The islands of
Japan are situated above the junction of four tectonic plates: The Pacific Plate,
The North American Plate, The Eurasian Plate and The Philippine Plate. Tectonic
Plates are large, moving slabs of rock which form the Earth’s lithosphere, or
crust. The lithosphere is divided into many plates, including eight major
plates. These lithospheric plates ride on the asthenosphere, commonly referred
to as the mantle, and move gradually (about 3 centimetres each year) across the
Earth’s surface relative to each other through convection currents.
Figure 2: Plate Tectonics
Figure 2: Plate Tectonics
These convection currents occur
in the mantle; a hot, molten layer inside the Earth. The Earth’s warm, iron
outer core heats the mantle material closest to it, forcing the liquid to
expand and become less dense. The mantle material near the crust is cooler and
denser and “sinks” towards the core, whilst the hot material near the core
rises to take the place of the cooler material. This continual, circular motion
drives the plates to slowly move. Evidence of this movement includes the
distribution of the continents and the formation of geological features such as
volcanos, hills and mountains, along the plate boundaries.
Figure 3: Convection Currents
Figure 3: Convection Currents
There are four types of plate
boundaries: divergent boundaries, convergent boundaries, transform boundaries
and subduction zones. Divergent boundaries occur where hot, rising magma pushes
the plates away from one another, forming features such as volcanos. Convergent
boundaries take place when two plates of similar density slide towards each
other and collide, creating structures such as mountains. Transform boundaries are
where two plates slide horizontally past each other neither destroying nor
creating new crust. Subduction zones are where two plates converge with the
thicker plate pushing the thinner plate down into the mantle in a process
called subduction. The majority of subduction zones are located around the Ring
of Fire, an area surrounding the Pacific Ocean where a large number of
earthquakes and volcanic eruptions occur. Japan is one of the countries located
in this Ring of Fire.
The 2011
Tohoku earthquake occurred in a subduction zone known as the Japan Trench. This
earthquake was caused when the thicker North American plate converged with the
thinner Pacific plate, forcing the thinner plate down into the Earth’s asthenosphere.
The two plates were rough and bonded together, building up large quantities of
energy. The pressure created by the collision of the two plates forced the rock
along the Japan Trench to break rapidly, releasing the energy and creating seismic
waves that caused the ground to shake. The area underground where the rock broke
is the focus of the earthquake. The focus of the Tohoku earthquake was at a
relatively shallow distance of 32 kilometres underground. The shallower the
focus, the higher the impact on the Earth’s surface.
Figure 6: Seismic Waves
Figure 6: Seismic Waves