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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 crust Continental crust
Average thickness
Density
Composition
8-16 km
30-150 km
3.2 g/cm3
2.7 g/cm3
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 1976 - an estimated 650,000 people were killed.