Geology Topics

Aa
A Hawaiian term for lava that has a rough, jagged, spiny, and generally clinkery surface.
Abbe Refractometer
An instrument for measuring directly the refractive index of liquids, minerals and gemstones.
Absolute Age
The geological age of a fossil, mineral, rock or event, generally given in years.
Abyssal Plain
Large area of extremely flat ocean floor lying near a continent and generally over 4000m deep.
Aclinic Line
The line through those points on the earth′s surface at which magnetic dip is zero.
Active Volcano
A volcano that is erupting.
Adhesive Tape
A tape with a sticky substance on one side.
Andesite
Volcanic rock characteristically medium dark in colour and containing 54 to 62 percent silica and moderate amounts of iron and magnesium.
Angle of Repose
The maximum angle at which loose material will come to rest when added to a pile of similar material.
Aquifer
A permeable region of rock or soil through which ground water can move.
Aquitard
A material of low permeability that greatly slows the movement of ground water.
Archaean
The geological eon covering about 3.9 billion years ago to 2.6 billion years ago.
Archaeozic
The geological period from 4.6 billion to 1.0 billion years ago.
Argentite
Naturally occuring Silver Sulphide. An important ore of silver.
Artesian Well
A well in which the water in the aquifer is under pressure that raises the water above the point that the well first encounters it.
Ash Flow
A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure.
Asthenosphere
The shell within the earth, some tens of kilometres below the surface and of undefined thickness.
Astronomic Theory of Glaciation
Theory based on the changing position of the Earth in its orbit around the Sun.
Barite
Most commonly found mineral of Barium Sulphate.
Basalt
Dark colored rocks formed by the rapid cooling and hardening of lava flows.
Beach
Temporary accumulations of sediments that collect between low and high tide marks.
Bedrock
A general term for solid rock that lies beneath soil, sediments, or other unconsolidated material.
Bischofite
Natural occuring mineral of Magnesium Chloride.
Brass Lump
The name miner′s would give to a large lump of fools gold.
Buddling Dish
A flat pan or vat used in washing ores.
Caldera
A large volcanic collapse depression, commonly circular or elliptical when seen from above.
Cambrian
The geological period from 570 million to 510 million years ago.
Carboniferous
The geological period from 362.5 million to 290 million years ago.
Cassiterite
Dark coloured mineral that is an important ore of Tin.
Cenozoic
The geological era covering about 65 million years ago to present day.
Central Volcano
A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.
Cinder Cone
The simplest type of volcano, built from particles and blobs of congealed lava ejected from a single vent.
Clay
Finely suspended earth mineral sometimes found as an impurity in water.
Composite Volcano
A steep volcanic cone built by both lava flows and airborne rocks and ash.
Compund Volcano
A volcano that consists of a complex of two or more vents.
Continental Rise
The portion of the continental margin that lies between the abyssal plain and the continental slope.
Continental Shelf
The portion of the continental margin that extends as a gently sloping surface from the shoreline seaward to a marked change in slope at the top of the continental slope.
Continental Slope
That part of the continental margin that lies between the continental shelf and the continental rise.
Cretaceous
The geological period from 144.2 million to 65 million years ago.
Dacite
Volcanic rock that characteristically is light in colour and contains 62% to 69% silica and moderate a mounts of sodium and potassium.
Devonian
The geological period from 408 million to 360 million years ago.
Dip Equator
Another name for Aclinic Line or Magnetic Equator.
Dormant Volcano
The term is used to describe a volcano which is presently inactive but which may erupt again.
Drainage Basin
The area of land drained by a river system.
Earthquake
The result of a sudden release of energy in the Earth′s crust that creates seismic waves.
Echo Sounding
Measurement of the depth of the ocean by directing a sonic or ultrasonic pressure wave vertically downward and determining the time taken before the echo is received.
Elutriation
The process of separating the finer particles of an earthy substance from the coarser and heavier by washing with water.
Eocene
The geological period covering about 56.5 million years ago to 35.5 million years ago.
Extinct Volcano
A volcano that is not presently erupting and is not likely to do so for a very long time in the future.
Fools Gold
Name given to Iron II Sulphide due to its colour and texture.
Foreshore
Lies between low and high tide marks.
Galena
A Lead Sulphide and the most common ore of lead.
Geochemistry
The study of materials and chemical reactions in rocks, minerals, magma, seawater, and soil.
Geothermal Energy
Energy made by using underground steam; the steam produces electricity.
Geothermal Gradient
The rate at which temperature increases with depth below the surface.
Geyser
Boiling hot water that is violently ejected from underground when some of the water is converted to steam.
Gibbsite
White crystalline mineral consisting of aluminum hydroxide; a constituent of bauxite and a source of alumina.
Glacier
A mass of ice, formed by the recrystallization of snow, that flows forward, or has flowed at some time in the past.
Global Warming
The prediction that climate will warm as a result of the addition to the atmosphere of humanly produced greenhouse gases.
Hadean
The geological eon covering about 4.6 billion years ago to 3.9 billion years ago.
Haematite
A common mineral; a form of iron oxide with the formula Fe2O3.
Hot Spot
A region of high heat flow.
Hot-Spot Volcanoes
Volcanoes related to a persistent heat source in the mantle.
Hydrosphere
All the water on the surface of a planet.
Hydrothermal
Pertains to hot water or the action of heated water, often considered heated by magma or in association with magma.
Hydrothermal Alteration
Alteration of rocks or minerals by the reaction of hot water with pre-existing rocks.
Ice Age
Aperiod characterised by a long term reduction in the temperature of the Earth′s climate.
Igneous Rocks
Rocks that are formed from melted rock that has cooled and solidified.
Interglacials
The warm periods between ice age glaciations.
Jurassic
The geological period from 208 million to 146 million years ago.
Lava
Hot liquid rock that flows out of a volcanic vent.
Leaching
the process by which soluble materials in the soil, such as salts and nutrients, are washed into a lower layer of soil or are dissolved and carried away by water.
Maar
A volcanic crater that is produced by an explosion in an area of low relief.
Magma
The melted rock produced inside the earth. When a volcano erupts, magma comes to the surface and is called lava.
Magma Chamber
An area filled with magma that lies beneath a volcano.
Magnetic Declination
The angle between magnetic north and true north at a given point on the Earth′s surface.
Magnetic Equator
The line through those points on the earth′s surface at which magnetic dip is zero.
Mercalli Scale
A measure of the intensity of an earthquake. This is a subjective value that varies depending on where the earthquake is observed.
Mesosphere
A zone in the Earth between 400 and 670 km below the surface separating the upper mantle from the lower mantle.
Mesozoic
The geological era covering about 245 million years ago to 65 million years ago.
Meteor
A meteoroid that has entered Earth′s atmosphere.
Meteorite
The remnants of a meteor after it has actually impacted the Earth.
Mineral
A naturally occurring inorganic substance having specified chemical composition and crystalline structure.
Miocene
The geological period covering about 23.5 million years ago to 5.2 million years ago.
Mohs
A hardness scale developed by F. Mohs in 1812 where the hardness of a material is compared with that of a mineral.
Monogenetic
A volcano built by a single eruption.
Niningerite
Rare naturally occuring mineral of Magnesium Sulphide.
Nitromagnesite
Naturally occuring Magnesium Nitrate.
Obsidian
A black or dark-coloured volcanic glass, usually composed of rhyolite.
Oligocene
The geological period covering about 35.5 million years ago to 23.5 million years ago.
Ordovician
The geological period from 510 million to 439 million years ago.
Palaeocene
The geological period covering about 65 million years ago to 56.5 million years ago.
Paleozoic
The geological era covering about 570 million years ago to 245 million years ago.
Permian
The geological period from 290 million to 245 million years ago.
Phanerozoic
The geological eon covering about 570 million years ago to present day.
Phreatic Eruption
An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks.
Phreatomagnetic
An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.
Pillow Lava
Interconnected, sack-like bodies of lava formed underwater.
Pitchblende
The most important Uranium ore.
Plate Techtonics
Motion of the plates that make up the surface of the Earth.
Pleistocene
The geological epoch covering about 1.64 million years ago to 10000 years ago.
Plinian Eruption
An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent.
Pliocene
The geological period covering about 5.2 million years ago to 1.64 million years ago.
Pluton
Igneous rock bodies that form at great depth.
Precambrian
The geological era covering about 4.6 billion years ago to 570 million years ago.
Proterozic
The geological period from 1.0 billion to 570 million years ago.
Proterozoic
The geological eon covering about 2.6 billion years ago to 570 million years ago.
Pumice
A light porous stone of mixed silicates.

