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Oceanic spreading ridge
Mohorovicic discontinuity, Moho
The boundary surface (between 25km and 60km deep beneath
the continents and between 5km and 8km deep beneath the ocean floor) that separates the Earth’s crust from the underlying mantle. Named after Andrija Mohorovicic (1857-1936), a Croatian seismologist. It is often just referred to as the Moho. At this depth the properties of the rocks change markedly, in part because the rocks are less brittle and able to move without fracturing.
Mud pool
A pool of heated water and mud that forms above some hot rock areas. Rainwater and melting snow fill the vent of a fumarole. As sulfur gases dissolve in the water, they make it very acid. The acid water weathers the surrounding rock, breaking the rock down, and creating mud. The mud acts like
a lid, sealing in the rising gases. The mud spurts up and splatters
when the pressure of the trapped gases becomes sufficient to push their way up through the mud.
Mud pools, unlike geysers and hot springs, change their character throughout the year depending on how much rainfall or snowmelt there has been.
Muscovite
A brown form of mica. It is
a common mineral in igneous rocks. The name muscovite comes from Muscovy glass, because it was once used instead of glass in Russia.
N
Normal fault
A fault in which one part of
the Earth’s crust has slipped down the face of another. It is the most common kind of fault and results from parts of the crust pulling apart. (Compare to: Reversed fault.) (See also: Rift, rift valley.)
O
Obsidian
A natural volcanic glass (rock). Glassy rocks are often associated with rapid cooling of very sticky acid lava containing water. This lava contains about 10% water, which keeps it mobile. When
the magma reaches the surface, the water boils off. As a result,
the magma increases rapidly in stickiness and, at the same time, cools. These are the conditions that produce volcanic glass. You cannot see through the glass because it contains too many tiny crystals. As a result, there is no clear path for light to pass through.
Oceanic spreading ridge
A fracture zone that occurs within the oceans and marks the place where two plates are pulling apart. It occurs where magma flows up from the mantle and is a common source of fissure eruptions. The Mid-Atlantic Ridge is the biggest example of a spreading ridge.
 Mud pool—Mud pools at Bumpass Hell, California.
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