Earth's lithosphere

Within the lithosphere, earthquakes occur, volcanoes erupt, mountains are built, and new oceans are formed. An understanding of the lithosphere's structure is needed in the search for oil and gas, for the prediction of earthquakes, and for the verification of nuclear test ban treaties.

Defining Terms

The lithosphere is the rigid outer shell of the earth. It extends to a depth of 100 kilometers and is broken into about ten major lithospheric plates. These plates “float” upon an underlying zone of weakness called the asthenosphere. The phenomenon is somewhat like blocks of ice floating in a lake. As lake currents push the ice blocks around the lake, so do currents in the asthenosphere push the lithospheric plates. The plates carry continents and oceans with them as they form a continually changing jigsaw puzzle on the face of the earth.

The word “lithosphere” is derived from the Greek lithos, meaning “stone” Historically, the lithosphere was considered to be the solid crust of the earth, as distinguished from the atmosphere and the hydrosphere. The words “crust” and “lithosphere” were used interchangeably to mean the unmoving, rocky portions of the earth's surface. Advances in the understanding of the structure of the earth's interior, resulting mostly from seismology, have forced the redefinition of old terms. “Crust” presently refers to the rocky, outer “skin” of the earth, containing the continents and ocean floor. “Lithosphere” is a more comprehensive term that includes the crust within a thicker, rigid unit of the earth's outer shell. To appreciate the reason for this redefinition, it is necessary to learn about the nature of the earth's interior. American geologist Joseph Barrell investigated the lithosphere and asthenosphere in a series of papers in 1914.

Earth's Interior

Except for the upper 3 or 4 kilometers, the earth's interior is inaccessible to humans. Therefore, indirect methods, such as studying earthquakes and explosions, are used to learn about the inside of the earth. Earthquakes and explosions, both conventional and nuclear, generate two types of energy waves: compressional (P) waves and shear (S) waves. P waves travel faster than do S waves and are generally the first waves to arrive at an observation station. The speed of a wave, however, depends on the rock through which it travels. When seismic waves encounter a boundary between two different rocks, some energy is reflected back, and some is transmitted across the boundary. If the rock properties are very different, the transmitted waves travel at a different speed and their travel path is bent, or refracted. This phenomenon can be illustrated by placing a pencil in a glass of water. Light in water travels at a speed different from that of light in air, so light is refracted, or bent, as it travels from water to air. Thus, the pencil appears to be bent. P and S waves are reflected and refracted as they travel through the earth. Waves following different paths travel at different speeds.

Since 1900, seismologists have studied P and S waves arriving at different locations from the same earthquake. They discovered three distinct layers in the earth: the crust, the mantle, and the core. The boundaries separating these layers show abrupt changes in both P- and S-wave speeds. These changes in wave speeds provide information about the earth's interior. Scientists studying the theory of traveling elastic waves, such as earthquake waves, related the speed of waves to the physical properties of the material through which they travel. It was found that S waves do not travel through liquids. From this finding, scientists concluded that the earth's core had a liquid outer region and a solid inner region. Other scientists measured the P- and S-wave speeds of many different rocks and provided clues to the kind of rocks found inside the earth.

The continental crust averages 30-40 kilometers thick and is divided into two main seismic layers. One layer, the upper two-thirds of the crust, has P- and S-wave speeds corresponding to those of granitic rocks. The speeds increase slightly in the bottom third of the continent, corresponding to rocks of basaltic composition. The average oceanic crust is 11 kilometers thick and is of basaltic composition. Beneath both continental and oceanic crust, the P- and S-wave speeds increase sharply. This boundary between the crust and mantle is called the Mohorovičić discontinuity, or Moho. The Moho marks a compositional change to a dense, ultramafic rock called peridotite.

The Asthenosphere

At an average depth of 100 kilometers, the S-wave speed decreases abruptly. It remains low for about 100-150 kilometers. This region is called the low velocity zone (LVZ). Laboratory experiments have shown that seismic-wave speeds, particularly those of S waves, decrease in rocks containing some liquid. The LVZ in the mantle indicates a zone of partial melting, perhaps 1-10 percent melt. The presence of the melt reduces the overall strength of the rock, giving the region its name, “asthenosphere,” from the Greek asthenes, meaning “without strength.”

The partially molten asthenosphere is very mobile, allowing the more rigid lithosphere above it to move about the earth's surface. The boundary between the lithosphere and the asthenosphere does not mark a change in composition; it marks a change in the physical properties of the rocks. The lithosphere defines this region of crust and mantle from the mantle region below by its seismic-wave speeds and its physical properties.

