Engineering geology

Plate Tectonics

Ever since the first reasonably accurate world maps were constructed in the 1600s, people have proposed models to explain the origin of Earth’s mountain belts, continents, ocean basins, rifts, and trenches.
For example, some people proposed that surficial processes, like catastrophic global floods, had carved our ocean basins and deposited mountains of gravel. Plate Tectonics

Others proposed that global relief was the result of what is now called tectonism: large-scale (regional, global) movements and deformation of Earth’s crust.
What kinds of tectonic movements occur on Earth, and what process(es) cause them?

Scientist and Hypothesis

German scientist, Alfred Wegener, noticed that the shapes of the continents matched up like pieces of a global jigsaw puzzle. In 1915, he hypothesized that all continents were once part of a single supercontinent, Pangea, parts of which drifted apart to form the smaller modern continents. However, most scientists were immediately skeptical of Wegener’s Continental Drift Hypothesis, because he could not think of a natural process that could force the continents to drift apart.

Shrinking Earth Hypothesis

These “anti-drift” scientists viewed continents as stationary landforms that could rise and fall but not drift sideways.
The anti-drift scientists argued that it was impossible for continents to drift or plow through solid oceanic rocks. They also reasoned that Earth was cooling from an older semi-molten state, so it must be shrinking. Their Shrinking Earth Hypothesis suggested that the continents were moving together, rather than drifting apart.

As Earth’s crust shrank into less space, flat rock layers in ocean basins would have been squeezed and folded between the continents (as observed in the Alps).
Two other German scientists, Bernard Lindemann (in 1927) and Otto Hilgenberg (in 1933), independently evaluated the Continental Drift and Shrinking Earth Hypotheses (Figure 1).

Plate Tectonics
Figure 1. Continental drift in 20 steps from 650 million years in the past to 250 million years in the future

Both men agreed with Wegener’s notion that the continents seemed to fit together like a jig-saw puzzle, but they also felt that the ocean basins were best explained by a new Expanding Earth Hypothesis (that they developed and published separately). According to this hypothesis, Earth was once much smaller (about 60% of its modern size) and covered entirely by granitic crust.

As Earth expanded, the granitic crust split apart into the shapes of the modern continents and basaltic ocean crust was exposed between them (and covered by the ocean).
During the 1960s more data emerged in favor of the Continental Drift Hypothesis. For example, geologists found that it was not only the shapes (outlines) of the continents that matched up like pieces of a Pangea jigsaw puzzle. Similar bodies of rock and the patterns they make at Earth’s surface also matched up like a picture on the puzzle pieces. Abundant studies also revealed that ocean basins were generally younger than the continents.

Harry Hess

An American geologist, Harry Hess, even developed a Seafloor-Spreading Hypothesis to explain this. According to Hess’ hypothesis, seafloor crust is created along mid-ocean ridges above regions of upwelling magma from Earth’s mantle. As old seafloor crust moves from the elevated mid-ocean ridges to the trenches, new magma rises and fills fractures along the mid-ocean ridge. This creates new crust while old crust at the trenches begins descending back into the mantle.

Harry Hess’ hypothesis was supported by studies showing that although Earth’s rocky body (geosphere) has distinct compositional layers (inner core, outer core, mantle, crust), it can also be divided into layers that have distinct physical behaviours. Two of these physical layers are the lithosphere and asthenosphere (Figure 2.).

Plate Tectonics
Figure 2. Three kinds of plate boundaries: divergent, convergent, and transform fault boundaries. White arrows indicate motions of the lithospheric plates. Half arrows on the transform fault boundary indicate relative motion of the two blocks on either side of the fault. The focus of an earthquake is the exact location where an earthquake occurred. Shallow focus earthquakes (0–69 km deep) are common along with all three kinds of plate boundaries, but intermediate focus earthquakes
(70–299 km deep) and deep-focus earthquakes (300–700 km deep) occur only in subduction zones (where lithospheric plates return to the mantle). Water from the subducted plate can lower the melting point of rock just above it at intermediate depths and lead to the formation of volcanoes.

The lithosphere (Greek lithos = rock) is a physical layer of rock that is composed of Earth’s brittle crust and brittle uppermost mantle called a lithospheric mantle. It normally has a thickness of 70–150 km but has an average thickness of about 100 km. The lithosphere rests on the asthenosphere (Greek asthene-s = weak), a physical layer of the mantle about 100–250 km thick that has plastic (ductile) behavior. It tends to flow rather than fracture.

Boundaries

Tectonic plates are plates of lithosphere that rest and move upon the weak asthenosphere. Zones of the abundant earthquake and volcanic activity are also concentrated along the unstable boundaries (plate boundaries) between rigid stable plates/sheets of lithosphere (lithospheric plates). Thus, by the end of the 1960s, a new hypothesis of global tectonics had emerged called the Plate Tectonics Hypothesis. It is now the prevailing model of Earth’s global tectonism.

According to the developing Plate Tectonics Model, the continents are parts of rigid lithospheric plates that move about relative to one another. Plates form and spread apart along divergent boundaries such as mid-ocean ridges (Figure 2.), where magma rises up between plates that are spreading apart. The magma cools to form new rock on the edges of both plates. Plates are destroyed along convergent boundaries, where the edge of one plate may subduct (descend beneath the edge of another plate) back into the mantle (Figure 2.) or both plates may crumple and merge to form a mountain belt.

Plates slide past one another along transform fault boundaries, where plates are neither formed nor destroyed (Figure 2).

