crust mass. Fragments have been excavated for our observation by eroded mountain belts and volcanic eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been the primary minerals found in this way. These and other minerals are refractory and crystalline at high temperatures; therefore, most settle out of rising magma either forming new crustal material or never leaving the mantle. Part of the upper mantle called the asthenosphere may be partially molten.

Oceanic crust: 0.099% of Earth's mass; depth of 0-10 km.
The oceanic crust contains 0.147% of the mantle-crust mass. The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000-km-long network of volcanoes, generates new oceanic crust at the rate of 17 km^3 per year, covering the ocean floor with basalt. Hawaii and Iceland are two examples of the accumulation of basalt piles.

Continental crust: 0.374% of Earth's mass; depth of 0-50 km.
The continental crust contains 0.554% of the mantle-crust mass. This is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust (both oceanic and continental) is the surface of the Earth and as such is the coldest part of our planet. Since cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).
The Lithosphere & Plate Tectonics

Oceanic Lithosphere- The rigid, outermost layer of the Earth comprising the crust and upper mantle is called the lithosphere. New oceanic lithosphere forms through volcanism in the form of fissures at midocean ridges which are cracks that encircle the globe. Heat escapes the interior as this new lithosphere emerges from below. It gradually cools, contracts and moves away from the ridge as on a conveyor belt traveling across the seafloor to subduction zones, a process called seafloor spreading. In time older lithosphere will thicken and eventually become more dense than the mantle below, causing it to descend (subduct) back into the Earth at a steep angle, cooling the interior. Subduction is the main method of cooling the mantle below 100km. If the lithosphere is young and thus hotter at a subduction zone, it will be forced back into the interior at a lesser angle.

Continental Lithosphere

The continental lithosphere is about 150 km thick with a low-density crust and upper-mantle that are permanently buoyant. Continents drift laterally along the convecting system of the mantle away from hot mantle zones toward cooler ones, a process known as continental drift. Most of the continents are now sitting on or moving toward cooler parts of the mantle, with the exception of Africa. Africa was once the core of Pangea, a supercontinent that eventually broke into todays continents. Several hundred million years prior to that the southern continents: Africa, South America, Australia, and Antarctica, as well as India were assembled together in what iscalled Gondwana.

Plate Tectonics

Plate tectonics involves the formation, lateral movement, interaction, and destruction of the lithospheric plates. Much of Earth's internal heat is relieved through this process and many of Earth's large structural and topographic features are consequently formed. Continental rift valleys and vast plateaus of basalt are created at plate break up when magma ascends from the mantle to the ocean floor
forming new crust and separating midocean ridges. Plates collide and are destroyed as they descend at subduction zones to produce deep ocean trenches, strings of volcanoes, extensive transform faults, broad linear rises, and folded mountain belts. Today Earth's
lithosphere is divided into eight large plates with about two dozen smaller ones that are drifting above the mantle at the rate of 5 to 10 cm per year. The eight large plates are the African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates. A few of the smaller plates are the Anatolian, Arabian, Caribbean, Cocos, Philippine, and Somali plates.

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