Mantle plumes and hotspots Not all volcanic activity can be related to present day active plate margins. The Pacific Ocean shows a number of sub-parallel chains of volcanic islands which run diagonally across the Pacific plate (see map below). Isotopic dating of the lavas making up the volcanic islands shows that in all cases, the age of the islands increases from SE to NW. The youngest islands in the chains are all volcanically active, but as the islands increase in age they become extinct. The islands are also much younger than the ocean crust that they are built on. Dating of islands in the Pacific chains gave an average plate movement of approximately 99mm per year over the last 90 million years, although there is no evidence to suggest that this rate of plate motion has been constant over time. Changes in the alignment of some of the volcanic chains preserves evidence of a change in the direction of plate motion about 45 million years ago

Sequences in the formation of a volcanic island above a mantle plume.

Oceanic crustContinental crust Average thickness8-16 km km Density3.2 g/cm g/cm 3 Composition Rich in Fe, Mg Poor in Si, Al Rich in Si, Al Poor in Fe, Mg

What causes an earthquake? The crust of the Earth is broken into 12 enormous 'tectonic' plates that stretch over entire continents and oceans. These plates of solid rock are in constant motion. Where they meet is known as a fault line. As these plates drag past or over each other, huge amounts of stress can build up. When the stress is too great the rocks force apart like an expanding spring.

Where are the most dangerous places for earthquakes? Major fault lines cross the planet and it is there that major earthquakes take place. Two of the most earthquake-prone faults are the San Andreas in California and the North Anatolian which runs along the north of Turkey. These are both more than 1000km long. Other areas of high earthquake activity are Japan, the Middle East and along the west coast of South America. All lie on the boundaries of the Earth's continental plates. How much stress is needed to trigger an earthquake? The amount of stress needed to trigger a following quake does not have to be great. Studies of over 20 faults have convinced many scientists that earthquakes can be triggered by as little as one eighth of the pressure required to inflate a car tire.

What is the difference between an earthquake and earthquake storm? An earthquake is a single isolated event in which the Earth's crust pulls apart due to the build up of stress. The phrase earthquake storm was coined by Stanford Professor of Geophysics, Amos Nur, and describes a series of earthquakes where one triggers another and then another. The phenomenon is based on a scientific theory known as the stress triggering model. Scientists believe that on some faults, as an earthquake strikes, the stress released does not simply disappear. Instead it must be redistributed to another region of the fault which sets off another earthquake. This series of earthquakes is known as an earthquake storm. Although still in its infancy the stress triggering model has identified possible series of earthquakes in California, Japan and Turkey.

1960 The largest ever earthquake is measured in Chile. At 9.5, it reaches the maximum value on the Richter scale. More than 2,000 are killed, 3,000 injured and 2 million made homeless

Asian Plate

Istanbul quake more likely but unpredictable

Earthquake Storms Earthquakes are among the most devastating natural disasters on the planet. In the last hundred years they have claimed the lives of over one million people. Earthquakes are destructive mainly because of their unpredictable nature. It is impossible to say accurately when a quake will strike but a new theory could help save lives by preparing cities long in advance for an earthquake. The surface of the Earth is made up of large 'tectonic' plates. These plates are in slow but constant motion. When two plates push against each other friction generates a great deal of energy. For this reason earthquakes occur most frequently on tectonic fault lines, where two plates meet. However these fault lines run for thousands of kilometres; predicting exactly where a quake will occur is nearly impossible

Stress lines In 1992, Dr Ross Stein was monitoring a large earthquake in a town in California called Landers. Three hours later, there was another quake 67km away at Great Bear. Stein believed that this was not simply an aftershock, instead he theorised the event at Landers had set off the earthquake at Big Bear. Stein believes that when an earthquake occurs the stress that has built up along the fault, is in part, transferred along the fault line. It is this energy transfer that causes other quakes to occur hours, days or months after the original.

What is the difference between an aftershock and an earthquake storm? All major earthquakes are followed by a series of aftershocks. These are smaller tremors that follow a main event. A series of aftershocks following a large earthquake is not considered to be a storm. An earthquake storm is when large independent earthquakes are triggered over long distances and over long periods of time. However aftershocks have helped scientists identify the mechanism which triggers an earthquake storm and have given them a clearer understanding of how the transfer of stress along a fault takes place. How many people die a year in earthquakes? There is no definitive figure for this but every year earthquakes can kill many thousands. In 1999, the Izmit earthquake in Turkey killed over 25,000. In January 2001 the Indian State of Gujarat was struck by a major earthquake killing over 30,000 people. The single most catastrophic quake of recent times was the Tungshan earthquake in China in an estimated 650,000 people were killed.