Global Positioning System

Most evidence for plate tectonics has come from the detailed observations, maps, and measurements made by field geologists studying Earth’s surface directly. However, some of the best modern evidence of lithospheric plate motions is now obtained remotely with satellites orbiting thousands of kilometres above Earth’s surface. Several different kinds of satellite technologies and measurement techniques are used, but the most common is the Global Positioning System (GPS).

The Global Positioning System (GPS) is a constellation of 24 satellites in orbit above Earth. These satellites transmit their own radio signals that can be detected by a fixed or hand-held GPS receiver. Most hand-held GPS receivers, including the ones used in many cell phones, are exact to meters or tens-of-meters of accuracy. The most expensive GPS receivers are much more exact. They are used to measure plate motions (movement of specific points over years) within fractions of a millimetre of accuracy.

What could cause plate tectonics?

Recall that geoscientists have historically tried to understand the cause of Earth’s oceans, mountains, and global tectonics by questioning if Earth could be shrinking, expanding, or staying the same in size. This question can be evaluated by studying Earth’s natural forces and faults in relation to those that you might predict to occur if the size of Earth were changing. By comparing your predictions to observations of the kinds of strains and faults actually observed, it is possible to determine if Earth’s size is changing and infer whether a change in Earth’s size could cause plate tectonics.

Earth Forces and the Faults They Produce

Three kinds of directed force (stress) can be applied to a solid mass of rock and cause it to deform (strain) by bending or even faulting (Figure 3.). Compression compacts a block of rock and squeezes it into less space. This can cause reverse faulting, in which the hanging wall block is forced up the footwall block in opposition to the pull of gravity (Figure 3). Tension (also called dilation) pulls a block of rock apart and increases its length. This can cause normal faulting, in which gravity pulls the hanging wall block down and forces it to slide down off of the footwall block (see Figure 3).

tectonics
Figure 3. Three kinds of stress (applied force, as indicated by arrows) and the kinds of strain (deformation) and faulting that they cause.

Shear smears a block of rock from side to side and may eventually tear it apart into two blocks of rock that slide past each other along a lateral or strike-slip fault (Figure 3). Plate tectonic forces can be understood by how the lithosphere is strained and faulted.

Mantle Convection as a Cause
of Plate Tectonics

While much is known about plate tectonics, and the plates have been identified and named (Figure 4), there has been uncertainty about how mantle rocks beneath the asthenosphere may influence this process. In the 1930s, an English geologist named Arthur Holmes speculated that the mantle may experience circular (convection cell) flow like a boiling pot of soup. He proposed that such flow could carry continents about the Earth like a giant conveyor belt.

This idea was also adapted in the 1960s by Harry Hess, who hypothesized that mantle flow is the driving mechanism of plate tectonics. New technologies provide an opportunity to evaluate this hypothesis. For example, seismic tomography now provides sound evidence that processes at least 660 kilometers deep inside the mantle may have dramatic effects on plate tectonics at the surface.

divergent boundaries
Figure 4. Earth’s lithospheric plates and their boundaries. Numerals indicate rates of plate motion in centimeters per year (cm/yr) based on satellite measurements (Courtesy of NASA). Divergent boundaries (red) occur where two adjacent plates form and move apart (diverge) from each other. Convergent boundaries (hachured with triangular “teeth”) occur where two adjacent plates move together. Transform fault boundaries (dashed) occur along faults where two adjacent plates slide past each other. Refer back to Figure 2. for another perspective of the three kinds of plate boundaries.

Seismic Tomography

Earth’s mantle is nearly 3000 km thick and occurs between the crust and the molten outer core. Although mantle rocks behave like a brittle solid on short timescales associated with earthquakes, they seem to flow like a very thick (viscous) fluid on longer timescales of days to years. Geologists use a technique called seismic tomography to detect this mantle flow.

The word tomography (Greek: tomos = slice, graphe = drawing) refers to the process of making drawings of slices through an object or person. Geologists use seismic tomography to view slices of Earth’s interior similar to the way that medical technologists view slices of the human body. The human body slices are known as CAT (computer axial tomography) scans and are constructed using X-rays to penetrate and image the human body. The tomography scans of Earth’s interior are constructed using seismic waves to penetrate and image the body of Earth.

Analyzing data

In seismic tomography, geologists collect data on the velocity (rate and direction) of many thousands of seismic waves as they pass through Earth. The waves travel fastest through rocks that are the densest and presumed to be coolest. The waves travel slower through rocks that are less dense and presumed to be warmer. When a computer is used to analyze all of the data, from all directions, it is possible to generate seismic tomography images of Earth.

These images can be viewed individually or combined to form three-dimensional perspectives. The computer can also assist in false coloring seismic tomography images to show bodies of mantle rock that are significantly warmer (red) and cooler (blue) than the rest of the mantle (Figure 5).

tomography
Figure 5. Seismic tomography image (horizontal slice) of Earth’s mantle at a depth of 350 km. Red false coloring indicates hot rock that is less dense and ascending. Blue false coloring indicates cooler rock that is static or descending. See text for discussion. (Courtesy of Paul J. Morin, University of Minnesota)

Evaluating Plate Tectonics and the hot spots

The Plate Tectonics Model is widely applied by geoscientists to help explain many regional and global features of the geosphere. Another regional feature of Earth is hot spots, centers of volcanic activity that persist in a stationary location for tens-of-millions of years. Geologists think they are either: a) the result of long-lived narrow plumes of hot rock rising rapidly from Earth’s mantle (like a stream of heated lava rising in a lava lamp), or b) the slow melting of a large mass of hot mantle rock in the upper mantle that persists for a long interval of geologic time.

Adapted from R.M. Busch, and D. Tassa

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