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Outline the formation of hotspots and explain their relationship to plate movement (8 marks)
The core of the earth is extremely hot with a temperature of over 6000C, this therefore warms
the asthenosphere which is semi molten as rock melts to create magma, creating convection
currents as the particles are warmed therefore rise and then they cool when the reach the upper
mantle and become denser so sink back toward the core and the process repeats. This raises a
mantle plume (hot, upswelling magma) towards the crust.
The crust swells in the broad region around a hotspot. The plume breaks through the crust at a
weak spot such as at a divergent margin, or some break through the surface due to partially
melting the lithosphere. They subsequently act as a continuous magma source, which rises up as it
is lighter than the overlying plate.
A hotspot is known as "anomalous volcanism" due to not occurring at a plate boundary but
instead within a plate at a weak spot above a mantle plume. They produce active volcanoes at the
earth's surface. As the plates move by the dragging and convection current theories, plates move
over the hotspot; as the hotspot erupts, its lava cools on the earth's surface creating shield
volcanoes from basaltic lava and eventually volcanic islands. As the plate moves over the static
hotspot, new volcanic islands are created, as they move further away from the hotspot they cool,
subside and become extinct. An example is the Hawaiian islands in the middle of the pacific plate
which has been active at least 70 million years, producing a volcanic chain that extends 6,000 km
across the northwest Pacific Ocean. The Pacific plate is moving over the hotspot at a rate of
10cm/year in a north west direction, the only island that is active is the one closest to the hot
spot: Hawaii, whilst the oldest islands such as Kauai are extinct as this is 70 million years old. This
is one way in which hot spots relate to plate movement.
A further relationship between plate movement and hot spots is the hot spot theory, suggesting
that the outward flow of viscous rock from the centre may create a drag force on the plates and
make them move.