Glossary terms
Plate tectonics and associated hazards
Aftershock – A smaller earthquake that occurs after a previous large
earthquake in the same area.
Ash – Dust-sized particles of rock produced by the explosive
eruption of some volcanoes. This material may be carried in the air for long
distances from the volcano which formed it.
Asthenosphere – Part of the earth’s mantle that lies below the lithosphere,
at depths between about 100 and 350km. The rock here is relatively soft because
of its high temperature and relatively low pressure. This enables it to move in
a plastic fashion. It is the layer upon which the tectonic plates move.
Batholith – A very large mass of igneous intrusive rock (often granite)
that form from cooled magma deep in the earth’s crust.
Composite volcano – large, steep-sided symmetrical cone-shaped volcano formed
from alternating layers of lava flows, volcanic ash, cinders, blocks and bombs.
Conservative plate margin – a type of plate margin where two
tectonic plates are moving past one another with no addition or destruction of
plate material
Constructive plate margin – a type of plate margin where new crust
is generated as the plates pull away from each other. These are found at
mid-oceanic ridges.
Continental crust – this is made of old, low density rocks such as granite. It is
generally 35-70km thick and mostly over 1500 million years old.
Continental drift – a hypothesis, proposed by Alfred Wegener, that today’s
continents are the result of the break-up of a single supercontinent. The
fragments then drifted to their present positions.
Convection current – Currents in the mantle that are driving force in the movement
of the tectonic plates. It is thought that they are initiated by hotspots deep
in the mantle.
Core – the innermost layers of the earth. The core comprises of two
concentric spheres. The inner core is believed to be made of solid iron and the
outer core, liquid iron.
Crater – a circular depression in the ground caused by volcanic
activity.
Crust – the outermost layer of the earth.
Destructive plate margin – A type of plate margin where crust is
destroyed as two plates converge. These are usually associated with island arcs
or young fold mountains.
Dormant – a description of the state of a volcano between eruptions
when it gives out very little gas and lava.
Dykes – steep, sheet-like intrusions, varying in thickness from a few
millimetres to tens of metres across. They occupy vertical weaknesses in the
rock. They often cut across rock bedding and form low ridges.
Epicentre – the point on the earth’s surface directly above the focus of
an earthquake.
Eruption – this is said to when a volcano gives off large quantities of
lava and gas.
Extinct – A description of a volcano that has not erupted for at least
25,000 years.
Extrusive activity -
Volcanic activity that results from magma reaching the surface.
Fault – a fracture in the earth’s crust that marks the point where
two adjacent masses of rock are moving in different directions
Focus – the point below the surface where an earthquake occurs
Geyser – a type of hot spring that erupts periodically, throwing a
column of hot water and steam into the air.
Hazard – the potential threat to humans from a naturally occurring
process or event.
Hot spring – a point where heated groundwater emerges onto the earth’s
surface.
Hotspot – an area deep within the mantle where the temperatures are
high enough to initiate convection. They are associated with spreading ridges
and isolated chains of volcanic islands found away from plate boundaries.
Igneous rock – A rock formed from the cooling magma
Intrusive activity – igneous activity that results from the movement of magma
within the crust
Island arc – a destructive plate boundary where oceanic crust is subducted
beneath oceanic crust
Lahar – a mudflow composed of pyroclastic material and water that
flows down from a volcano, usually along a river valley
Lava – molten rock expelled by a volcano during an eruption
Magma – molten rock that is found beneath the surface of the earth
Mantle – the layer of the earth between the core and the crust
Moment magnitude scale – a scale used to measure the size of earthquakes in terms of
the energy released.
Oceanic crust – the type of crust that underlies the ocean basins. It is
generally between 5 km and 10 km thick, composed predominantly of basic igneous
rock.
Palaeomagnetism – a record of the history or the earth’s magnetic field,
persevered in magnetic minerals in volcanic rocks.
Plate tectonics – the theory that states that the earth’s crust is made up of
several rigid plates moving relative to one another.
Plume – a hot column of magma which rises up from deep within the
earth
Rift valley – a long, deep valley found in the centre of a spreading ridge.
It is formed between parallel faults where a block of the crust has sunk down.
Seafloor spreading – the theory that the ocean floor is moving away from the
mid-oceanic ridge and across the deep ocean basin, to disappear beneath
continents and island arcs.
Seismic waves – shock waves caused by sudden movement along a fault.
Shield volcano – a large, low angled volcano composed of layers of low
viscosity basaltic lava.
Sill – an igneous intrusion between bedding planes of sedimentary
rock layers
Subduction – the process whereby one crustal plate descends below another.
This occurs at destructive plate margins.
Surface waves – seismic waves that travel along the surface of the earth.
They include Rayleigh waves and Love waves.
Tectonic plates – a series of rigid sections of the earth’s crust. They float
on the upper mantle and move relative to one another
Tephra – any type of rock fragment that is forcibly ejected from a
volcano during an eruption
Tsunami – sea wave that can be generated by undersea earthquakes,
volcanic eruptions and landslides into the sea
Vent – an opening, or rupture, in the earth’s surface which allows
hot magma, ash and gases to escape from below the surface
Volcanic bombs – rocks that are more than 5mm in diameter that are thrown into
the air by a volcanic eruption
Volcanic explosive index (VEI) – a scale used to measure the
explosiveness of volcanoes
GEOGRAPHY HWK! (ST.MARKS)
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