Why Earthquakes and Volcanoes Happen at Plate Edges

0:00Why do earthquakes and volcanoes happen where they do?

0:04Each yellow dot is a strong earthquake. Each red triangle is a volcano.

0:11Most line up along the edges of tectonic plates, the giant slabs of rock that form Earth's outer shell.

0:18The busiest edge circles the Pacific Ocean: the Ring of Fire.

0:24Slice through one edge: an ocean plate moves toward a continental plate, a few centimetres a year.

0:32The plates don't slide smoothly. They stick, and stress builds up.

0:38When the stress gets too great, the rock snaps. That is an earthquake.

0:44The energy spreads as seismic waves, vibrations through the rock.

0:49The rock breaks at the focus, underground. The point on the surface right above it is the epicentre.

0:57Magnitude measures earthquake size. Is magnitude 6 only a little stronger than 5?

1:04No. Each step up the scale releases about 32 times more energy.

1:11So magnitude 7 releases about 1000 times more energy than 5.

1:17Here the ocean plate sinks into the mantle, the hot rock below the plates: a subduction zone.

1:25The sinking plate carries water down. Water lowers the melting point of mantle rock, so some melts into magma.

1:34Magma is less dense than the rock around it, so it rises and erupts at a volcano.

1:42A few volcanoes, like Hawaii, sit far from plate edges, above hot spots where hot mantle rock rises.

1:51So most earthquakes and volcanoes happen at plate edges, where plates snap and sinking plates make magma.