Plate tectonics explains why our oceans open and continents move throughout Earth’s history. This theory also explains why we have tall mountain ranges, valleys, and deep ocean trenches. However, what does all of this have to do with the geologic hazards and disasters that impact our society daily? In this section, we will explore how we can predict, and even come to expect, certain types of disasters near plate boundaries.

The Ring of Fire

If you have ever had a unit on earthquakes and volcanoes in school or watched a documentary on the subject, chances are good that you may have heard of a region called the “Ring of Fire.” Not to be confused with the Johnny Cash song, the Ring of Fire is enormous! It extends for around 40,000 km around the coasts that border the Pacific Ocean – from New Zealand to the Philippines, Japan, Alaska, the western coast of the United States, Mexico, and all the Western Coast of South America. The Ring of Fire is infamous for having heightened volcanic and earthquake activity, and that is a direct consequence of tectonic activity.

Convergent plate boundaries involving subduction primarily occur throughout the Ring of Fire. The release of volatiles, such as water, from the subducting slab enables the overriding lithosphere to melt easier, and volcanic arcs form.

Volcanoes are not the only consequence of subduction zones. When cold, the rigid lithosphere sinks into the asthenosphere, stress from that movement will build up in the rocks over long periods of time. There is a limit to the amount of stress a rigid object, such as a rock, can tolerate before it breaks. Once the rock does rupture under the stress, it releases the energy it built up over those many years in the form of a seismic wave – an earthquake occurs.

Because this energy is released from a plate that underwent tremendous stress deep beneath the Earth’s surface, the earthquakes at subduction zones can be as devastating as 9.0 in magnitude. Some of these earthquakes originate under seawater, which can result in giant tsunamis, such as the 2004 Indian Ocean Earthquake and the 2011 Tōhoku Earthquake in Japan.

Gigantic Mountain Ranges

Subduction is not the only tectonic process responsible for the world’s geologic hazards! When two continental plates collide at a convergent boundary, the lithosphere is too buoyant to subduct, and large mountain chains build upward toward the sky. We know that Mount Everest is the Earth’s tallest mountain (above water, that is), and as part of the Himalayas, a convergent plate boundary between the Indian and Eurasian Plates has built it.

Because there is no subduction at this type of boundary, however, the continental lithosphere does not melt easily. There is almost no volcanic activity around continental-continental convergent boundaries. Nevertheless, there are still the same compressional stresses acting between the plates, and those stresses will slowly build upon rigid rocks in the lithosphere until they break and release seismic waves. That means that like the other convergent boundaries, regions such as the Himalayas experience earthquakes. The lack of subduction may not allow these earthquakes to originate as deeply, but these earthquakes are often moderate, and sometimes severe, in magnitude.

A far greater mountain range than the Himalayas can be found underwater, and it is produced by a different tectonic process. The Mid-Atlantic Ridge was the first example of a Mid-Ocean Ridge discovered by scientists, and is 16,000 km (10,000 miles) long. Divergent boundaries are sites of magma upwelling from the mantle because the tensional stress has thinned the lithosphere enough to allow the molten plume to penetrate the surface.

A divergent boundary, such as a rift or a spreading center at the Mid-Atlantic Ridge, hosts active volcanism on the axis of the ridge. Furthermore, new lithosphere is always being formed at a divergent boundary and pushing the old crust aside. This motion causes the older crust to fracture and rupture, which will produce earthquakes. The lithosphere at divergent boundaries is not very thick, and the earthquakes at these locations are shallow (< 30 km), but the magnitude can vary from low to moderate.

Shifting Ground

Transform boundaries will slide one segment of the lithosphere relative to another, with results that are observable in our lifetime. However, most of that sliding motion is not something that happens on a day-to-day basis! As stress builds up at the plate boundary, these large units of rock will resist moving. Therefore, that stress will build over many years. Once the plates cannot tolerate any more stress, their rocks fracture, and the plate releases the energy by snapping past one another. Two things happen:

1. The two tectonic plates move past one another in large amounts
2. The energy previously stored within two plates is released as a shallow, but moderately powerful earthquake.

Video 2.8.1.  How does shear break the ground? (0:10).

Keep in mind that there is no subduction at transform boundaries, and there are also no lithosphere creation/destruction processes. Because there is no opportunity for the lithosphere to melt from transform plate motion, there are few, if any, active volcanoes along a transform plate boundary.