active volcanoes: volcanoes that are presently erupting or have recently erupted asthenosphere: the hot, semifluid upper part of the Earth’s mantle batholith: intrusive rock mass that measures more than 100 kilometres across composite volcanoes: volcanoes that alternate in quiet lava flows and explosive eruptions. Thick lava blocks the vent of the volcano, building up pressure until an explosive eruption deposits rock fragments and lava around the vent. This builds a cone with steep sides that weathers slowly. Most volcanoes that are active today are of this type. continental shelf: the submerged edge of a continent that slopes very gently until dropping to the sea floor crust: the outer layer of the Earth, made mostly of solid rock dormant volcanoes: volcanoes that have not erupted in the last 20 years epicentre: the point on the Earth’s surface directly above the original disturbance inside the Earth’s crust erosion: the process of moving weathered rock or soil from one place to another extinct volcanoes: no longer active. Extinct volcanoes have not erupted for thousands of years and show no sign of future eruption. extrusive rock: igneous rock that forms when lava cools above the Earth’s surface igneous: rock that forms when magma or lava cools to a solid inner core: the solid centre of the Earth intrusive rock: igneous rock that forms when magma cools below the Earth’s surface lava: a mixture of molten rock and gases from a volcano that has reached the surface lithosphere: the rocks and soil of the outer layer of the Earth magma: a very hot mixture of molten rock and gases just below the Earth’s surface that has come from the mantle layer below the Earth’s outer crust. mantle: the thick layer inside the Earth, below the crust. Most of the mantle is solid rock, although the upper part is molten rock called magma. ocean ridges: long mountain ranges of solidified lava that flowed from cracks in the Earth’s crust on the sea floor. The mountains spread and form new crust. ocean trench: a long, narrow opening in the ocean floor outer core: the liquid layer surrounding the solid inner core of the Earth Richter scale: a scale used to measure the strength of earthquakes seismograph: an instrument used to record the time and strength of earthquake tremors shield volcanoes: the lava flows easily across the Earth’s surface and forms a low gently sloping mound. The mound, called the cone, can grow very large, for example, Mauna Loa, an island in Hawaii. stratosphere: the layer of the atmosphere above the troposphere. The upper stratosphere contains the ozone that shields the Earth from much of the dangerous ultraviolet radiation from the sun. submarine volcano: a volcano that rises from the ocean floor tremors: movements inside the Earth’s crust felt on the surface as vibrations tsunamis: large, often destructive sea waves caused by an underwater earthquake volcanic bomb: a large rock fragment that is blown out of erupting volcanoes volcanoes: natural openings in the Earth’s crust connected to areas of molten rock deep inside the crust. The heat is thought to be due to radioactivity of rocks in the Earth’s interior.
