Glossary
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ice age
An interval of time in which the climate was colder than it is today, glaciers occasionally advanced to cover large areas of the continents, and mountain glaciers grew; an ice age can include many glacials and interglacials.
iceberg
A large block of ice that calved off the front of a glacier and dropped into the sea.
icehouse period
A period of time when the Earth’s temperature was cooler than it is today and ice ages could occur.
ice-margin lake
A meltwater lake formed along the edge of a glacier.
ice-rafted sediment
Sediment carried out to sea by icebergs.
ice sheet
A vast glacier that covers the landscape.
ice shelf
Sea ice that surrounds and protrudes outward from land.
ice stream
A portion of a glacier that travels much more quickly than adjacent portions of the glacier.
ice tongue
The portion of a valley glacier that flowed out into the sea.
igneous rock
Rock that forms when hot molten rock (magma or lava) cools and freezes solid.
ignimbrite
Rock formed when deposits of pyroclactic flows solidify.
inactive fault
A fault that last moved in the distant past and probably won’t move again in the near future, yet is still recognizable because of displacement across the fault plane.
inactive sand
The sand along a coast that is buried beneath a layer of active sand and moves only during severe storms or not at all.
incised meander
A meander that lies at the bottom of a steep-walled canyon.
inclusions
pieces of one rock that are incorporated into another.
index minerals
Minerals that serve as good indicators of metamorphic grade.
induced seismicity
Seismic events caused by people.
industrial minerals
Minerals that serve as the raw materials for manufacturing chemicals, concrete, and wallboard, among other products.
inequant
A term for a mineral grain whose length and width are different lengths.
inertia
The tendency of an object at rest to remain at rest.
infiltrate
Seep down into.
injection well
A well in which a liquid is pumped down into the ground under pressure so that it passes from the well back into the pore space of the rock or regolith.
inner core
The inner section of the core 5,155 km deep to the Earth’s center at 6,371 km, and consisting of solid iron alloy.
inselberg
An isolated mountain or hill in a desert landscape created by progressive cliff retreat, so that the hill is surrounded by a pediment or an alluvial fan.
insolation
Exposure to the Sun’s rays.
interglacial
A period of time between two glaciations.
interference
The overlap of swells that may lead to giant waves.
interior basin
A basin with no outlet to the sea.
interlocking texture
The texture of crystalline rocks in which mineral grains fit together like pieces of a jigsaw puzzle.
internal process
A process in the Earth system, such as plate motion, mountain building, or volcanism, ultimately caused by Earth’s internal heat.
intertidal zone
The area of coastal land across which the tide rises and falls.
intertropical convergence zone
The equatorial convergence zone in the atmosphere.
intraplate basins
basins that develop in the interiors of continents where crustal stretching began but didn't get very far before it stopped.
intraplate earthquakes
Earthquakes that occur away from plate boundaries.
intrusive contact
The boundary between country rock and an intrusive igneous rock.
intrusive igneous rock
Rock formed by the freezing of magma underground.
ionosphere
The interval between 50 and 400 km distance from the Earth containing abundant positive ions.
iridium
a very heavy element found only in extraterrestrial object.
iron catastrophe
The proposed event very early in Earth history when the Earth partly melted and molten iron sank to the center to form the core.
isobar
A line on a map along which the air has a specified pressure.
isograd
A line along a pressure-temperature graph along which all points are taken to be at the same metamorphic grade.
isostasy (or isostatic equilibrium)
The condition that exists when the buoyancy force pushing lithosphere up equals the gravitational force pulling lithosphere down.
isostatic compensation
The process in which the surface of the crust slowly rises or falls to reestablish isostatic equilibrium after a geologic event changes the density or thickness of the lithosphere.
isotherm
Lines on a map or cross section along which the temperature is constant.
isotopes
Different species of a given element that have the same atomic number but different atomic weights.