The Earth's Structure

Planet Earth consists of three distinct layers, and can be pictured much like the skins of an onion. They are called: 1)the lithosphere, 2) the mantle, and 3) the core. The outermost layer, called the lithosphere, covers the Earth like a very thin skin. Below that is a thick layer called the mantle. The innermost region is the core.

The Lithosphere:

The outermost layer, the lithosphere, is about 50 miles (80 kilometers) thick. The lithosphere includes the crust which varies in thickness from place to place. Under the oceans, the crust is approximately 3 miles (5 kilometers) thick, but under the continents the average thickness is 19 miles (31 kilometers).

The Mantle:

The next layer, the mantle, is about 1,740 miles (2,800 kilometers) thick and consists of three regions: the upper mantle, the intermediate mantle, and the inner mantle. The mantle is composed of solid rocky material.

The Core:

The innermost layer, the core, extends from the Earth's center out to a radius of about 2,160 miles (3,480 kilometers). The core is believed to consist of two layers: 1) the inner core which has a radius of about 780 miles (1,255 kilometers) and is solid and 2) the outer core which is composed of liquid. The core is believed to consist of mostly metallic iron and nickel.

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Types of Rocks

Rocks can vary a lot in appearance. Although there can be a lot of differences in rocks, they all belong to one of three basic groups: 1) Igneous rocks, 2) Sedimentary rocks, and 3) Metamorphic rocks. All rocks are composed of one or more of the minerals forming the earth's crust.

Following is a brief description of the three major classes of rock:

Igneous rocks:

The word igneous comes from the Latin word ignis, which means fire. Igneous rocks result from the cooling and solidification of molten matter from the earth's interior. Examples of igneous rock include granite, obsidian, and pumice.

Sedimentary rocks:

The word sedimentary comes from the Latin word sedimentum, which means settling. Sedimentary rocks originate from the cementation of sediments that result from the erosion of older rocks. The characteristic feature of sedimentary rocks is their stratification or layering. Examples of sedimentary rock include chalk, clay, coal, limestone, sandstone, and shale.

Metamorphic rocks:

The word metamorphic comes from the Greek word meta, meaning change, and morphe meaning form, hence: change in form. Metamorphic rocks originate from existing rocks that are altered by extreme heat and pressure within the earth. Examples of metamorphic rock includes marble (altered limestone), quartzite (changed sandstone), and slate (metamorphisized shale).

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Volcanoes

Pacaya Volcano in Guatemala (courtesy of U.S. Geological Survey)

A volcano is an opening in the surface of the Earth through which magma (molten rock) , gases and ash erupt. Magma forms as a result of radioactive heating and melting of the rocks below the surface. Volcanic material moves toward the surface through channelways called volcanic conduits. Volcanoes are usually cone-shaped as a result of the built-up erupted material.

On Earth there are about 500 active volcanoes. Volcanoes occur mainly near plate tectonic boundaries and are very common around the Pacific basin. About 75% of all active volcanoes are situated in what is called the Pacific Ring of Fire.

Volcanoes are described as extinct, dormant, or active. An extinct volcano is one that is not erupting and is not likely to erupt in the future. A dormant volcano is one that is currently inactive, but has erupted within historic times and is likely to do so in the future. Of course, an active volcano is one that erupted recently.

Volcanic eruptions may be violent or mild. The most spectacular displays occur when blasts of steam are shot from the volcano. When volcanoes are born in the sea, the eruptions may be more violent than those on land because of the contact between the molten rock and water. Earthquakes, lightning, tsunamis, and strong whirlwinds have been know to result from volcanic eruptions. Mild eruptions occur when molten rock pushes through long cracks in the Earth's crust and floods the surrounding areas.

Volcanoes are a very powerful force of nature. Some of the more deadly volcanoes are shown below:

Killer Volcanos

Location	First Eruption (year)	Latest Eruption (year)	Comments
Pelee, Martinique	1792	1932	30,000 people killed in 1902 eruption
Katma, Alaska, USA	1912	1962	In 1912 eruption, 50 feet of ash built up near volcano base
Java, Indonesia	1000	1967	10,000 killed in 1586 and 5000 killed in 1919 eruptions
Krakatau, Indonesia	1680	1980	36,000 people killed from tidal wave caused by volcano
Ruiz, Columbia	1595	1988	22,000 people killed from mudflow caused by volcano

There are three main types of volcanoes: shield volcanoes, stratovolcanoes, and cinder cones.

Shield volcanoes

Shield volcanoes are low in height and are broad. They are formed by very fluid molten rock that cannot build up steep sides. Over thousands of years, however, these cones can become quite large. The Hawaiian Islands are composed of shield volcanoes that have built up from the sea floor to the surface some 3 miles (5 kilometers) above. Hawaii is the largest lava structure in the world.

Stratovolcanoes

Stratovolcanoes are composed of alternating layers of lava and pyroclastic material. Pyroclastic material is various sized, hot particles that are thrown from a volcano. When a quiet lava flow ends, it creates a seal of solidified lava that plugs the volcano opening. Pressure then builds up and a violent blast of pyroclastic material occurs. This cycle repeats many times.

Cinder cone

A cinder cone is a cone shaped hill that is composed mostly of cinder-sized pyroclastics. The sides of the cone are often quite steep and become less steep near the base of the cone.

A very interesting scientific tid-bit is that volcanoes have also been found at other places in our solar system. In 1971, the Mariner 9 space craft revealed that Mars has the largest volcano in the solar system (called Olympus Mons). In 1979, Voyager 1 photographed at least eight active volcanoes on Io, one of the moons of Jupiter. In 1991, the Magellan spacecraft provided evidence of volcanoes on Venus.

