SUPPORTING MATERIAL

Video: Plate Boundaries & Tectonic plates (IRIS)

Studying Earth’s Interior

Drilling Deep holes have been drilled in the continents and in the ocean floor (Figure 14.2a). Samples extracted from these holes, and measurements taken in these holes, provide constraints on the structure of the uppermost part of the Earth. But even the deepest hole on Earth, drilled in Russia, penetrates only about 15 km deep, a mere 0.2% of the Earth’s radius.

Electrical conductivity The manner in which electricity conducts through subsurface Earth materials depends on rock composition and on the presence of fluids (water and oil near the surface, magma at depths). Electrical conductivity measurements can detect the presence of partially molten rock (rock that is starting to melt, so that it consists of solid grains surrounded by films of melt) in the mantle and crust.

Earth’s density By measuring the ratio between the gravitational force generated by a mountain (of known dimensions and composition) and the gravitational force generated by the whole Earth, geoscientists determined the mass of the Earth and, therefore, its average density. Knowledge of density limits the range of possible materials that could comprise the Earth.

Earth’s shape The Earth is a spinning sphere. Thus, Earth must be largely solid inside, for if its insides were liquid, its spin would cause the planet to be disc-shaped. Similarly, if density were uniform through the planet, Earth’s spin would cause the planet to be more flattened than it actually is. Thus, the shape of the Earth requires that it have a core that is denser than the surrounding mantle and crust.

Exposed deep crust In ancient mountain belts, the combined process of faulting, folding, and exhumation (removal of overlying rock) have exposed very deep crustal levels. In fact, outcrops in the interior of large mountain ranges contain rocks that were once at depths of 20 to 50 km in the crust. In a few localities geologists have even found rocks brought up from about 100 km depth (these rocks contain ultra-high pressure phase coesite). Study of these localities gives a direct image of the geology at depth.

Geochemistry Rocks exposed at the surface were, ultimately, derived by extraction of melt from the mantle. Thus, study of the abundance of elements in rocks at the surface helps to define the range of possible compositions of rocks that served as the source of magma at depth.

Gravity field Measurements of variations in the strength of Earth’s gravity field at the surface give a clue to the distribution of rocks of different density below the surface, for denser rocks have more mass and thus cause greater gravitational attraction. Gravity measurements can indicate where the lithosphere is isostatically compensated and where it is not.

Lab experiments Laboratory studies that determine the velocity of seismic waves as a function of rock type, under various conditions of pressure and temperature, allow geoscientists to interpret the velocity versus depth profile of the Earth, and to interpret seismic-refraction profiles. Sophisticated studies of elastic properties of minerals squeezed in diamond anvils and heated by lasers even allow the study of materials at conditions found in the lower mantle or core.

Lithospheric flexure The lithosphere, the outer relatively rigid shell of the Earth, bends (“flexes”) in response to the addition or removal of a surface load. For example, when a huge glacier spreads out over the surface of a continent during an ice age, the surface of the continent bends down, and when the glacier melts away, the surface slowly rises or “rebounds.” When the lithosphere bends down, the underlying asthenosphere must flow out of the way, and when rebound occurs, the underlying asthenosphere flows back in. Thus, the rate of sinking or rebound depends on the rate at which the asthenosphere moves, and thus on the viscosity (resistance to flow) of the asthenosphere. As another example, the shape of a lithosphere where it bends down into the mantle at convergent plate boundaries (subduction zone) provides insight into flexural strength (“bendability”) of the plate. Thus, studies of flexure give insight into the rheology of crust and mantle (e.g., they tell us if it is it elastic, viscous, or viscoelastic).

Magnetic anomalies A magnetic anomaly occurs where the measured strength of the Earth’s magnetic field is greater or lesser than the strength that would occur if the field were entirely due to the Earth’s internal field (caused by the flow of iron alloy in the outer core). Anomalies occur because of the composition of rock in the crust, or due to the polarity of the magnetic field produced by tiny grains of iron-bearing minerals in a rock.

Meteorites Meteorites are chunks of rock or metal that came from space and landed on Earth. Some are relict fragments of the material from which planets first formed, while others are fragments of small planets that collided and broke apart early in the history of the solar system. Thus, some meteorites may be samples of material just like that which occurs inside the Earth today.

Ophiolites An ophiolite is a slice of oceanic crust that was thrust over continental crust during collisional orogeny, and thus is now exposed on dry land for direct examination by geologists. The complete set of rocks consists of basalt, dikes, gabbro and peridotite. Study of ophiolites gives us an image of the structure of the oceanic crust.

