Page last updated at 09:11 GMT, Thursday, 1 October 2009 10:11 UK

The Pacific 'Ring of Fire'

In the wake of devastating - and unconnected - earthquakes and tsunamis in Indonesia and Samoa, the BBC News website looks at the so-called Pacific "Ring of Fire" - the zone of major seismic activity which has one of the world's most active fault lines.

Map shows ring of fire and tectonic plates

Circling the Pacific Basin, on the bottom of the sea bed, lie a dramatic series of volcanic arcs and oceanic trenches.

The zone - known as the "Ring of Fire" - is notorious for frequent earthquakes and volcanic eruptions, and coincides with the edges of one of the world's main tectonic plates.

More than half of the world's active volcanoes above sea level are part of the ring.

Intense geological activity

By the 1960s, scientists had developed a theory called plate tectonics explaining the locations of volcanoes and other large-scale geologic features.

According to tectonic theory, the surface of the Earth is made up of a patchwork of massive rigid plates, about 80km (50 miles) thick, which float in slow motion on top of the Earth's hot, pliable interior.

The plates change size and position over time, moving at speeds of between 1cm and 10cm every year - about the speed at which fingernails grow.

New sea bed is constantly being created in the middles of the oceans - flowing out as hot lava, and rapidly cooling on contact with cold deep sea water.

To make room for the continual addition of new ocean crust, all the Earth's plates move. And as they move, intense geologic activity occurs at the plate edges.

At the edges, one of three things may occur:

  • the plates can be moving away from each other, leaving space for new ocean floor
  • some plates are moving towards each other, causing one to submerge beneath the other
  • other boundaries slide past each other without much disturbance.

Tectonics and earthquakes

Parts of the plate boundary that slide past one another in opposite directions - such as the San Andreas Fault - cause minor earthquakes.

A man, assisted by his friend, carries his injured daughter after the tsunami hit Aceh, Indonesia. Photo: 26 December 2004
The 2004 tsunami killed some 230,000 people in the Indian Ocean area

The faults may also create cliffs or scarps thousands of feet high on the ocean bed.

But where one oceanic plate collides with and is forced deep into the Earth's interior, the subsumed plate encounters high temperatures and pressures that partially melt solid rock.

Some of this newly-formed magma rises to the Earth's surface and erupts, forming chains of violent volcanoes - like the Ring of Fire.

These narrow plate-boundary sites, known as subduction zones, are also associated with the formation of deep ocean trenches and big earthquakes.

When there is an earthquake under the sea, one side of the ocean floor suddenly drops downward, beneath the top edge of the subducting plate.

The resulting vertical fault will generate a tsunami - much as a wave machine in a swimming pool will generate one.

The movements of the plates usually allow little warning for those at risk in coastal areas.

One warning of a tsunami is that there is a rush of water away from the coastline - but this predictor may mean the forthcoming seismic wave is only minutes away.

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