Lithospheric plates

Lithospheric plates are large, distinct, plate-like segments of brittle rock. They are composed of upper mantle material and oceanic or continental crust. The seven major and numerous minor plates fit together to form the outer crust of the earth. The seven major plates include the Eurasian plate, the North American plate, the South American plate, the Pacific plate, the African plate, the Australian plate, and the Antarctic plate.

Plate Structure and Composition

The lithosphere is the sphere of stone or outer crust of the earth. It is composed of seven major plate-like segments and numerous smaller ones. These lithospheric plates fit together in jigsaw-puzzle fashion. The recognition of the existence of these plates and their distinct boundaries has led to the theories of plate tectonics and seafloor spreading.

Lithospheric plates are layered. The bottom layer is the rigid upper portion of the mantle. The upper mantle is composed of dense, grayish green, iron-rich rock. Some plates have another solid layer of oceanic crust; this crustal rock is composed primarily of basalt. Some plates consist of only upper mantle and a thin covering of oceanic crust, while other plates have mantle material, oceanic crust, and continental crust. The continental crust is primarily granitic and is less dense than the basalt of the oceanic crust. Until recently, it was assumed that all plates had a continuous layer of oceanic crust and that continental crust was an additional layer, riding on the top. That no longer appears to be the case. The continental crust may be underlain by areas of oceanic crust in a discontinuous fashion, but the two crustal types are actually complexly intermingled.

The upper crustal rocks range from 12 kilometers thick over the ocean plains to more than 30 kilometers thick on the continental masses. The Mohorovičić discontinuity defines the boundary between the crust and the upper mantle. This boundary is recognized because seismic waves suddenly accelerate at it. The lithospheric plates, including the rigid upper mantle, are 75 to 150 kilometers thick. They float on the asthenosphere, which is a deeper portion of the mantle. The rock of the asthenosphere is under such pressure and increased temperature that it has little strength and can readily flow in much the same fashion as warm candle wax. The contact between the plates and the asthenosphere is marked by a sudden decrease in the speed of seismic waves.

The ability of the lithospheric plates to float on the asthenosphere is a key to understanding them. In much the same way that ice floats on water, the plates float on the material below them. Ice is able to float because it is less dense than water. As ice forms, it crystallizes and expands to fill more space. A given volume of ice has less density than the same volume of water. The density of the different layers of the earth increases toward the solid iron and nickel core. The lower mantle floats on the outer core, the asthenosphere floats on the lower mantle, and the lithospheric plates float on the asthenosphere.

Plate Margins

The plates fit together along margins. There are generally considered to be only three types of plate margins: ridges, trenches, and transform faults. Ridges, such as the Mid-Atlantic Ridge, are characterized by rifts or spreading centers. Trenches are margins where one plate is being forced below another and are the deepest areas of the ocean floor. Transform faults, such as the San Andreas fault in California, are areas where two plates are sliding alongside each other. The complex interactions of the lithospheric plates have led to the formation of the continents as they now exist. The plate margins do not necessarily follow the continental outlines. Continents may be composed of more than one plate. All the rocks and minerals that are on or near the surface are located on these plates. Geologic processes such as mountain building, earthquakes, and volcanism can be observed at or near the plate margins.

Formation of Crustal Material

Both types of crustal material—oceanic and continental—form through crystallization. This process is dependent on time, temperature, and pressure. As a molten material cools, a complex series of reactions occurs. The denser minerals crystallize early in the cooling of a molten material. If there is sufficient time in the cooling process, these early-formed dense minerals will gradually react with the remaining molten materials to form less dense minerals.

At the divergent plate margin, melting of the upper mantle gives rise to a silicate magma rich in iron and magnesium. This magma intrudes along fractures to be emplaced in the ocean floor as dikes and erupts to the surface as lava flows. The mafic material thus formed is called “basalt” and makes up the ocean floor. As new material is added by injection into the basalt of the ocean floor, it must push the existing material out of the way. Thus, a new sea floor is added at the spreading centers, which is made up of progressively older material away from the spreading center. In contrast, continents are composed of mostly granitic material that is formed from silicate magmas that are low in iron and magnesium and high in alkali elements such as sodium and potassium. These granites are less dense than basalt.

If a basaltic oceanic plate collides with lighter continental crustal material, the continental crust will ride up over the oceanic plate, and the oceanic plate will be pushed down into the hotter mantle, where it will be assimilated back into the mantle. This convergent margin is marked by a deep oceanic trench on the ocean side of the collision zone. The descending oceanic plate is known as a subduction zone.

When two plates of continental material collide, neither can be subducted. If they do not begin to slide alongside each other, the compressive forces will form mountains. These mountains cannot rise higher than their isostatic balance. They must either be eroded by wind and water or sink back into the asthenosphere. The eroded pieces of rock, called sediment, are transported to lower areas called basins. As the sediment becomes more deeply buried, the pressure of overlying sediments causes them to lithify or become sedimentary rock. These sedimentary rocks have considerable pore space between the individual grains of sediment or silt and, therefore, are not very dense. They become additional continental crust material.

If the sedimentary rocks are buried deep enough, the increased temperature and pressure will begin a process known as metamorphism. During metamorphism, the original minerals in the sedimentary rock react with each other to form new minerals that are stable in the new environment. As temperature and pressure increase with depth, the rock becomes more and more like granite. If the temperature reaches high enough, the rock may melt and become magma.

