Marine Geology Phần 9 ppt

MUD VOLCANOES

In the western Pacific Ocean, about 50 miles west of the Mariana Trench, the

world’s deepest depression, lies a cluster of large seamounts 2.5 miles below

the surface of the sea in a zone about 600 miles long and 60 miles wide.The

undersea mountains were built not by hot volcanic rock as with most Pacific

seamounts but by cold serpentine, which is a soft, mottled green rock similar

Figure 195 An

unusual lightning strike of

a plume of water in the

ocean.

(Photo courtesy U.S. Navy)

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Marine Geology

to the color of a serpent, hence its name. Serpentine is a low-grade meta￾morphic rock and the main mineral of asbestos. It originates from the reac￾tion of water with olivine, an olive-green, iron- and magnesium-rich silicate

that is a major constituent of the upper mantle.

The erupting serpentine rock flows down the flanks of the seamounts

similar to lava from a volcano and forms gently sloping structures. Many of

these seamounts rise more than 1 mile above the ocean floor and measure as

much as 20 miles across at the base, resembling broad shield volcanoes such

Mauna Loa (Fig. 196), which built the main island of Hawaii. Drill cores taken

during the international Ocean Drilling Program in 1989 showed that ser￾pentine not only covers the tops of the seamounts but also fills the interiors.

Several smaller seamounts only a few hundred feet high are mud volca￾noes, resembling those in hydrothermal areas on land (Fig. 197). They are

Figure 196 The

Mauna Loa Volcano,

Hawaii.

(Photo courtesy USGS)

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Rare Seafloor Formations

composed of mounds of remobilized sediments formed in association with

hydrocarbon seeps, where petroleum-like substances ooze out of the ocean

floor. Apparently, sediments rich in planktonic carbon are “cracked” into

hydrocarbons by the heat of Earth’s interior. Even drill cores recovered around

hydrothermal fields smell strongly of diesel fuel.

Mud volcanoes exist in many places around the world. They usually

develop above rising blobs of salt or near ocean trenches.The mud comprises

peridotite that is converted into serpentine and ground down into rock flour

called fault gouge by movement along underlying faults.The mud volcanoes

appear to undergo pulses of activity interspersed with long dormant periods.

Many seamounts formed recently (in geologic parlance), probably within the

last million years or so.

A strange mud volcano that spews out a slurry of seafloor sediments

mixed with water lies beneath the chilly waters of the Arctic Ocean. It is a

half-mile-wide circular feature that lies 4,000 feet deep and is covered by an

unusual layer of snowlike natural gas called methane hydrate.The underwater

volcanic structure is the first of its kind found covered with such an icy coat￾ing draped across a warm mud volcano. Methane hydrate is a solid mass

formed when high pressures and low temperatures squeeze water molecules

into a crystalline cage around a methane molecule.Vast deposits of methane

hydrate are thought to be buried in the ocean floor around the continents and

represent the largest untapped source of fossil fuel left on Earth.

Figure 197 Mud

volcanoes and acidulated

ponds northwest of

Imperial Junction,

Imperial County,

California.

(Photo by Mendenhall,

courtesy USGS)

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Marine Geology