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Iowa State University - Geology terms Category: Education > University Date & country: 18/11/2013, USA Words: 782

granitization
A metamorphic process by which solid rock is converted into granite by the addition or removal of material, without passing through a magmatic stage. compare metasomatism .

gravitational heating
Planetary heating caused by the conversion of potential energy into heat. Associated with the iron catastrophe .

gravitational moisture
Water in the zone of aeration that is moving down toward the zone of saturation.

graywacke
() ) A variety of sandstone characterized by angular-shaped grains of quartz and feldspar, and small fragments of dark rock all

greenhouse gase
s Gases (primarily water and carbon dioxide, but also a variety of sulfur and nitrogen compounds and gaseous hydrocarbons) that trap the Sun

greenstone
An altered or metamorphosed mafic igneous rock that owes its dark color to the presence of chlorite, epidote, or amphiboles.

greenstone belt
A region of greenstones, one of two characteristic regions within cratons .

groin
A wall built out from the shore, usually at perpendicular to it to trap sand carried by longshore currents .

groove
A broad, deep, generally straight furrow carved in bed rock by the abrasive action of debris embedded in a moving glacier. Larger and deeper than a glacial striation.

ground moraine
Till deposited from main body of glacier during ablation.

ground water
Water beneath the Earth

ground water table
see water table .

guyot
see seamount

habit
A general term for the outward appearance of a mineral, defined by the relative sizes and arrangement of characteristic crystal faces.

Hadean
The oldest eon in Earth history, extending from the origin of the Earth to about 3.9 billion years ago.

half-life
The amount of time that it takes for one half of an original population of atoms of a radioactive isotope to decay.

hanging valley
A valley whose mouth is high above the floor of the main valley to which it is tributary. Usually, but not always, the result of mountain glaciation.

hanging wall block
The body of rock that lies above an inclined fault plane. compare foot wall block

hardness
Resistance of a mineral to scratching, determined on a comparative basis by the Mohs scale .

hardpan
A general term for a relatively hard layer of soil at or just below the ground surface, cemented by silica, iron oxide, calcium carbonate, or organic matter. compare caliche , claypan, fragipan.

head
(hydraulic head) The level to which ground water in the zone of saturation will rise.

heat flow
The amount of thermal energy leaving the Earth per cm2/sec.

heave
In mass movement the upward motion of material by expansion as, for example, the heaving caused by freezing water.

hiatus
A gap or interruption in the continuity of the geologic record either because the record was never formed or because it was destroyed by erosion. It represents the time interval spanned by an unconformity .

high level nuclear waste
Radioactive waste from defense activities of the U. S. government and from spent fuel rods from nuclear reactors.

hinge fault
A fault along which there is increasing offset or separation along the strike of the fault plane, from an initial point of no separation.

hoodoo
A column or pillar of rock produced by differential weathering in a region of sporadic heavy rainfall, commonly facilitated by joints and by rock layers of varying hardness.

Hooke
A statement of elastic deformation, that strain is directly proportional to stress.

horn
The sharp spire of rock formed as glaciers in several cirques erode into a central mountain peak.

hot spot
A region of high heat flow on the Earth

humus
The generally dark, more or less stable part of the organic matter in a soil, so well decomposed that the original sources cannot be identified.

hydraulic conductivity
Measure of permeability in Earth materials.

hydraulic gradient
The slope of the water table. Measured by the difference in elevation between two points on the slope of the water table and the distance of flow between them.

hydraulic head see head
.

hydrograph
Graph of variation of stream flow over time.

hydrologic system
(or hydrologic cycle) The pattern of water circulation from the ocean to the atmosphere to the land and back to the ocean.

hydrolysis
A decomposition reaction involving water, in which hydrogen ions (H+) or hydroxyl ions (OH-) replace other ions. The result is a new residual mineral. Example: the addition of water to orthoclase produces kaolinite and releases K+ and silica into solution.

ice sheet
A broad, mound-like mass of glacier ice that usually spreads radially outward from a central zone.

ice shelf
A floating ice sheet extending across water from a land-based glacier.

icecap
A small ice sheet.

igneous rock
A rock that has crystallized from a molten state.

inclined bedding
(cross-bedding) Bedding laid down at an angle to the horizontal, as in many sand dunes.

inclined fold
A fold whose axial plane is inclined from the vertical, but in which the steeper of the two limbs is not overturned. compare overturned fold.

inclusion
(xenolith) A fragment of older rock caught up in an igneous rock.

index fossil
A fossil that identifies and dates the strata in which it is typically found. To be most useful, an index fossil must have broad, even worldwide distribution and must be restricted to a narrow stratigraphic range.

index minera
l A mineral formed under a particular set of temperature and pressure conditions, thus characterizing a particular degree of metamorphism.

inertia
The tendency of a body to resist acceleration . A moving body tends to keep moving at a constant speed in the same direction, and a stationary body tends to remain in one place, unless acted upon by an outside force.

inner core
The solid innermost part of the core with a diameter of a little over 1,200 km.

intensity
A measure of the size of an earthquake in terms of the damage it causes.