Seismic reflection Geoscientists have developed methods for sending artificial seismic waves (vibrations generated by explosions or by large vibrating trucks) down into the crust and upper mantle. These waves reflect off boundaries between layers in the subsurface and then return to the surface. Sophisticated equipment measures the time it takes for this process to occur and, from computer analysis of this data, geoscientists can create cross sections of the subsurface that reveal formation contacts, folds, faults, and even the Moho.

Seismic refraction When a seismic wave reaches the boundary between two layers, some of the energy reflects, or bounces off the boundary, while some refracts, meaning that it bends as it crosses the boundary. Studies of refracted waves can be used to define the velocity of seismic waves in a layer. Such studies provide insight into the composition and dimensions of subsurface layers.

Seismic-wave paths By studying the paths that earthquake waves follow as they pass through the Earth, and the time it takes for the waves to traverse a distance, geoscientists can identify subsurface layer boundaries and layer characteristics.

Seismic tomography Seismic computer techniques, similar to those used when making medical “CAT scans,” allow geoscientists to create a three-dimensional picture of seismic velocity as a function of location in the crust, mantle, and inner core. These images can be interpreted in terms of variation in material properties controlled by temperature and/or chemistry.

Xenoliths Xenoliths (from the Greek xeno, meaning foreign or strange) are preexisting rocks that have been incorporated in a magma and brought to or near the Earth’s surface when the magma flows upward. Some xenoliths are fragments of the deep crust and/or upper mantle, and thus provide samples of these regions for direct study.

More Exploration

Vectors and Plate Velocities

Let’s describe relative plate motion with the use of vectors. Imagine two plates, A and B, that are moving with respect to each other. We define the relative motion of plate A with respect to plate B by the vector AWB, where:

AΩB = ωk Eq. S11.1

In this equation, ω is the angular velocity and k is a unit vector parallel to the rotation axis.

For most discussions of plate kinematics, however, it is easier for us to describe motion in terms of the linear velocity, v, as measured in centimeters per year at a point on a plate. For example, we can say that New York City, a point on the North American Plate, is moving west at 2.5cm/y with respect to Paris, a point on the Eurasian Plate. Note that we can only describe v if we specify the point at which v is to be measured. If we know the value of AΩB as the relative plate motion, we can calculate the value of v at a point. Specifically, v is the vector cross product of AΩB and the radius vector (ri) drawn from the center of the Earth to the point in question. This relation can be represented by the equation:

v = AΩB X ri Eq. S11.2.

As in any vector cross product, this equation can be rewritten as:

v = ri sin θ Eq. S11.3

where θ is the angle between ri and the Euler axis (Figure 14.20d).

FIGURE S11.1. Linear velocity as a function of angle θ. ri is the radius vector to a point on the Earth’s surface. Since θ2 > θ1, V2 > V1. [14.20]

We see from the preceding equations that v is a function of the distance along the surface of the Earth between the point at which v is determined and the Euler pole. As you get closer to the Euler pole, the value of θ becomes progressively smaller, and at the pole itself, θ = 0°. Since sin 0° = 0, the relative linear velocity (v) between two plates at the Euler pole is 0 cm/y. To picture this relation, think of a Beatles vinyl record playing on a turntable, or a bicycle wheel. When the record or wheel spin at a constant angular velocity, the linear velocity at the center is zero and increases toward the outside (Figure S11.2).

Figure S11.2. Linear velocity increases from the center (0) to edge of a rotating wheel with constant angular velocity. Compare arrows 1 and 2 traveling different distances with same revolution around the rotation axis.

Returning to plate motions, note that the maximum relative linear velocity occurs where sin θ = 1 (i.e., at 90° from the Euler pole). What this means is that the relative linear velocity between two plates changes along the length of a plate boundary. For example, if the boundary is a mid-ocean ridge, the spreading rate is greater at a point on the ridge at 90° from the Euler pole than it is at a point close to the Euler pole. Note that in some cases, the Euler pole lies on the plate boundary, but it does not have to be on the boundary. In fact, for many plates the Euler pole lies off the plate boundary.