Lithospheric Plates

Seismic-wave speeds and earthquake distribution provide information about the lithospheric plates and the boundaries between them. Like the earth's crust, lithospheric plates are not the same everywhere. For example, the Pacific plate contains primarily oceanic crust, the Eurasian plate is mostly continental, and the North American plate contains both continental and oceanic crust. The lithosphere is thinnest at spreading centers, or regions where two plates are moving away from each other, such as the Mid-Atlantic Ridge and the East Pacific Rise. Here, the asthenosphere is close to the surface and the melt portion pushes upward, separating the plates and creating new lithosphere. Shallow earthquakes occur as the new crust is cracked apart. In areas such as western South America or southern Alaska, two plates are coming together, with the oceanic lithosphere being thrust under the continental plate. Earthquakes occur as deep as 700 kilometers as one plate slides under the other. Along the California coast, two plates slide past each other along faults that cut through the lithosphere. Earthquakes are common, and the faults can move several meters at a time. Where two continental plates, India and Eurasia, have collided, the crust is highly faulted and 65 kilometers thick. Earthquakes in and near the Himalaya are numerous, often occurring along deep fault zones.

Although earthquakes are most common along plate boundaries, they can also occur within lithospheric plates. Some earthquakes are related to newly forming boundaries. The Red Sea is believed to be a recently formed spreading center pushing the Arabian Peninsula and Africa apart. Some earthquakes result from the movement along ancient geologic faults buried within the crust. The causes of some earthquakes, however, such as the one in 1886 in Charleston, South Carolina, remain unknown.

Upper and Lower Lithosphere

Structural details within plate regions cannot be determined by earthquake studies alone. P and S waves generated by explosions are reflected and refracted by layers within the lithospheric plates. Regional studies show the upper lithosphere to be highly variable. In mountainous regions, such as the Appalachians or the Rocky Mountains, the continental crust is thicker than average and shows much layering. In the midcontinent and the Gulf of Mexico regions, the crust consists of thick layers of sediments and sedimentary rocks. Oil companies, combining the data from many controlled explosions, discovered petroleum and natural gas within these layers from the changes in P- and S-wave speeds. Other regional seismic studies have found ancient geological features deep within the crust. Similarities in the seismic structure between these and other known features can uncover potential sites of much-needed natural resources. The discovery of the oil fields of northern Alaska was prompted by the area's structural similarity to the Gulf of Mexico, a known source of oil and gas.

The seismic structure of the lower lithosphere is less well known. Early studies show that it is also highly variable and that crustal structures are often related to features deep in the lithosphere. Much work, however, remains in unraveling the details of the lithosphere.

Study of Seismic Waves

Scientists use a number of seismic techniques to study the lithosphere. They use P and S waves generated by earthquakes that travel through the earth (body waves) and along the earth's surface (surface waves). Reflection and refraction seismology use seismic waves generated by explosions to study the continental and oceanic lithosphere. Data from experimental studies of rocks are used to relate seismic speeds to specific kinds of rocks. Computers help analyze the vast amounts of seismic data and are used to develop models to aid in the understanding of the earth.

The use of P and S waves from earthquakes is the oldest method of studying Earth's structure. The times at which P and S body waves, reflected and refracted by the layers in the earth, arrive at different distances from the same earthquake are related to the average speed at which the waves travel. The arrival times of surface waves also depend on the layer speeds. Using seismic waves from many earthquakes, seismologists can determine the seismic structure of the lithosphere.

In regions with numerous earthquakes, seismologists record P and S waves using many portable seismographs, instruments that record seismic-wave arrivals. The scientists can then determine a more detailed regional structure. Earthquakes, however, do not occur regularly everywhere on the earth. Until an average regional structure is known, it will be difficult to determine the precise location and time of an earthquake.

Explosions as a source of seismic waves to study crustal structure have been developed and used extensively by the oil industry. With an explosive source, the location and time of detonation can be precisely controlled. Two basic techniques using artificial sources are reflection seismology and refraction seismology. Refraction seismology studies the arrivals of waves that are refracted, or bent, by the layers in the crust. The scientist determines an average velocity structure for an area by recording the time the first waves arrive at receivers located varying distances from the explosion. To determine deep structure, the distance between the explosion and the receivers must be very large. Reflection seismology allows a deeper look into the crust by studying reflections from many different layers. The seismic-wave receivers do not need to be placed as far from the source as they must in refraction studies. The reflection technique combines the results from many explosions, producing a picture of the earth's layers. This method is used extensively in the search for oil and gas. The techniques of reflection and refraction seismology have been applied to the lithosphere. Long reflection and refraction profiles have been acquired over geologically interesting but little-understood regions.

