2.2: Plate Tectonics

Plates and Plate Motion

The lithosphere (the upper rigid mantle plus the crust) is divided into several pieces known as plates that can move around on the Earth’s surface, all in different directions and at different rates. The behavior of these plates: their motions and their interactions with each other is known as plate tectonics, where the word “tectonics” refers to the deformation of rocks. The distributions of the plates are shown on the figure below; the seven major ones are as follows:

• North American Plate
• Eurasian Plate
• Pacific Plate
• African Plate
• South American Plate
• Indo-Australian Plate (includes Australia and India Plates)
• Antarctic Plate

Figure \(\PageIndex{1}\): The Approximate Extents and Rates of Motion of the Earth’s Tectonic Plates

Some of the minor plates include the Juan de Fuca, Cocos and Nazca Plates along the western edge of N. America, the Caribbean Plate, and the Scotia, Arabia and Filipino Plates. There are several other smaller plates not shown on this map.

The generalized plate motions are shown in the figure with black arrows. Plate motions range from roughly 1 to 10 cm/yr (larger arrows mean the plate is moving faster there). As can be seen on the map, parts of individual plates are moving in different directions and at different rates. That’s not because the plates are squeezing and stretching (although that does happen near to plate boundaries) but because the plates are all moving in a rotational way, each around a different rotational axis. The North America Plate is moving counterclockwise around an axis in the southern hemisphere, so the rate of motion is greater in the far north than in the south, and it changes from “towards the northwest” in the east to “towards the southeast” in the west.

Plate Boundaries

At plate boundaries the interaction between plates can be: convergent (moving towards each other), divergent (moving away from each other), or transform (sliding past one another). A convergent boundary is illustrated on Figure \(\PageIndex{2}\). In this case a plate comprised of oceanic crust is moving towards one comprised of continental crust. Because oceanic crustal is denser than continental crust, the oceanic plate gets pushed down—or subducted—beneath the continental plate. That has some important implications, as shown. First, is that there is friction between the two plates, which results in periodic earthquakes, some of which can be very large. Second, is that the subducted oceanic crust introduces water into the mantle. This water mixes with the hot rocks of the asthenosphere which reduces their melting temperature (so more melting takes place) creating magma that moves towards surface. This magma can erupt at the surface as lava and form a volcano as seen in the figure.

Figure \(\PageIndex{2}\): Depiction of Some of the Processes Taking Place at an Ocean-Continent Subduction Zone. The red stars represent where earthquakes may start.

As an oceanic plate converges with a continental plate, as shown in the figure above, it is possible that a continent or island will be moving along with that oceanic lithosphere and that the two areas of continental lithosphere will eventually collide. This scenario is illustrated in the figure below. In this situation the continental lithosphere cannot be subducted because it isn’t sufficiently dense to be pushed down into the mantle. As the continents collide the sediments that had accumulated along their edges get pushed up to form fold-belt mountains and the older crustal rocks also get deformed and pushed up. The leading edge of the subducted oceanic lithosphere eventually breaks off and descends into the mantle.

Figure \(\PageIndex{3}\): Depiction of Processes Taking Place at a Continent-Continent Convergence Zone

A divergent boundary exists where two plates are moving apart from each other, likely in response to convection in the mantle. This means that there is slow upward movement of mantle rock along the boundary. As the hot mantle rock moves upward it experiences reduced pressure which allows some of it (about 10%) to melt. This produces metal-rich magma that erupts at surface (in this case on the sea floor) as lava. New oceanic crust is made in this way.

Figure \(\PageIndex{4}\): Depiction of Processes Taking Place at a Divergent Boundary Between Two Oceanic Plates

Figure \(\PageIndex{5}\): Three Different Types of Plate Boundaries in the Western US and Canada

Some plate boundaries are shown in plan view on Figure 2.2.5. The Juan de Fuca Ridge, where the Juan de Fuca and Pacific Plates are moving away from each other and new oceanic crust is being made, is an example of a divergent boundary. The Cascadia Subduction Zone is an example of a convergent boundary. Here the Juan de Fuca Plate is pushing down underneath the North America Plate, resulting in earthquakes and volcanoes.

The third type of plate boundary—a transform boundary—where two plates are moving side-by-side relative to each other, also exists in this region. An example is the San Andreas Fault, which forms the boundary between the North America and Pacific Plates through California. The relative motion of these two plates is shown with small white arrows. There have been large earthquakes along this boundary, and there are frequent smaller ones. It is important to note that this type of plate boundary produces earthquakes up to about an 8.0 on the Richter scale. The larger events (9.0 and greater) only occur with subduction.

