C1bL13 Earthquakes and Volcanoes

Key Words

Continental Plate - a tectonic plate on which land is found.

Earthquake - a sudden release of energy that comes about when rocks shift.

Hot spot - a place where molten rock comes through to the Earth's Surface.

Magma - hot molten rock.

Oceanic plate - tectonic plate mostly under the ocean.

Plate boundaries - where plates meet.

Subduction zone - where an oceanic plate is being forced under a continental plate.

Tsunami - a massive wave caused by an undersea earthquake.

Vent - where magma comes out of a volcano.

Volcano - where magma erupts from the Earth to form a mountain consisting of ash and rocks.

Grade E

The tectonic plates are in constant motion, driven by heat from the middle of the Earth.  This comes about through radioactive processes.  The plates are as in the picture below:

Gary Hincks (Science Photo Library)

When plates come together:

• Oceanic plates slide under the thicker continental plates;

• Continental plates crumple to form mountains;

• Plates can slide along each other.

Earthquakes happen mostly (but not always) at plate boundaries.  Underwater earthquakes can result in highly destructive tsunamis.  Volcanoes are also found at plate boundaries.

Grade C

Volcanoes are found by plate boundaries.  Magma is forced up through the Earth's surface from an underground chamber.

Wesley Bocxe

Pressure builds up in the underground magma chamber, until the magma breaks though in an eruption.  A plume of hot rocks and ash rises above the vent.  Notice the typical cone shape of the mountain.  Occasionally the volcano collapses to form a large crater called a caldera.

Gary Hincks (Science Photo Library)

Sometimes the plates can be pushed apart to allow molten rock to come to the surface at a hot spot.  Volcanic islands are made.  As the plates move, the volcanoes become inactive, then extinct.  There are chains of islands where this has happened.

A subduction zone is formed when a dense oceanic plate is forced under a less dense continental plate.  Violent volcanic eruptions can occur.

Grade A

 Subduction zones occur when dense oceanic plates are forced under less dense continental plates that ride over the top. 

Jon Lomberg

At subduction zones there are strong earthquakes.  Water from the oceans is taken down, and combined with magma, causes dangerously explosive volcanic eruptions.  In 1883 Krakatau in Java exploded.  The mountain was destroyed.  21 km³ of rock was ejected from the volcano, and the explosion was considered to be equivalent to a 200 megatonne nuclear bomb.  The explosion was heard 5000 km away.  Some archaeologists think that an earlier eruption of Krakatau led to the dark ages.