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Fault Boundary Transformations Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Greenstone

Metamorphosed lava and sediments from volcanic eruptions in the Precambrian period were formed early on when the Earth’s crust was warmer and more malleable. The large crustal plates still floated freely, and were added to by huge, violent explosions of lava that blew through cracks and holes in the new crust.

Rock formed in this way is known as greenstone . After ash and lavas bubble through seawater and groundwater of temperatures between 150 and 300°C, greenstone is formed. The typical green color comes from high amounts of chlorite. Greenstone rock contains most of the world’s gold. Most gold mines around the world are ancient playgrounds of volcanic activity. Greenstone belts in southeast Africa are about 19 km thick and roughly three billion years old.

Ophiolites

Ancient ocean floor sediments that have turned to rock and pushed up through cracks in the continents are known as ophiolites . However, before plate tectonics was explained, geologists couldn’t figure out how these rocks, usually found on the seafloor, came to be located on land.

It was not until samples gathered by submarines and deep-sea drilling were studied more closely that scientists figured out this mystery. They found that ophiolites are formed when the oceanic crust that has been smoothed and smashed against the continents is carried along with seafloor spreading and then shoved up onto the land. This process has been going on for a long time. Some of the ophiolites samples studied are thought to be around 3.6 billion years old.

Ophiolites, with veins of rich ores and mineral deposits, are found in many of the mountain ranges of the continents.

Blue Schists

Blue schists are metamorphosed rock of subducted oceanic crust forced back into the mantle at subduction areas of the ocean floor or forced up onto the continents. There are also green schists that contain larger amounts of chlorite and epidote and are formed by low-temperature, low-pressure metamorphosed volcanic rock.

Measurements

With the development of sonar and highly sensitive imaging instruments during World Wars I and II, the timing of plate tectonics was right. The world’s ocean depths were determined and ridges and trenches discovered. Plate motions could be drawn accurately with much less guesswork.

The use of the Global Positioning System (GPS) (the same system that allows some automobiles to know exactly where they are on a road trip) uses the radio signals of an encircling network of 27 GPS satellites, each with a highly precise atomic clock on board. A ground-based radio receiver gathers the signals from 4 to 7 satellites at the same time and identifies the differences in the movement time from each satellite. A component of the receiver uses the time differences to locate the receiver to within 1 cm. Along the San Andreas fault in southern California, there are nearly 300 GPS monitoring stations constantly checking satellite signals for small displacements in local landforms.

NASA’s Space Shuttle and the International Space Station also provide valuable, real-time imaging. Through precise measurements, geologists have been able to accurately calculate the spreading of the Mid-Atlantic ridge to within a centimeter and the slow closing of the Pacific Ocean through subduction.

Plate movements are used by geologists to help to predict possible earthquakes and volcanic eruptions. This “early warning system” gives scientists around the world, one more way to protect entire populations from Mother Nature’s occasional temper tantrums.

Practice problems of this concept can be found at: Plate Tectonics Practice Test

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