interlobate moraine
Ridge formed along junction of adjacent glacier lobes.

intrusive
Pertaining to igneous rocks or features formed by the emplacement of magma in pre-existing rock.

ion
An atom that has an electrical charge, by virtue of having gained or lost electrons. see cation, anion

ionic radius
The effective distance from the center of an ion to the edge of its electron cloud.

ionic substitution
The replacement of one or more ions in a crystal structure by others of similar size and electrical charge. Example: Fe2+ is interchangeable with Mg2+ in most ferromagnesian minerals.

iron catastrophe
The period in the Hadean eon during which much of the iron in outer portions of the Earth migrated toward the center of the planet, producing the core and releasing large amounts of gravitational heat.

ironpan
A hardpan in which iron oxides are the primary cementing agents.

island arc
A curved belt of volcanic islands lying above a subduction zone. compare continental arc.

isochemical reaction
A reaction in which chemical constituents of a rock are rearranged to form a new mineral assemblage, but no material is added to or lost from the rock as a whole. Applied generally to diagenetic or metamorphic environments.

isoclinal fold
A fold in which the limbs are parallel.

isograd
A line on a map joining points at which metamorphism took place under similar temperature and pressure conditions, as indicated by rocks belonging to the same metamorphic facies . Generally, the line separates two adjacent metamorphic zones, as indicated by specific .

isoseismal line
A line on a map joining points of equal earthquake intensity.

isostasy
The condition of equilibrium, comparable to floating, of units of the lithosphere above the asthenosphere .

isostatic change in sea level
A sea level change due to change in load on Earth

isotope
Atoms that differ in atomic mass number , but not in atomic number , are called isotopes. For example, oxygen (atomic number 8) may have an atomic mass number of 16, 17, or 18, depending on whether it has 8, 9, or 10 neutrons. It therefore has three isotopes.

jasper
A red variety of chert , its color coming from minute particles of included hematite.

jet flow
Flow in which fluid moves at high speed in jet-like surges as does water in free fall over a falls.

jetty
Similar to a groin but built to keep sand out of a harbor entrance.

joint
A surface of fracture in a rock, without displacement parallel to the fracture.

juvenile hydrothermal fluid
A hot fluid, largely water, presumed to have been released from a magma.

kame
Stratified drift deposited in depressions and cavities in stagnant ice and left as irregular, steep sided hills when the ice is melts.

kame terrace
Stratified drift deposited between wasting glacier and adjacent valley wall. Stands as a terrace when glacier melts.

karst
A landscape that develops from the action of ground water in areas of easily soluble rocks. Characterized by caves, underground drainage and sinkholes.

kettle
Depression in ground surface formed by the melting of a block of glacier ice buried or partially buried by drift.

komatiite
An ultramafic rock with a non-cumulate texture, presumed to be extrusive.

laccolith
A concordant igneous intrusion with a flat floor and a convex upper surface, usually less than 8 km across and from a few meters to a few hundred meters thick at its thickest point.

lag time
The delay in the response of stream flow between precipitation and flood peak.

lahar
A mudflow composed chiefly of pyroclastic material on the flanks of a volcano.

laminar flow
Fluid flow in which flow lines are distinct, and parallel and do not mix. compare turbulent flow .

lateral continuity
The extent of a rock unit over a considerable but definite area.

lateral moraine
Moraine formed by valley glaciers along valley sides.

laterite
A highly weathered red soil rich in iron and aluminum oxides. Typically formed in a tropical to temperate climate where intense chemical weathering is common.

Laurasia
The northern portion of the late Paleozoic supercontinent called Pangea.

lava
Molten rock that flows at the Earth

lava dome
A steep-sided rounded extrusion of highly viscous lava squeezed out from a volcano and forming a dome-shaped or bulbous mass above and around the volcanic vent. The structure generally develops inside a volcanic crater.

lava flood
(plateau basalt) A term applied to large areas of basaltic lava presumably extruded from fissures.

lava lake
A lake of lava, usually basaltic, in a volcanic caldera.

layered complex
An intrusive igneous body in which there are layers of varying mineral content.

levees
Banks of sand and silt along stream bank built by deposition in small increments during successive floods.

limestone
A sedimentary rock composed mostly of the mineral calcite, CaCO3.

linear dune
Long, straight dune with slip faces on each side.

lineation
A general term applying to any linear feature in a metamorphic rock.

liquefaction
The transformation of a soil from a solid to a liquid state as the result of increased pore pressure.

lithic sandstone
see graywacke.

lithification
The process by which an unconsolidated deposit of sediments is converted in to solid rock. Compaction, cementation and recrystallization are involved.

lithophile
Said of an element that has a greater chemical affinity for silicate rocks than for sulfides or for a metallic state. Example: Aluminum.

lithosphere
The rigid outer shell of the Earth. It includes the crust and uppermost mantle and is on the order of 100 km in thickness.

lithostatic stress
The confining (non-directed) pressure imposed by the weight of overlying rock.

littoral current
see longshore current.

load
Of a stream, the amount that it carries at any one time.

loess
Deposits of wind-borne dust.