Overview of Plate Tectonics

Introduction

Plate tectonics is a revolutionary mid-twentieth (20th) century geologic model that attempts to explain and unify a number of seemingly unrelated geological phenomena, such as deformation of the Earth's crust, earthquake distribution, continental drift, and mid-ocean ridges. Plate tectonics states that the Earth's surface is broken into many different sized plates that are slow moving and able to change size over time. Most importantly, intense geologic activity occurs along these plate boundaries. An overview of the Earth's structure, plate boundaries, and plate motions is essential to understand the basic principles of plate tectonics. Once these principles are understood, a more in-depth discussion can be given concerning plate tectonics specific to the Puget Sound region.

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Structure of the Earth

Different forces within the Earth can sometimes put a strain on the rocks beneath the surface, until those strained rocks essentially break and release waves of energy throughout the Earth known as seismic waves. The majority of our understanding of the Earth's internal structure is based on seismic wave activity. Seismic waves traveling through the Earth will have different velocities and directions depending on the type of material density. Repetitive studies of velocities and directions of seismic waves traveling through the Earth have given rise to the theory that the Earth is divided into different regions. The three main concentric regions of the Earth are the crust, the mantle, and the core (See Figure 1). The crust, mantle, and core are distinguished from each other based on marked differences (changes in velocity and or direction are often referred to as discontinuities) observed in seismic wave behavior at particular depths.

Figure 1. Internal regions of the Earth. [Courtesy of U.S. Geological Survey]

The crust is the outermost layer of the Earth consisting mainly of rock. There are two major types of crusts: oceanic and continental. Continental crust is found under continents. Oceanic crust lies under the ocean and is typically much thinner and denser than continental crust. The crust can range in depth from a few kilometers under deep oceans to about one-hundred kilometers (100 km) under continents.

The mantle makes up the largest volume of the Earth. The mantle can be divided into upper and lower portions. The upper mantle starts at the base of the crust (~100 km) and continues to an approximate depth of 660 km. The lower mantle extends from the 660 km seismic wave discontinuity to the approximate 2900 km discontinuity (where the "core" begins).

There are two regions found within the crust and mantle that play a significant role in most geological activity; the lithosphere and asthenosphere. The lithosphere is a region that includes the crust and a portion of the upper mantle. The region is collectively considered the strong solid outer layer of the Earth. Its thickness can range from 50-300 km. The asthenosphere lies directly beneath the lithosphere and behaves like a very thick fluid. The asthenosphere runs from about 300 km deep to the 660 km discontinuity (start of the lower mantle). According to plate tectonics, the lithosphere is broken into many plates that are in motion and move relative to one another over the underlying asthenosphere.

The core of the Earth can be divided into outer and inner portions. The outer core extends from the base of the lower mantle (2900 km) to a depth of approximately 5100 km. The inner core extends from the base of the outer core (~5100 km) to the center of the Earth (~6400 km). Seismic wave activity indicates that the outer core is a liquid composed mainly of iron and the inner core is a solid that is close to the solid-liquid temperature.

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Plate Boundaries

Plate boundaries are also known as seismic boundaries because earthquakes occur along their edges. In fact, since earthquakes are so closely related to plate boundaries, mapping earthquake distribution has helped mark the presence of plate boundaries. Each boundary is distinguished by its distinct movement and there are three main types of plate (seismic) boundaries: divergent, convergent, and transform (See Figure 2).

Divergent boundaries occur where two plates are moving away from each other. A new crust is created when the plates move apart, as magma (molten rock) pushes its way up from the mantle. The results of divergent boundaries are what are known as ocean ridges. Volcanoes and shallow earthquakes are often associated with these ocean ridges. The Mid-Atlantic Ridge is an example of a divergent boundary (See Figure 3). This submerged mountain range extends from the Arctic Ocean to beyond the southern tip of Africa.

Transform boundaries, also known as strike-slip boundaries, occur where two plates slide past one another horizontally. The surface (crust) of the Earth is maintained with transform boundaries as material is generally neither created nor destroyed. It should be noted, however, that small amounts of magma do slowly emerge along the fault surface over time. Transform boundaries are characterized by shallow earthquakes that result from the plate motion. The San Andreas Fault in California is a good example of this type of boundary (See Figure 4).

Figure 4. Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. (Photograph by Robert E. Wallace, U.S. Geological Survey.) [Courtesy of U.S. Geological Survey]

Convergent boundaries occur when two plates move toward each other. There are three general types of convergent boundaries: oceanic-continental, oceanic-oceanic, and continental-continental (See Figure 5). When two plates collide, one plate will often be subducted beneath or pulled under the other. The more dense plate is usually the one subducted, therefore at oceanic-continental boundaries, the more dense oceanic plate is generally subducted. The majority of destruction to the Earth's crust is due to subduction. Trenches are the deepest parts of the ocean floor and occur at and are formed by the subducting plate. As the plate is subducted, it begins to melt and break apart. The partially molten material works its way back to the Earth's surface where new volcanoes are formed or active volcanoes are sustained. At continental-continental convergent boundaries, also known as collisional boundaries, two continental plates collide, resulting in a general folding and uplift of the plate material. An example is the formation of the Himalayas where the Asia continent and India sub-continent began converging over 45 million years ago.

