Assigned Reading: History and Structure of Earth

I. Earth Formation: The Earth and solar system are roughly 4.6 billion years old, having formed as a homogeneous mass of undifferentiated material over a period of about 500 million years through the accretion of cosmic material: gas, dust and meteorites. Much of this material originated in the exploded remains of earlier stars.

II. Differentiation: The primordial Earth was homogeneous, but contained radioactive materials that released heat as they decayed. Some of these, such as 26Al, (an isotope of aluminum) were very short-lived and decayed rapidly releasing enough heat to partially melt the Earth early in its history. When this occurred, most heavy materials such as iron and nickel sank to the center, while lighter ones such as silicates (compounds of silicon and oxygen) floated to the surface. The process of radioactive decay continues today, although at a much slower rate. Its heat keeps limited parts of Earth molten, and drives the geologic forces that continuously reshape the Earth's surface. The net result of these processes is that today's Earth, unlike the primordial Earth is stratified into distinct layers.

The layers of the Earth can be broken down by two different criteria: by composition and by mechanical properties. Compositional and mechanical layers do not correspond perfectly. Do not mix these systems up!

III. Compositional layers of the Earth:

  1. Core: The densest region, located at the center of the Earth, it is composed largely of iron, nickel, and other heavy elements in both liquid and solid states.

  2. Mantle: The solid region surrounding the core. It is less dense, consisting of minerals that combine large amounts of iron, magnesium, silicon, and oxygen. For various reasons, mantle rock in a locality may occasionally melt. This is the source of the magma that erupts from volcanoes and mid-ocean ridges. Despite this, the mantle is mostly solid.

  3. Crust: The thin outer region of the solid Earth consists mostly of light silicate rocks, rocks that combine large amounts of silicon, aluminum, and oxygen, but have relatively little iron, and magnesium.

IV. Physical layers of the Earth:

  1. Inner Core: The nickel-iron center of the Earth. Although hot enough to melt, it is kept solid by the immense pressure of overlying material.

  2. Outer Core: Above the inner core, nickel, iron, and other heavy core materials form a molten metallic sea. Currents of molten metal flowing through the outer core form the dynamo that generates Earth's magnetic fields.

  3. Asthenosphere: Above the outer core lies a thick region of rock that is solid but under such pressure from the weight of overlying rock that it deforms ductilely rather than breaking. Vast convection currents seem to slowly move through the ductile asthenosphere.

  4. Lithosphere: The outer mechanical layer of the Earth, consisting of rock that is under relatively little pressure, so that it cracks and breaks rather than flowing ductilely. The lithosphere is divided into rigid "plates" that move like rafts on the flowing asthenosphere. It corresponds to the crust and uppermost part of the mantle.

V. Plate Tectonics: The continued production of heat through radioactive decay has resulted in the ongoing convection of the mantle and associated movement of lithospheric plates. As the plates move toward, past, and away from one another, their margins interact to produce regions of magma extrusion, volcanoes, oceanic trenches, faults, and other features. The stresses imparted to these plates by their movement has also caused them to buckle and fold, causing regional uplift and subsidence. Plate tectonics is the study of the structure and movement of lithospheric plates.

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