Because the relative velocity between two plates can be described by a vector, plate velocity calculations obey the closure relationship, namely:

AΩB = AΩB + BΩC Eq. S11.4

Using the closure relationship, we can calculate the relative velocity of two plates even if they do not share a common boundary. For example, to calculate the relative motion of the African Plate with respect to the Pacific Plate, we use the equation:

Africa Ω Pacific = Africa Ω S.America +
S.America Ω Nasca + Nazca Ω Pacific Eq. S11.5

An equation like this is called a vector circuit. At this point, you may be asking yourself the question, how do we determine a value for AΩB in the first place? Actually, we can’t measure AΩB directly. We must calculate it by knowing ω, which we determine, in turn, from a knowledge of v at various locations in the two plates or along their plate boundary. We can measure values for v directly along divergent and transform boundaries.

As an example of determining v along a divergent boundary, picture point P on the Mid-Atlantic Ridge, the divergent boundary between the African and the North American Plates. To determine the instantaneous Euler pole and the value for v at point P, describing the motion between these two plates, we go through the following steps:

• First, since v is a vector, we need to specify the orientation of v, in other words, the spreading direction. To a first approximation, the spreading direction is given by the orientation of the transform faults that connect segments of the ridge, for on a transform fault, plates slip past one another with no divergence or convergence. Geometrically, a transform fault describes a small circle around the Euler pole, just as a line of latitude describes a small circle around the Earth’s geographic pole. Therefore, the direction of v at point P is parallel to the nearest transform fault.

• Second, we need to determine the location of the Euler pole describing the motion of Africa with respect to North America. Considering that transform faults are small circles, a great circle drawn perpendicular to a transform fault must pass through the Euler pole, just as geographic lines of longitude (great circles perpendicular to lines of latitude) must pass through the geographic pole. So, to find the position of the Euler pole, we draw great circles perpendicular to a series of transforms along the ridge, and where these great circles intersect is the Euler pole.

• Third, we need to determine the magnitude of v. To do this, we use the age of oceanic crust on either side of point P. Since velocity is distance divided by time, we simply measure the distance between two points of known, equal age on either side of the ridge to calculate the spreading velocity across the ridge. This gives us the magnitude of v at point P.

On continental transform boundaries, like the San Andreas Fault in California, the magnitude of v can be determined by matching up features of known age that formed across the fault after the fault formed, but are now offset by the fault. We cannot directly determine relative motion across a convergent boundary, so the value of v across convergent boundaries must be determined using vector circuits.

READING

Anderson, DL, 1989. Theory of the earth. Blackwell Scientific Publications: Boston. Bolt, B. A., 1982. Inside the earth. Freeman: San Francisco.
Brown, GC, and Musset, AE, 1992. The inaccessible earth. Chapman and Hall: London.
Butler, RJ, 1992. Paleomagnetism: Magnetic domains to geologic terranes. Blackwell: Boston.
Condie, KC, 1997. Plate tectonics and crustal evolution (fourth edition). Butterworth: Oxford.
Condie, KC, 2001. Mantle plumes & their record in earth history. Cambridge University Press: Cambridge.
Cox, A, and Hart, RB, 1986. Plate tectonics: how it works. Blackwell Scientific Publications: Oxford.
Davies, GF, 1992. Plates and plumes: dynamos of the earth’s mantle. Science, 257, 493–494.
Fowler, CMR, 1990. The solid Earth: an introduction to global geophysics. Cambridge University Press, Cambridge.
Hawkesworth, CJ, Cawood, PA, Dhuine, B, 2016. Tectonics and crustal evolution. GSA Today, 26, 4-11.
Keary, P, and Vine, FJ, 1990. Global tectonics. Blackwell Scientific Publications: Oxford. Lillie, R. J., 1999. Whole earth geophysics. Prentice Hall: Upper Saddle River.
McFadden, PL, and McElhinny, MW, 2000. Paleomagnetism: continents and oceans. Academic Press.
Moores, EM (ed.), 1990. Shaping the earth—Tectonics of continents and oceans. Freeman: New York, 206 pp.
Nance, RD,Worsley, TR, and Moody, JB, 1986. Post- Archean biogeochemical cycles and long-term episodicity in tectonic processes. Geology, 14, 514–518.
Oreskes, N (ed.), 2003. Plate tectonics: an insider’s history of the modern theory of the earth. Westview Press: Boulder, CO.
Tackley, PJ, 2000. Mantle convection and plate tectonics: toward an integrated physical and chemical theory. Science, 288, 2002–2007.
Turcotte, DL, and Schubert, G, 2014. Geodynamics. Cambridge University Press: Cambridge.
Windley, BF, 1995. Evolving continents. Wiley: Chichester.

Back to Contents

Back to PSG&T Home

Last update: 28-apr-17