Types of Plate Boundaries

Lithospheric plates move around the globe in different directions and come in many different shapes and sizes. Their rate of movement is millimeters to a few centimeters per year, similar to the rate that your fingernails grow. Motion between tectonic plates can be divergent, convergent, or transform.

• On divergent boundaries plates are moving away from each other.
• On convergent boundaries plates are moving toward each other.
• Along transform boundaries, plates are sliding past each other.

Earth’s crust comes in two different types: oceanic crust, and continental crust (Table 2.1). The type of crust on each plate determines the geologic behavior where the plates interact because of differences in the density of the two types. The density difference is especially important when plates collide. When denser plates of oceanic lithosphere (i.e., plates with oceanic crust on top) collide with another plate, the denser plate gets jammed under the other, and forced into the mantle along a subduction zone. Plates of continental lithosphere (with continental crust on top) are not dense enough to slide down into the mantle (imagine trying to force a cork under water), so when they smash together, the result is a mountain belt formed from a broad zone of broken and folded rocks

Taking into account the type of lithosphere (oceanic or continental) and the interaction between the plates results in six different plate tectonic boundary scenarios with characteristic landforms (Table 2.2). These landforms can be spotted on satellite photos and maps.

The geographic locations of earthquakes mark plate tectonic boundaries, but they can also tell us about the type of boundary if we look at how deep the earthquakes are. For the snapping action to happen, rocks have to be relatively cold and stiff. These conditions are present in the lithosphere, but not in mantle rocks. That means earthquakes tend to be at depths shallower than 30 km, except along subduction zones, where lithosphere is being driven into the mantle. In those regions, earthquakes have been measured all the way to nearly 800 km depth. The deep earthquakes occur within subducted lithosphere because it bends and flexes as it is forced into the mantle. This means that we can use the depth and geographic locations of earthquakes to map out the shape of slabs of lithosphere that that have long since disappeared into the mantle.

Volcanoes

Volcanoes are present where melted rock reaches Earth’s surface. In spite of diagrams you might see depicting the mantle as a red-hot lava colour, Earth’s mantle is almost entirely solid rock. The mantle is much hotter than would normally be necessary to melt rocks, but the rocks remain solid because they are under immense pressure. For volcanoes to happen, special conditions are necessary, and those conditions can be present along some types of plate tectonic boundaries.

One way to melt rocks that are very hot, but under too much pressure to become liquid, is simply to decrease the pressure. Along divergent margins where Earth’s lithosphere is being stretched and thinned, the thinning reduces the weight of overlying rocks pressing down on the mantle. This reduces the pressure enough that some of the minerals in the mantle will begin to melt, eventually leading to volcanic eruptions.

The other way to melt rocks that are very hot but under too much pressure is to change how they respond to pressure. By adding water, rocks can be made to melt even at high pressures. (The temperature must still be high enough, which is why you don’t have to worry about rocks turning to lava in the rain.)

Water is part of the atomic structure of some minerals, and when those minerals are under high pressures, they transform and release the water. Water-rich minerals are present in oceanic lithosphere, and forcing it deep enough into the mantle will trigger the transitions that release water into surrounding mantle rocks, causing them to melt. The melt rises and pools, eventually leading to volcanoes forming on the overriding plate.

Volcanoes do not form by these means (decreasing pressure or adding water) along transform margins or continent-continent convergence zones. Neither type of boundary has a mechanism for subducting oceanic lithosphere, and neither decreases the pressure on the mantle beneath. In fact, the thick mountain belts that build up in continent-continent convergence zones actually increase the pressure on the mantle beneath.

Landforms and Topography

The activity along plate tectonic boundaries generates structures like chains of volcanic mountains on land or underwater, broad non-volcanic mountain belts, and rift valleys. Some of these can be spotted in a satellite photo because of a change in vegetation, evidence of higher elevation (e.g., ice, snow), the presence of a long trough-like lake, or offsets in other geographic features. We can also use topographic data—measurements of the elevation of Earth’s surface—to detect these features.

Topographic information (or bathymetric information, when we’re talking about water depths) can be conveyed on a graph as a profile (i.e., a line showing the “landscape” of hills and valleys along a specific stretch of terrain). But they can also be shown on maps as a series of lines marking different elevations, or as gradients of colour. Maps of the ocean floor often use darker shades of blue where the ocean is deeper, and lighter shades where it is shallower.