Two smaller transform boundary segments are shown (as red lines) on the map, between the segments of the Juan de Fuca Ridge. In both cases the Juan de Fuca Plate is moving relative to the Pacific Plate, and there are frequent small earthquakes along these boundaries.

Hazards Associated With Plate Boundaries

The sections above address the geologic hazards related to plate tectonics, but it is worth looking at them again, in a more centralized section.

Volcanic Activity

Regarding plate boundaries specifically, there either needs to be a gap at the boundary the magma can exploit or melt needs to be generated somehow and rise to the surface, through the crust.

When two plates move away from one another at divergent boundaries, a space is created that must be filled. Molten material is created below this boundary due to decompression melting as seen in Figure \(\PageIndex{4}\). The material rises, fills the gap, and cools to form new crust. When this happens out in the ocean, new ocean floor is created. This does not affect humans as this is not where we live. When this happens on land, it eventually pulls the continental crust apart enough an ocean forms, but in the meantime the crust thins, and bits of molten material rises to the surface and volcanoes form.

When two plates move toward one another at convergent boundaries, two things can happen. In the case where both plates are made of continental crust, land is pushed up and mountains form (Figure \(\PageIndex{3}\)), but there is no volcanic activity. In the case where at least one of the plates is made of oceanic crust, the plate subducts and magma is generated (Figure \(\PageIndex{2}\)) resulting in the formation of a chain of volcanoes.

When two plates slide past one another at transform boundaries there is no gap to fill, and molten material is not generated. Accordingly, there is no volcanic activity at these boundaries.

To summarize whether various plate boundaries are volcanically active:

• Divergent boundaries - yes
• Convergent boundaries without subduction - no
• Convergent boundaries with subduction - yes
• Transform boundaries - no

Earthquakes

Since earthquakes are generated on active faults, and active faults primarily exist at plate boundaries, this is where 90% of earthquakes occur. When looking at divergent boundaries, since the processes are happening in the uppermost part of the crust (Figure \(\PageIndex{4}\)), the earthquakes generated tend to occur at shallow depths. At convergent boundaries, especially those with subduction, earthquakes occur at shallow and deep depths. This is because the stresses associated with those collisions are happening deeper into the Earth's interior (figures \(\PageIndex{2}\) and \(\PageIndex{3}\)). This is also the type of plate boundary that generates the largest earthquakes (>9.0M). Transform boundary earthquakes are typically quite shallow and can be sizable (~8.0M) but do not reach the magnitude of the largest convergent boundary events.

To summarize earthquake activity by plate boundary type:

• Divergent boundaries - shallow; smaller in magnitude
• Convergent boundaries - shallow to deep; very large in magnitude
• Transform boundaries - shallow; large in magnitude (but not as large as convergent boundary events)

Mass Wasting

Mass wasting is discussed in detail in a future chapter. For now, you can think of mass wasting as synonymous with landslides. These events are driven by gravity, so for them to occur land must be uplifted above the surrounding area, so the land has somewhere to slide to. Divergent boundaries are unique because as you can see in Figure \(\PageIndex{4}\), a little ridge develops at the spreading center. This uplifted area is susceptible to mass wasting. You can see in figures \(\PageIndex{2}\) and \(\PageIndex{3}\), convergent boundaries produce mountains. These uplifted areas are prone to mass wasting. Transform boundaries that just produce pure sliding motion do not result in any uplift or down drop of land, so they are not particularly susceptible to mass wasting. However, it is common for bends in these boundaries to occur. If they bend in the "right" direction, the boundary can experience compression and mountains can uplift. In this case, mass wasting can occur.

Figure \(\PageIndex{6}\): Depiction of a bend along a transform boundary. The red arrows on the plate on the right and green arrows on the plate on the left show the transform motion of the plates overall. Where the bend occurs, the green and red arrows point at one another showing where compression occurs and mountains form.

To summarize whether various boundaries are susceptible to mass wasting:

• Divergent boundaries - yes
• Convergent boundaries - yes
• Transform boundaries - sometimes

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0, from a public domain base map, “Slabs,” created by US Geological Survey
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{5}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{6}\): GeoAsh, CC BY-SA 4.0