Geologic Glossary

- J -

joint

A narrow crack in rock along which there has been no significant movement of either side. Joints commonly form in parallel sets.

- K -

karst

A distinctive landscape (topography) that can develop where the underlying bedrock, often limestone or marble, is partially dissoved by surface or ground water.

kyanite

An aluminum-rich, blue to light green silicate mineral. Kyanite forms in metamorphic rocks at moderate temperature and high pressure.

- L -

lahar

A type of mudflow that originates on the slopes of volcanoes when volcanic ash and debris becomes saturated with water and flows rapidly downslope.

lamination

Very thin layers of less than 1 cm thickness.

landslide

Downslope movement of rock, soil, and mud.

land-use

Present and historical uses of land, such as for agriculture, mining, recreation and grazing.

lava

Magma that reaches the Earth's surface through a volcanic eruption. When cooled and solidified, forms extrusive (volcanic) igneous rock.

lawsonite

A metamorphic mineral that forms only under very high pressure. It is a calcium aluminum silicate and usually forms microscopic crystals.

lichenometry

A dating method that uses the growth rate of certain lichen species as an indicator of the age of the surface the lichen is growing on.

limestone

A sedimentary rock made mostly of the mineral calcite (calcium carbonate). Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

limonite

A mineral composed of iron oxides and water. Rust. Very common in many rocks after weathering at the Earth's surface. Imparts brown or yellow colors to many rocks.

lineament

A linear (relatively straight) topographic feature or features such as a fault, line of dense vegetation, or a chain of aligned volcanoes.

lineation

Parallel arrangement of elongate minerals or groups of minerals. To envision lineation, imagine packages of spaghetti or pencils.

lithification

The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.

lithosphere

The outer layer of solid rock that includes the crust and uppermost mantle. This layer, up to 100 kilometers (60 miles) thick, forms the Earth's tectonic plates. Tectonic plates float above the more dense, flowing layer of mantle called the asthenosphere.

loess

A wind-blown deposit of sediment made mostly of silt-sized grains.

luster

The appearance of the reflection of light from the surface of a mineral. Luster is described as metallic, glassy, dull, etc.

- M -

mafic

A term used to describe minerals or igneous rocks that are rich in iron and/or magnesium. Mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.

magma

Molten rock. Magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals. Molten rock that flows out onto the Earth's surface is called lava.

magma chamber

A body of molten rock and solid crystal mush beneath the Earth's surface. When this chamber cools and solidifies, it is called a pluton.

magnetic anomalies

See magnetic reversals.

magnetic reversals

Earth's magnetic field occassionally "flips" or reverses polarity. This means that, if a polarity reversal happened today, your compass would point south instead of north!

magnetite

Iron oxide mineral (Fe₃O₄). Usually tiny black, metallic crystals. Magnetite will attract a magnet and sometimes, in a rock, a hiker's compass needle.

magnitude

A measure of the total amount of energy released by an earthquake.

magnetometer

Device for measuring magnetism.

mantle

The layer of the Earth below the crust and above the core. The uppermost part of the mantle is rigid and, along with the crust, forms the 'plates' of plate tectonics. The mantle is made up of dense, iron and magnesium rich (ultramafic) rock such as dunite and peridotite.

marble

A metamorphic rock of made of calcium carbonate. Marble forms from limestone by metamorphic recrystallization.

mass wasting

Movement of rock and soil downslope under the influence of gravity.

matrix

Fine-grained material surrounding larger grains in a sedimentary rock.

mechanical weathering

The mechanical break-up or disintegration of rock into smaller fragments.

mélange

Mixture of rocks formed by tectonic disruption, such as multiple faulting, which brings disparate rock types together. Usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.

metaconglomerate

Metamorphosed conglomerate.

metamorphic rock

A rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids.

More Information

mica

Group of silicate minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water. All micas form flat, plate-like crystals. Crystals cleave into smooth flakes. Biotite is dark, black or brown mica; muscovite is light-colored or clear mica.

micaceous

A general term for mica-rich rocks.

midden

In geology, a mound of organic debris or organic-rich soil created by an animal. In archeology, a mound of human refuse.

migmatite

"Mixed rock". A metamorphic rock that forms in one of two ways. The metamorphic rock may be heated enough to partially melt, but not completely. The molten minerals resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dikes.

mineral

A naturally occurring chemical compound or limited mixture of chemical compounds. Minerals generally form crystals and have specific physical and chemical properties which can be used to identify them.

mineralization

The formation of minerals. New minerals may be added to fractures and empty spaces in a rock or by replacing preexisting minerals with different ones.

mineralogy

The study of minerals.

Miocene

An Epoch that includes the time interval of about 23.7 to 5.3 million years ago.

Moho

The boundary separating the base of the Earth's crust and the top of the mantle. The Moho occurs at a depth of 5-10 kilometers beneath oceanic crust and about 35-65 kilometers below continental crust. The term moho is an abbreviation for Mohorovicic discontinuity, named for Andrija Mohorovicic, a Croatian seismologist.

moraine

A hill-like pile of rock rubble located on or deposited by a glacier. An end moraine forms at the terminus of a glacier. A terminal moraine is an end moraine at the farthest advance of the glacier. A lateral moraine forms along the sides of a glacier. See till

morphology

The study of shape or form. See geomorphology.

mud

Wet clay and silt-rich sediment.

mudflow

Same as debris flow.

mudstone

A very fine-grained sedimentary rock formed from mud.

muscovite

One of the mica family of minerals. Muscovite is light-colored or clear mica, sometimes called isingglass.

- N -

nonconformity

An type of unconformity in which young sedimentary rocks lie on top of older metamorphic or intrusive igneous rocks.

nonsilicate minerals

A mineral without silicon (Si). See silicate minerals.

normal fault

A fault that drops rock on one side of the fault down relative to the other side.

normal polarity

A magnetic field produced by the Earth that is the same as exists today.