Study of Rock Properties

Seismic waves are vibrations traveling around and through the earth. Because of friction, these vibrations eventually stop, and seismic waves no longer travel. Earthquakes and explosions generate waves that vibrate at many frequencies. The earth slows each frequency differently. As a seismic wave travels through different rocks, the shape of its vibrations recorded on a seismograph is related to the properties of the rocks through which it travels. The analysis of seismic waveforms has shown differences between waves generated by earthquakes and by explosions.

To understand the lithosphere, it is necessary to know about rocks. Using a hydraulic press, scientists squeeze rocks in the laboratory to pressures and heat them to temperatures present deep within the earth. They then measure the rocks' physical properties at these conditions. Experimentally measured P and S speeds are compared to wave speeds determined from earthquakes and explosions to infer the kind of rocks and the conditions that exist within the earth. The complexity of the lithosphere, however, does not allow simple answers.

Computer Modeling

To aid the scientists in their studies, computers are used to develop models—simplified representations—of the earth. By making changes in the model, the scientist can study changes in computed seismic properties and compare them to the observed earth properties. Changes in the model are made to resemble the earth more closely. In modeling the lithosphere, scientists incorporate data from a wide range of sources, such as earthquake studies, experimental rock studies, and geologic maps. The computer allows the earth scientist to test more complex models in an effort to provide a better understanding of the lithosphere.

Significance

For the earth scientist, increased knowledge of the seismic structure of the lithosphere helps in unraveling the processes by which geologic features are formed. The movement of the lithospheric plates about the earth creates mountain ranges, causes earthquakes, and devours or creates ocean basins. Because much of the earth is inaccessible, seismic waves generated by earthquakes and explosions are used to look deep within the earth to provide a picture of the earth's structure.

Increased knowledge of the lithosphere is important to the average person for three reasons: First, earthquakes are caused by movements between and within the lithospheric plates. Every year, lives are lost and millions of dollars in damage occur because of earthquakes and earthquake-related phenomena. Detailed knowledge of the lithosphere helps scientists understand where and how earthquakes occur. This information can lead to regional assessment of the potential for earthquakes and earthquake-related damage. Knowledge of the earthquake potential of a region can result in the improvement of local building codes and the evaluation of existing emergency preparedness plans. Earthquake-hazard assessment can also aid in prediction by determining the probability of future earthquake occurrence. Some success in long-term predictions has been seen in Japan and China. Eventually, the increased understanding of the lithosphere may lead to the short-term prediction of earthquakes.

Second, detailed knowledge of lithospheric structure will lead to the discovery of potential sites of needed natural resources, such as oil, gas, and coal; metals, such as iron, aluminum, copper, and zinc; and nonmetal resources, such as stone, gravel, clay, and salt. Scientists are beginning to unravel the relationship of tectonic features to the formation of many mineral deposits. Detailed knowledge of the structure of the lithosphere from seismic studies can uncover deeply buried features that may provide new sources for critically needed resources.

Finally, scientists require detailed information on the seismic structure of the lithosphere to locate and identify earthquakes and nuclear explosions. More structural information will also lead to better identification of the differences between these two types of seismic wave sources. An accurate and reliable means of distinguishing between earthquakes and nuclear explosions is critical for the verification of any nuclear test ban treaty.

Principal Terms

asthenosphere: the partially molten weak zone in the mantle directly below the lithosphere

basalt: a dark-colored igneous rock containing minerals, such as feldspar and pyroxene, high in iron and magnesium

crust: the rocky, outer “skin” of the earth, made up of the continents and ocean floor

granite: a light-colored igneous rock containing feldspar, quartz, and small amounts of darker minerals

mantle: the thick, middle layer of the earth between the crust and the core

Mohorovičić discontinuity (Moho): the boundary between the crust and the mantle, named after the Croatian seismologist Andrija Mohorovičić, who discovered it in 1909

peridotite: an igneous rock made up of iron- and magnesium-rich olivine, with some pyroxene but lacking feldspar

reflected wave: a wave that is bounced off the interface between two materials of differing wave speeds

refracted wave: a wave that is transmitted through the interface between two materials of differing wave speeds, causing a change in the direction of travel

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