You can imagine that the crust of the earth is like a very thin skin floating on a big ball of molten rock. Like a puzzle, it consists of different parts called tectonic plates that move very slowly. Most volcanoes occur where two plates meet:
When two plates move apart causing a gap, hot molten rock - called lava - rises up between them. This type of volcano occurs on the ocean floor, so it is mostly invisible. Only if the amount of magma is big enough, it rises above the surface of the ocean and an island is created. A long time ago, Iceland was formed this way.
If two plates collide and one plate is forced beneath the other plate, the friction causes the first to melt and magma rises up. Only a small part of the volcanoes on earth are formed like this, but their eruptions are the most violent and dangerous ones.
Sometimes volcanoes also form in the middle of the plates, in so-called hotspots. These are places that are connected to the hot mantle of the earth by channels. The Hawaiian volcanoes belong to this group.
Earthquakes
Those areas, where the tectonic plates meet or move alongside each other, are also those areas most frequently hit by earthquakes. Earthquakes occur when the plates rub against each other and regularly move in small jerks.
The ocean west of Indonesia is one such active earthquake region. The most recent earthquake took place on December 26, 2004. The earthquake, with a magnitude of 9.0 on the Richter Scale, unleashed a tsunami that ravaged the coastline along the entire Indian Ocean.
Another area frequently shook by earthquakes is the St. Andrews fault in the state of California in the USA. In San Francisco, houses are especially built to withstand earthquakes.
Geysers
The gaps between the tectonic plates are also home to geysers. A geyser is a special kind of hot spring. It erupts periodically and sends out a fountain of hot water and steam.
Geysers only occur where the groundwater level is high and the ground temperature is at the same time very high. Such high temperatures - must be over 100 degrees centigrade - are only found where the red-hot lava is located just below the surface of the earth.
The only place in Europe that has geysers is Iceland.