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lateral fault: see thrust fault.
laterite: a surface deposit containing a high
proportion of iron.
lava: molten rock material extruded onto the surface of the earth.
lava bomb: see volcanic bomb.
law of superposition: the principle that
younger rock is deposited on older.
limestone: a carbonate sedimentary rock composed of more than half calcium carbonate.
lithosphere: that part of the crust and upper mantle which is brittle and makes up the tectonic plates.
lode: a mining term for a rock containing many rich ore-bearing minerals. Similar to vein.
Love wave, L wave: a major type of surface earthquake wave that shakes the ground surface at right angles to the direction in which the wave is travelling. It is named after A.E.H. Love, the English mathematician who discovered it.
lustre: the way in which a mineral reflects light. Used as a test when identifying minerals.
magma: the molten material that comes from the mantle and which cools to form igneous rocks.
magma chamber: a large cavity melted in the earth’s crust and filled with magma. Many magma chambers are plumes of magma that have melted their way from the mantle to
the upper part of the crust. When a magma chamber is no longer supplied with molten magma, the magma solidifies to form a granite batholith.
mantle: the layer of the earth between the crust and the core. It is approximately 2900 kilometres thick and is the largest of the earth’s major layers.
marginal accretion: the growth of mountain belts on the edges of a shield.
mass extinction: a time when the majority of species on the planet were killed off.
matrix: the rock or sediment in which a fossil is embedded; the fine-grained rock in which larger particles are embedded, for example, in a conglomerate.
mechanical weathering: the disintegration of a rock by frost shattering/ice wedging.
mesa: a large detached piece of a tableland.
Mesozoic, Mesozoic Era: the geological era between the Palaeozoic and the Cenozoic eras. It lasted from about 225 to 65 million years ago.
metamorphic aureole: the region of contact metamorphic rock that surrounds a batholith.
metamorphic rock: any rock (e.g. schist, gneiss) that was formed from a pre-existing rock by heat and pressure.
meteorite: a substantial chunk of rock in space.
micas: a group of soft, sheet-like silicate minerals (e.g. biotite, muscovite).
mid-ocean ridge: a long mountain chain on the ocean floor where basalt periodically erupts, forming new oceanic crust.
mineral: a naturally occurring inorganic substance of definite chemical composition (e.g. calcite, calcium carbonate).
More generally, any resource extracted from the ground by mining (includes metal ores, coal, oil, gas, rocks, etc.).
mineral environment: the place where a mineral or a group of associated minerals form. Mineral environments include igneous, sedimentary, and metamorphic rocks.
mineralisation: the formation of minerals within a rock.
Modified Mercalli Scale: a scale for measuring the impact of an earthquake. It is composed of 12 increasing levels of intensity, which range from imperceptible, designated by Roman numeral I, to catastrophic destruction, designated by XII.
Mohorovicic discontinuity: the boundary surface that separates the earth’s crust from the underlying mantle. Named after Andrija Mohorovicic, a Croatian seismologist.
Mohs’ Scale of Hardness: a relative scale developed to put minerals into an order. The hardest is 10 (diamond), and the softest is 1 (talc).
monoclinic: a crystal system in which crystals have 2 axes all at right angles to one another, and each axis is of unequal length.
mould: an impression in a rock of the outside of an organism.
mountain belt: a region where there are many ranges of mountains. The term is often applied to a wide belt of mountains produced during mountain building.
mountain building: the creation of mountains as a result of the collision of tectonic plates. Long belts or chains of mountains can form along the edge of a continent during this process. Mountain building is also called orogeny.
mountain building period: a period during which a geosyncline is compressed into fold mountains by the collision of two tectonic plates. Also known as orogenesis.
mudstone: a fine-grained, massive rock formed by the compaction of mud.
nappe: a piece of a fold that has become detached from its roots during intensive mountain building.
native metal: a metal that occurs uncombined with any other element.
natural gas: see petroleum.
normal fault: a fault in which one block has slipped down the face of another. It is the most common kind of fault and results from tension.
nueé ardente: another word for pyroclastic flow.
ocean trench: a deep, steep-sided trough in the ocean floor caused by the subduction of oceanic crust beneath either other oceanic crust or continental crust.
olivine: the name of a group of magnesium iron silicate minerals that have an olive colour.
order: a level of biological classification between class and family.
Ordovician, Ordovician Period: the second period of geological time within the Palaeozoic Era. It lasted from about 500 to 430 million years ago.
ore: a rock containing enough useful metal or fuel to be worth mining.
ore mineral: a mineral that occurs in sufficient quantity to be mined for its metal. The compound must also be easy to process.
organic rocks: rocks formed by living things, for example, coal.
orthoclase: the form of feldspar that is often pink in colour and which contains potassium as important ions.
orogenic belt: a mountain belt. orogeny: a period of mountain building.
Orogenesis is the process of mountain building and the creation of orogenic belts.
orthorhombic: a crystal system in which crystals have 3 axes all at right angles to one another but of unequal length.
outcrop: the exposure of a rock at the surface of the earth.
overburden: the unwanted layer(s) of rock above an ore or coal body.
oxide minerals: a group of minerals in which oxygen is a major constituent. A compound in which oxygen is bonded to another element or group.
Pacific Ring of Fire: the ring of volcanoes and volcanic activity that circles the Pacific Ocean. Created by the collision of the Pacific Plate with its neighbouring plates.
pahoehoe lava: the name for a form of lava that has a smooth surface.
Palaeozoic, Palaeozoic Era: a major interval of geological time. The Palaeozoic is the oldest era in which fossil life is commonly found. It lasted from about 570 to 225 million years ago.
palaeomagnetism: the natural magnetic traces that reveal the intensity and direction of the earth’s magnetic field in the geological past.
pegmatite: an igneous rock (e.g. a dyke) of extremely coarse crystals.
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