MAR: Glossary

aphanitic
A term used to describe the texture of igneous rocks in which individual crystals are too small to be distinguished by eye without the use of magnification.

asthenosphere
The layer of the earth distinguished by its plastic behavior that lies between 100-200 km and 250-500 km below the surface of the earth, below the lithosphere and above the mesosphere. Plastic materials are deformed permanently by stresses imparted by pushing, pulling or the burden of weights.
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basalt
A dark, aphanitic rock commonly composed of Ca-plagioclase and pyroxene, with or without amphibole or olivine. In hand-sample, this rock is fine-grained, dark-gray to black in color, and otherwise plain.

basin and range
The Basin and Range is an area of southwest North America extending from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. This province is characterized by multiple north-south trending linear mountain ranges alternating with intervening valleys. The linear valleys are separated from the linear mountain ranges by normal faults. The Basin and Range topographic pattern is the result of processes that stretch the crust sideways.
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continental crust
The type of crust that makes up continents. Although a wide range of rocks make up the continents, the average composition of continental crust is that of the igneous rocks andesite or diorite. The thickness of continental crust ranges between 35-70 km.

core complex
A core complex is an area that exhibits three key features together - a domal core, hanging wall blocks, and detachment fault. The domal core is a large dome-shaped mountain composed of rocks which originally occured much deeper within the earth. Rocks of the domal core are part of the footwall exposed along special types of normal faults called detachment faults. In contrast to the domal core, rocks of the hanging wall block are much shallower in origin. Core complexes can be found on land and in the ocean.

corrugation
Corrugation is a surface topographic feature that looks like the linear ripples on ridged potato chips. The linear corrugations are a few tens of meters high and are spaced every 1 to 2 km apart. Corrugations may be formed in the same way as a striations, by scraping that takes place along fault planes.

crust
The outer layer of the earth distinguished by its silica-rich composition with thicknesses ranging between ~5-45 km. The crust generally occurs in two types - continental and oceanic, each distinguished by particular composition, density, and thickness characteristics.
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detachment fault
A detachment fault is a special kind of normal fault. Typically, a portion of a detachment fault dips at a shallow angle.

domal massif
A domal massif is a smooth rounded mountain marked by linear surface features called corrugations, where deep rocks are exposed on the surface or seafloor
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fault
A plane of breakage in rock where blocks of rock on either side of the break have been displaced.

footwall
The rocks on the side of a fault plane on which you could stand.

fracture zone
Part of the linear topographic features that run perpendicular to oceanic spreading centers separating adjacent segments of midocean ridges. The parts that are the fracture zones extend beyond the part that is called the transform fault. The transform fault section is the part between the two adjacent ridge segments. There is very little or no opposing motion of rocks on opposite sides of a fracture zone.
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gabbro
A phaneritic rock commonly composed of pyroxene, Ca-plagioclase and olivine. In hand-sample, this rock is fine-grained.

geothermal gradient
The change in the earth's temperature with depth.

gravity
The force of attraction exerted by objects with mass.
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hanging wall
The rocks on the side of a fault plane from which you could hang.

heat flow
The rate at which heat flows out of an area over a given amount of time.
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knot
A unit of velocity equal to one nautical mile per hour. Knots are usually used as the unit for speeds at sea such as ship speed, wind speed, and ocean current speed.
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lithosphere
The layer of the earth distinguished by its rigid behavior that occupies the layer between the surface and ~100 km below the surface of the earth. The crust and part of the mantle are contained within the lithosphere. Rigid or brittle materials are not permanently deformed by stresses imparted by pushing, pulling or the burden of weights; however, they may break.
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magma
Molten rock which lies below the earth's surface.

magnetometer
An instrument that measures the earth's magnetic field.

mantle
The middle layer of the earth between the crust and core, extending down to ~2,900 km in depth, distinguished by its silica-poor, and iron and magnesium-rich composition in comparison with the crust.

Mid-Atlantic Ridge
The major oceanic spreading center or midocean ridge that runs down the center of the North and South Atlantic Oceans.

mid-ocean ridges
See oceanic spreading center.
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normal fault
A plane of breakage in rock where rocks on either side of the break have been displaced such that the hanging wall has fallen relative to the footwall.
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oceanic crust
The type of crust that makes up the oceans. Unlike the larger variety of rocks found in the continental crust, oceanic crust is mostly basalt. The thickness of oceanic crust is usually less than 8 km.

ophiolite complex
A vertical pattern of rock types associated with the oceanic crust. From top-to-bottom, the rock types are: 1) Crustal section 6 km thick total, a - top half km pillow basalts, b - 1 km sheeted dikes, c - 3-4 km gabbro, d - thin ultramafic layers; 2) Exposed mantle rocks: peridotite (source of olivine for serpentine).
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paleomagnetic anomaly
A deviation of the magnetic characteristics of rock material from designated normal values.

partial melting
The melting of part of the crystals or chemical constituents of rock, without melting all of the components. Since crystalline solids of different types melt at different temperatures, there are depths at which only part of a rock melts.

peridotite
A phaneritic rock commonly composed of olivine, pyroxene and sometimes, garnet. This rock is usually dark-colored or green if it's entirely composed of olivine.

phaneritic
A term used to describe the texture of igneous rocks in which individual crystals are large enough to be seen by eye, without the use of magnification.
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reverse fault
A plane of breakage in rock where rocks on either side of the break have been displaced such that the hanging wall has risen relative to the footwall.

rifting
See seafloor spreading.
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seafloor spreading
The process whereby the tectonic plates on either sides of midocean ridges move apart in opposite directions and new seafloor is created at the ridge.

serpentinite
A low-grade metamorphic rock composed primarily of the mineral serpentine. Serpentinite is the alteration product of the igneous rock peridotite. Metamorphism can take place when hydrothermal solutions interact with peridotites, causing alteration of the component mineral olivine to serpentine.

side-scan sonar
An instrument that uses soundwaves to map the texture of the seafloor. The instrument operates by sending beams of sound out sideways. A receiver on then captures and records sound waves which are reflected from the seafloor. The instrument measures 1) the time it takes for sound to bounce back to determine distance to the seafloor and 2) the strength of the returned sound to determine the roughness of the seafloor. Generally, steep hard surfaces reflect sound strongly and rough or soft surfaces may not reflect sound as well.

striation
Linear scrape marks made as two fault blocks slip past each other.

strike-slip fault
A plane of breakage in rock where rocks on either side of the break have been displaced sideways such that the hanging wall and footwall haven't moved vertically with respect to each other.
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transform fault
A special case of the strike-slip fault that occurs between two tectonic plates. Transform faults are commonly associated with the line between two offset ridge segments on midocean ridges. The San Andreas Fault is a transform fault.
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watch
A work shift during which part of a ship's crew is on duty, usually alternating every four hours.
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