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series: the rock layers that correspond to an epoch of time.
shadow zone: the region of the earth that experiences no shocks after an earthquake.
shaft: a vertical tunnel that provides access or ventilation to a mine.
shale: a fine-grained sedimentary rock made of clay minerals with particle sizes smaller than 2 microns.
shield: the ancient and stable core of a tectonic plate. Also called a continental shield.
shield volcano: a volcano with a broad, low- angled cone made entirely of lava.
silica, silicate: silica is silicon dioxide. It is a very common mineral, occurring as quartz, chalcedony, etc. A silicate is any mineral that contains silica.
sill: a tabular, sheet-like body of intrusive igneous rock that has been injected between layers of sedimentary or metamorphic rock.
Silurian, Silurian Period: the name of the third geological period of the Palaeozoic Era. It began about 430 and ended about 395 million years ago.
skarn: a mineral deposit formed by the chemical reaction of hot acidic fluids and carbonate rocks.
slag: waste rock material that becomes separated from the metal during smelting.
slate: a low-grade metamorphic rock produced by pressure, in which the clay minerals have arranged themselves parallel to one another.
slaty cleavage: a characteristic pattern found in slates in which the parallel arrangement
of clay minerals causes the rock to fracture (cleave) in sheets.
species: a population of animals or plants capable of interbreeding.
spreading boundary: a line where two plates are being pulled away from each other. New crust is formed as molten rock is forced upwards into the gap.
stock: a vertical protrusion of a batholith that pushes up closer to the surface.
stratigraphy: the study of the earth’s rocks in the context of their history and conditions of formation.
stratovolcano: a tall volcanic mountain made of alternating layers, or strata, of ash and lava.
stratum: (pl. strata) a layer of sedimentary rock.
streak: the colour of the powder of a mineral produced by rubbing the mineral against a piece of unglazed, white porcelain. Used as a test when identifying minerals.
striation: minute parallel grooves on crystal faces.
strike, direction of: the direction of a bedding plane or fault at right angles to the dip.
Strombolian-type eruption: a kind of volcanic eruption that is explosive enough to send out some volcanic bombs.
subduction: the process of one tectonic plate descending beneath another.
subduction zone: the part of the earth’s surface along which one tectonic plate descends into the mantle. It is often shaped in the form of an number of arcs.
sulphides: a group of important ore minerals (e.g. pyrite, galena, and sphalerite) in which sulphur combines with one or more metals.
surface wave: any one of a number of waves such as Love waves or Rayleigh waves that shake the ground surface just after an earthquake. See also Love waves and Rayleigh waves.
suture: the junction of 2 or more parts of a skeleton; in cephalopods the junction of a septum with the inner surface of the shell wall. It is very distinctive in ammonoids and used to identify them.
S wave, shear or secondary seismic wave:
this kind of wave carries energy through the earth like a rope being shaken. S waves cannot travel through the outer core of the earth because they cannot pass through fluids. See also P wave.
syncline: a downfold of rock layers in which the rocks slope up from the bottom of the fold. See also anticline.
system: see geological system. tableland: another word for a plateau.
See plateau.
tectonic plate: one of the great slabs, or plates, of the lithosphere (the earth’s crust and part of the earth’s upper mantle) that covers the whole of the earth’s surface. The earth’s plates are separated by zones of volcanic and earthquake activity.
Tertiary, Tertiary Period: the first period of the Cenozoic Era. It began 665 and ended about 1.6 million years ago.
thrust fault: see reversed fault.
transcurrent fault: see lateral fault.
transform fault: see lateral fault.
translucent: a description of a mineral that allows light to penetrate but not pass through.
transparent: a description of a mineral that allows light to pass right through.
trellis drainage pattern: a river drainage system where the trunk river and its tributaries tend to meet at right angles.
trench: see ocean trench.
Triassic, Triassic Period: the first period
of the Mesozoic era. It lasted from about 225 to 190 million years ago.
triclinic: a crystal system in which crystals have 3 axes, none at right angles or of equal length to one another.
tsunami: a very large wave produced by an underwater earthquake.
tuff: a rock made from volcanic ash. unconformity: any interruption in the
depositional sequence of sedimentary rocks.
valve: in bivalves and brachiopods, one of the separate parts of the shell.
vein: a sheet-like body of mineral matter (e.g. quartz) that cuts across a rock. Veins are often important sources of valuable minerals. Miners call such important veins lodes.
vent: the vertical pipe that allows the passage of magma through the centre of a volcano.
vertebrate: an animal with an internal skeleton.
vesicle: a small cavity in a volcanic rock originally created by an air bubble trapped in the molten lava.
viscous, viscosity: sticky, stickiness. volatile: substances that tend to evaporate or
boil off of a liquid.
volcanic: anything from, or of, a volcano. Volcanic rocks are igneous rocks that cool as they are released at the earth’s surface – including those formed underwater; typically have small crystals due to the rapid cooling, e.g. basalt, andesite and rhyolite.
volcanic bomb: a large piece of magma thrown out of a crater during an eruption, which solidifies as it travels through cool air.
volcanic eruption: an ejection of ash or lava from a volcano.
volcanic glass: lava that has solidified very quickly and has not had time to develop any crystals. Obsidian is a volcanic glass.
volcanic plug: the solidified core of an extinct volcano.
Vulcanian-type eruption: an explosive form of eruption without a tall ash column or pyroclastic flow.
water gap: a gap cut by a superimposed river, which is still occupied by the river.
weather, weathered, weathering: the process of weathering is the mechanical action of ice and the chemical action of rainwater on rock, breaking it down into small pieces that can then be carried away. See also chemical weathering and mechanical weathering.
wind gap: a gap cut by a superimposed river, which is no longer occupied by the river.
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