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Mass Wasting

The formal definition of mass wasting is the movement of regolith downslope by gravity without the aid of a stream, a glacier, or wind. Regolith is a particle that is loosened by weather and ceases being bedrock. In simpler terms, mass wasting occurs when loose particles, such as rocks or soil, slide down a slope as a result of gravity. There are many different kinds of mass wasting. Large scale mass wasting can cause very large amounts of damage. Examples of this include the huge mud slides and slump that often occur on the California coast. Some of the more common types of mass wasting are discussed below:

Rock fall and debris fall

This is the rapid vertical fall of a rock mass from a cliff or steep hill.

Rockslide and debris slide

This is rapid sliding of a rock mass down a slope. At the bottom of the hill, heaps of rock are usually found.

Slump

Slump is the downward slipping of the side of a hill. When this happens, the entire side of a hill breaks loose and stays mostly intact as it slides down the hill.

Debris flow

This is the rapid down hill flow of loose debris, such as stone or soil. It usually looks like a small portion of a hill broke loose and slid down the hill.

Mudflow

This is a debris flow where the debris has the consistency of mud and contains a large amount of water.

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Earthquakes

Shaking movements of the Earth's surface are called earthquakes. Most earthquakes are caused by the sudden release of stress along a fault, or fracture, in the Earth's crust. Forces creating movement along the fault must first overcome friction along the fault surface. When this occurs the fault walls move suddenly, which releases a lot of energy and causes an earthquake.

Earth quakes are dangerous for the following reasons:

1. The ground motion caused by earthquakes can severely damage or destroy buildings.
2. They cause severe fires because the movement of the earth breaks gas lines and electrical lines. In the famous earthquakes that struck San Francisco in 1906 and Tokyo / Yokohama in 1923, it is estimated that 90% of the damage was caused by fires.
3. Earthquakes cause severe landslides. Many homes, roads, and other structures are destroyed as a result of these slides.
4. Tidal waves caused by earthquakes can cause massive damage and loss of life.

It is estimated that each year the Earth experiences thousands of earthquakes. Fortunately most of these are quite small. However some have been massive killers as shown below:

Killer Earthquakes

Location	Year	Number of Deaths
Egypt / Syria	1201	1,000,000
Shaanxi Province, China	1556	830,000
Calcutta, India	1737	300,000
Jave, Indonesia	1883	100,000
Gansu Province, China	1920	200,000
Tokyo / Yokohama, Japan	1923	130,000
Northwestern Iran	1990	60,000

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The San Andres Fault

The San Andres fault is a network of faults that extends more than 600 miles (965 km). The fault runs from Northwest California to the Gulf of California. Many earthquakes have occurred (and will occur) in California as a result of this fault.

The fault is caused because two plates, the American Plate and the Pacific Plate (see plate tectonics), are moving past each other. These plates are moving at a rate of several centimeters a year. Scientists estimate that in 10 million years, Los Angeles will move as far north as San Francisco. In 60 million years, Baja California and a portion of California will entirely separate from the continental U.S. and be entirely surrounded by water.

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Plate Tectonics

Would you believe that many geologists believe that about 200 million years ago there was a single gigantic continent called Pangaea? They believe that over very long periods of time plate movements have split Pangaea into the continents and islands that we recognize today. This theory is called Plate Tectonics. Plate Tectonics is also referred to as Continental Drift. The theory indicates that the Earth's crust is divided into different regions or plates that slowly move. The Earth's surface is composed of a dozen large plates and several small ones. The continents are located on the plates so they move as well. Geologists believe that earthquakes, volcanic activity, and mountain-building processes are caused by the movement of these plates. Where these plates meet there are generally lines of earthquakes and volcanic activity. Following are some of the activities that occur on plate boundaries:

Plates moving apart

At the mid-ocean ridges, plates are moving apart. When this happens molten rock or magma pushes up from the depths. This molten material forms new crust as it becomes welded to the trailing edges of the plates. When a continent straddles such a rift it is split apart, forming a new ocean area. The Red Sea and the Gulf of California are good examples of this process.

Colliding Plates

Subduction (meaning pushed under) zones occur when two plates collide. In the deep ocean trenches a plate's edge dives into the mantle where it is reabsorbed. When subduction occurs on a continent a mountain chain is formed. The Andes are a good example of this process.

Sliding plates

When two plates slide past each other there are usually violent earthquakes along faults and sites where they grind past each other in opposite directions. The San Andreas Fault in California is a good example of this process.

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Geysers

A Geyser is a hot spring from which water and steam are ejected periodically. Geysers can shoot up to a height of several hundred feet. Following is an explanation of how scientists believe that geysers form.

Old Faithful

Geysers occur in areas where there was previous volcanic activity. As a result, the rocks that are located deep underground are still quite hot. When groundwater seeps into the ground it travels down a natural crooked rock tube or pathway that extends deep into the ground. The water is heated because it is in contact with the hot rocks. Now it gets a little more complicated. Because the water is deep in the ground it is under a lot of pressure. When water is under pressure, the boiling point temperature rises and the water becomes superheated. At the bottom of the tube or pathway the pressure is the greatest and the boiling temperature is the highest. When this condition occurs, a slight decrease in pressure or a slight increase in temperature will cause the bottom water to boil. The steam that is produced develops enough pressure to eject a small amount of water to the surface. This reduces the pressure on the water in the tube which causes it to be quickly converted to steam and an eruption occurs.

The most famous geysers are found in Iceland, New Zealand; and Yellowstone National Park, USA. Old Faithful, in Yellowstone Park, is the most famous geyser in the United States. Old Faithful erupts every 30 to 90 minutes and rises to heights of 115 to 165 feet (35 to 50 meters). With each eruption it is estimated that about 10,000 to 12,000 gallons of water are discharged.

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