Multidisciplinary Study of Lithospheric Plates

Lithospheric plates fit together to form the crust or rock surfaces of the earth. The study of the surface of the earth and its composition is an extremely broad subject, including many of the subdisciplines of geology and oceanography. The study of the earth's surface and extraction of economic minerals have been undertaken since humankind's earliest times. Flint and obsidian used in tool making were early trade items; mining geology and mineralogy are almost as old. The early Greeks and Romans wrote books on geology.

Humans have used minerals and metallic minerals since prehistory. Much knowledge of the earth is essentially a by-product of what was learned during the search for minerals, mineral ores, and gems. Something as simple as the formation of a nail requires iron ore and carbon. Mining geologists assay ores looking for economic deposits, and mineralogists study minerals.

Geophysicists bounce sound waves through the earth to determine subsurface structures. With the use of seismographs, they listen to earthquakes to pinpoint their locations. They also measure the gravity and magnetic field of specific areas of the earth. Petroleum geologists search for oil and gas by drilling into the earth's surface. Their interpretation of drill cuttings and core samples provides information about ancient environments. Volcanologists study volcanoes. They employ lasers to measure any minute movements on the surface of a volcano. They also use seismographs to detect earthquakes that may signal an onset of volcanic activity. Because of the potential devastation of volcanoes, prediction has become increasingly important. Petrologists examine rocks to understand the earth processes that formed them. Their primary tools are the scanning electron microscope and X-ray diffraction machines.

Geochemists analyze the chemical composition of rocks and minerals and the reactions that may have caused their formation and dissolution. Paleontologists study fossilized life-forms, while paleoecologists study ancient environments. Planetary scientists investigate meteorites and moon rocks to increase understanding of the earth and its lithospheric plates. Much of what is known about the mantle material is a result of the study of meteorites.

Study of Oceanic Crust

Much early geologic work was done on the more readily accessible continental crust. Recently, scientists have made considerable progress in the study of the oceanic crust. Early exploration of the ocean floor was through simple depth measurements from ships. Sailors lowered a weighted line over the side of a ship and physically measured the depth to the sea floor. The echo sounders developed in the early 1900s allowed for more rapid measurements of the ocean depths. In time, continuous profiles of the sea floor were made. Instead of the featureless plain that was expected, oceanic ridges, deep trenches, and numerous submerged volcanoes appeared.

Dredging is an old but ongoing method of sampling the surface of the ocean floor. The deep-diving bathysphere paved the way for bathyscaphes and other high-technology submersibles. Much recent work has been done with television cameras. A major find was made by a geologist in the late 1970s. A seafloor volcanic vent actually had life-forms subsisting on the chemically rich waters near it. Until this time, it had been assumed that all life on the earth was dependent on photosynthesis. This initial television discovery of chemosynthetic life-forms shocked the scientific community.

Drilling on the ocean floor has been accomplished by drill ships such as the Glomar Challenger. The cores of the deep ocean floor indicated a much younger oceanic crust than had been expected. Much that was learned about the oceanic crust simply did not fit with the scientific theories of the day. Serious rethinking had to be done, and many theories had to be radically changed.

Significance of Plate Interactions

Lithospheric plates are brittle rocks that float on the hot, plastic asthenosphere. They fit together to form the crust of the earth. Each step people take is either on the surface of a lithospheric plate or on something that is directly or indirectly made from one. Weathered surface rock provides the soil in which plants grow. The plants provide a breathable atmosphere and sustain animal life. The interaction of lithospheric plates leads to earthquakes, volcanic activity, and tidal waves. These impressive geologic displays have caught the human imagination since the earliest times.

Since the lithospheric plates form the solid surface of the earth, in a real sense everything humans touch is related to them. Even something as unlikely as plastic is made from petroleum products extracted from the earth's crustal rocks. Coal, oil, and gas are burned to provide heat and electricity. Minerals extracted from lithospheric plates become the gold that makes jewelry and crowns for teeth and is part of circuit boards and computer chips. The minerals and compounds extracted from the lithospheric plates provide the iron for skyscrapers, cars, and car fuel. Coal, oil, and gas are formed by complex interactions of ancient plant life during the rock-forming processes that have occurred during the formation of upper portions of the lithospheric plates. The surface of the earth and its ongoing geologic processes also affect the weather.

The study of the composition and motion of lithospheric plates has created nearly all the body of knowledge in the field of geology. Numerous subdisciplines have arisen to study specific areas of geology. Study of the oceanic crust is relatively new, and recent discoveries are changing commonly accepted views of the earth. As views change, more discoveries seem to become possible. As more is learned about the earth's surface, views must be altered and upgraded to explain the phenomena observed.

Principal Terms

asthenosphere: a layer of the mantle in which temperature and pressure have increased to the point that rocks have very little strength and flow readily

density: the mass of a given volume of material as compared to an equal volume of water

felsic: rocks composed of the lighter-colored feldspars, such as granite

isostasy: the balance of all large portions of the earth's surface when floating on a denser material

lithosphere: the outermost portion of the globe, including the mantle above the asthenosphere

mafic: rocks composed of dark, heavy, iron-bearing minerals such as olivine and pyroxene

mantle: the layer of the earth between the crust and the outer core

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