Figure 5. The three main types of convergent boundaries. [Courtesy of U.S. Geological Survey]

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Plate Motions

The types of interactions that occur along the plate boundaries and the structure of the Earth have been described previously. However, how do we know that plates are moving and that interactions at the plate boundaries change over time? What is moving the plates over the surface of the Earth? The plates move relative to one another in a spherical cycloid motion. The motion of the plates can be complicated to describe and visualize because of the spherical (ball) nature of the earth. However, the motion of each plate can be described if the following variables are known: the position of the pole of rotation, the direction of relative motion, and the magnitude of the angular velocity. Imagine a spinning bicycle tire; it is spinning about a center point or its pole of rotation. The tire is moving in a direction, however, and any point on the tire may have a different motion compared to the overall direction of the bicycle. That is, a point on the tire will be moving up or down while the bicycle is moving forward so the direction becomes relative based on the reference of the observer. The magnitude of the angular velocity describes how fast the tire is spinning about its pole of rotation. This analogy simplifies the plate motion significantly, because a bicycle tire is spinning only in contact with the ground, while the plates are in contact with other plates that have different poles of rotation, different relative motions and angular velocities.

How do we know the plates move?
It has been observed since the late 18th century that the continents of today can be made to fit together like pieces from a puzzle (See Figure 6). South America and Africa were prime pieces for the continental puzzle around which the idea of Continental Drift was formulated. Continental drift is the idea that continents freely move over the Earth's surface and change their relative positions to one another. In the early 1900s a German meteorologist by the name of Alfred Wegener provided the first solid evidence for the idea of continental drift. One piece of evidence Wegener noted was that South America, Africa, India, and Australia had almost identical rocks and fossils from the late Paleozoic (360 to 248 million years ago). Wegener hypothesized that these continents were once joined to form a supercontinent known as Pangea, which over geological time, moved apart to their present day positions. The observations of identical looking rocks and similar fossilized species have been scientifically collaborated using geologic dating techniques (age based on radioactive decay rate), proving that when the rocks of South America and Africa were formed the two continents were connected.

In addition to matching the edges of continents together, we also know the plates as demonstrated by the results of research conducted on paleomagnetism, or ancient magnetism. When magma is brought to the Earth's surface either at a spreading ridge or via a volcano, the cooling lava records the Earth's magnetic field. In particular, rocks containing magnetite and hematite minerals have preserved this magnetic alignment. The magnetic poles do periodically reverse: that is, magnetic north becomes magnetic south. The newly formed rocks have recorded the patterns of magnetic changes, often called magnetic striping. The magnetic striping is most commonly found at the oceanic ridge spreading centers where magma is being brought to the surface and adding to the plate and pushing the older material out away from the spreading ridge (See Figure 7).

Figure 7. Paleomagnetism patterns offshore of the Pacific Northwest. The striping pattern results when new rock forms under different magnetic field conditions (pole reversals). In this picture the magnetic patterns are colored by age. [Courtesy of U.S. Geological Survey]

What causes the plates to move?
Many experts have historically debated what drives the Earth's plates, but most agree that plate motions are related to thermal convection in the mantle (Condie 1997). Thermal convection is a circulation pattern driven by the rising of hot material and the sinking of cold material. Hot material has a lower density and thus rises, whereas cold material has a higher density and thus sinks. Convection cells are created within the mantle as hot materials rise from the core, move laterally, cool, and then descend in a cycle. Thermal convection within the mantle partially results from this heating at the base (near the core) and cooling at the surface (near the crust). So even though the mantle is considered a solid, there are zones within the mantle that experience extreme temperature differences, creating mantle portions that lie within the solid state and some that lie within the liquid state. Subducting plates are major contributors to thermal convection patterns within the mantle. Indeed, most plate movement is due mainly to what is known as "slab-pull" forces. Slab pull forces are created when plates descend into the mantle at subduction zones. A subducting plate sinks because it is denser than the surrounding mantle. The thermal convection cell arises as the cold lithosphere (subducting plate) sinks into the hot mantle.

References

Condie, Kent C. 1997. Plate Tectonics and Crustal Evolution. Butterworth-Heinemann. Oxford. 4th edition.

Plummer, Charles C., David McGeary, and Diane H. Carlson. 1999. Physical Geology. WCB McGraw-Hill. 8th edition.

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