Earthquakes and stress. Most earthquakes are caused by lithospheric plates moving against each other. Since lithosphere is brittle, when it needs to move it doesn't flow, it breaks.

Stresses build up as tectonic motions push plates together, apart, or past each other. At first, frictional forces hold the pieces of crust together. When the stresses build up enough energy, they overcome the frictional forces, and the two pieces of crust slip past each other. The energy is released, causing an earthquake.

Earthquakes and stress. When tectonic stresses overcome frictional forces, there is an earthquake.

Earthquakes occur at all of the three types of plate boundaries. A small percentage of quakes occur away from plate boundaries, at intraplate locations.

About 95% of earthquakes take place at plate boundaries. Convergent plate boundaries have among the most and most severe quakes. Transform plate boundaries also have severe earthquakes.

About 80% of all earthquakes occur around the Pacific Ocean and 15% in the Mediterranean-Asiatic belt. The remaining 5% take place at divergent plate boundaries or in intraplate locations.

Earthquake locations. Each dot represents the epicenter of an earthquake between 1963 and 1998. Most of the dots outline plate boundaries, with a small percentage occurring intraplate.

Pacific Ring of Fire. Earthquakes line the rim of the Pacific Ocean.

The Pacific is the site of so much earthquake activity because plate boundaries line the ocean basin. Most are convergent plate boundaries, but a few are transform plate boundaries. Volcanoes are also common along the convergent plate boundaries.

Pacific plate boundaries. Subduction zones and transform faults line the Pacific Ocean basin. The red lines are earthquake fault zones, including oceanic trenches; the red dots are volcanoes.

Earthquake focus. Earthquakes have a point of origin.

The focus of an earthquake is the point in the Earth's crust where the energy is released.

The point on Earth's surface directly above the focus is the epicenter. The epicenter is often the position where damage is greatest.

Focus and epicenter. The epicenter of an earthquake is directly above the focus, where the greatest movement occurs.

Earthquake focus depths. The focus of an earthquake can be from shallow to deep, depending on where the lithosphere breaks.

Shallow focus earthquakes are the most common because most of the interactions between lithospheric plates take place at or near the Earth's surface. They are the most destructive because they are the closest to where people live.

Intermediate focus earthquakes are found between around 70 and 300 km depth. Deep focus earthquakes are deeper than 300 km.

Earthquake locations. Red dots indicate shallow focus earthquakes and are found at divergent, transform and convergent plate boundaries. Green dots indicate intermediate and blue dots indicate deep focus earthquakes.

Plate tectonics and earthquake depths. Plate boundaries have their own earthquake depths.

All earthquakes at divergent and transform plate boundaries are shallow because plate interactions are at the surface.

Convergent plate boundaries have earthquakes from shallow to deep. The subducting plate is brittle and causes earthquakes along its depth. When the plate becomes so warm that it is no longer brittle, the earthquakes cease.

Earthquakes along a subducting plate. The locations of earthquakes along a subducting plate at the Japan Trench in cross section view and in map view.

Earthquakes at divergent plate boundaries. Earthquakes occur at divergent plate boundaries in the oceans or on continents.

At mid-ocean ridges the crust is new and thin so the earthquakes are small and shallow. On land, such as in the East African rift, the quakes are also shallow, but they can cause more damage because the crust is thicker, allowing more stress to build up.

Earthquake locations. Volcanoes and earthquakes are located in the East African Rift where a divergent plate boundary is separating eastern Africa from the main continent.

Earthquakes at transform plate boundaries. The best known earthquake fault in the United States is the San Andreas Fault.

The San Andreas Fault is a transform plate boundary between the Pacific and North American plates. Earthquakes along the San Andreas Fault are shallow and they can be extremely large. Stresses along much of the fault may build up to extremely high levels before they overcome frictional forces. When the massive chunks of continental crust finally move, they usually do so in an enormous jolt.

San Andreas Fault. At this transform boundary, the Pacific plate is moving northwest relative to the North American plate, which is moving southeast.

Volcanoes and plate tectonics. Volcanoes are found in many of the places where there is earthquake activity.

Unlike earthquakes, which are found at all three types of plate boundaries, there must be melting for there to be volcanoes. Mantle melts at divergent and convergent plate boundaries, but not at transform boundaries.

Most volcanoes are located around the Pacific Ring of Fire. Convergent plate boundaries produce the island arc volcanoes of the western and northern Pacific, and the continental (volcanic) arc volcanoes of the eastern Pacific.

Eruptions at divergent plate boundaries are usually through fissures rather than volcanic cones since the crust spreads apart before a cone can form.

Locations of active volcanoes. The locations of active volcanoes coincide with earthquake locations. Volcanic eruptions are how the Pacific Ring of Fire earned its name.

Natural resources and tectonic processes. Many natural resources were formed along ancient plate boundaries.

Metallic minerals, such as gold, silver, copper, lead and zinc, are found at subduction zones, current and ancient. Metallic minerals form in and near magma chambers and so mineral deposits are often located near volcanoes.

Some of the magma bodies do not reach the surface. They cool below the surface and form plutons, which heat and supply fluids to the surrounding rock. Ore deposits form around these magma bodies in veins or disseminated through the rock. In California, mineral deposits, such as the gold the '49ers were after, formed from fluids that came off of the granitic magma bodies that now form the Sierra Nevada mountain range.

Ancient ocean crust contains mineral resources. Iron, copper, nickel, zinc and other metals are deposited at mid-ocean ridges. Seawater circulates through hot lava and deposits minerals along the ridge. Much later this ridge may be exposed on land and mined for its ore deposits.

Fun fact: The California Gold Rush was the result of gold metal that formed hundreds of millions of years ago when California was a convergent plate boundary. Subduction created a line of volcanoes tens of millions of years ago. The magma chambers that cooled beneath the volcanoes rose up to become the Sierra Nevada Mountains and the fluids that came off the magma created abundant mineral deposits.

Ore deposits. Ore deposits form around magma chambers either as veins of metal rich fluids that travel through the surrounding rock, as disseminated ore minerals that infiltrate the rock, or as a deposit that comes off the magma itself (pegmatite).