Pyroclastic
Fragmented rock material formed by a volcanic explosion or ejection from a volcanic vent.
Pyroclastic Flow
A hot, dry, fast-moving (10-100m/s) and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions or from the collapse of a lava dome.
Pyroclastic Surge
Turbulent, low-density cloud of hot rock debris and gases that moves over the ground surface at high speed. Lower density than Pyroclastic Flow.
Quaternary
The geological period covering about 2 million years ago to the present.
Rhyolite
Volcanic rock that characteristically is light in colour, contains 69% silica or more, and is rich in potassium and sodium.
Richter Scale
A measure of the magnitude of an earthquake at its epicentre. Each point increase on the Richter scale is a 30 times increase in the magnitude.
Seismograph
An instrument for detecting and recording the intensity, direction, and duration of a movement of the ground caused by an earthquake or explosion.
Shield Volcano
A gently sloping volcano in the shape of a flattened dome and built almost exclusively of gentle lava flows.
Silurian
The geological period from 439 million to 409 million years ago.
Soil Bulk Electrical Conductivity
Conductivity of electricity through soil.
Soil Heat Flux
The amount of heat flowing into a cross-sectional area of soil per unit time.
Soil Volumetric Water Content
The proportion of a volume of soil that is water.
Spring
Occurs at the intersection of the water table with the ground surface.
Stalactite
A cone shaped deposit of minerals hanging from the roof of a cavern.
Stalagmite
A post of dripstone growing up from a cave floor.
Stratovolcano
A steep volcanic cone built by both lava flows and airborne rocks and ash.
Strombolian Eruption
A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater.
Sylvine
Naturally occuring potassium chloride.
Tensile Fracture
A fracture caused by tensional stress.
Tephra
Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.
Tertiary
The geological period from 65 million to 1.64 million years ago.
Thermal Spring
A spring whose temperature is 6.5°C or more above mean annual air temperature.
Triassic
The geological period from 245 million to 208 million years ago.
True Meridian
A great circle through the geographical poles.
Tsunami
A giant wave set in motion by a large earthquake under or near the ocean that causes the ocean floor to shift vertically.
Volcanic Ash
Lava from a volcano that is in the form of chunks smaller than an apple, about the size of marbles.
Volcanic Bomb
A large lump of lava thrown out of a volcano during an eruption.
Volcanic Dust
Lava in the form of small, tiny particles thrown from a volcano during an eruption.
Volcanoes
Vents or fissures in the earth's crust through which gases, molten rock, or lava , and solid fragments are discharged.
Wentworth-Udden Particle Scale
A common system of grading the size of grains in a rock or sediment.

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