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Fault Boundary Transformations Help (page 2)

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

Igneous Activity

For millions of years, interior magma has bubbled up from the Earth’s mantle only to cool, turn solid, and add to the depth of the crust. When held back for any length of time, the pressure increases to an extreme point until it blasts violently through volcanoes to form new rock along their sides. The comings and goings of magma is called igneous activity , from the Latin word, ignis or fire. Magma that erupts from a volcano is called lava , which when cool turns into volcanic rock . This rock name comes from the Roman God of fire, Vulcan. The study of volcanoes is sometimes called vulcanism .

Rock from magma that bubbles up more slowly and never reaches the surface is called plutonic rock . It was named after the Greek God of the underworld, Pluto. Plutonic rock spends most, if not all, of its lifetime deep within the Earth.

Continental Shields

All continents are made of new and old rock. When Pangea fractured into several chunks and began drifting around the face of the globe, land originally side by side drifted hundreds and thousands of miles apart. It was this “sameness” of rock types, in far flung areas of the world, that got geologists thinking that all land must have been together in one piece originally.

Further study of these ancient areas showed that the lowest level of crustal rocks, known as granulites , formed a kernel around which the continents developed. These dome-shaped structures or shields have very little sedimentary deposits and only thin soils.

Ten to twelve continental shields have been discovered containing ancient rocks. The largest of these are the Canadian Shield in North America and the Fennoscandian Shield in northern Europe. The western one-third of Australia has been found to be part of an ancient shield.

Every continent has an area of ancient unchanged rock known as a continental shield . These stable, shield areas have experienced very little change. Since the original Precambrian eon formed continents millions of years ago, continental shields have only felt minor bending and gentle erosion compared to highly stressed plate margins. Surrounding the continental shields are flat, sediment containing continental platforms .

Cratons

In addition to the continental shields, geologists find areas of rock that form an edge or frame along the rim of the shields. These edge areas are called platforms . When shields are framed by a platform area, it is known as a craton .

Cratons are made up of pieces of continents that have not been affected by major changes since Precambrian times.

The four billion-year-old metamorphosed granite, known as the Acasta Gneiss found in the Northwest Territories of Canada, shows that the first kernels of continental crust were around even during the earliest formation of the Earth.

When chunks of granitic crust combined into stable, solid kernels drifting about on the malleable mantle, they provided a place for cooled bits of rock to buildup. The first cratons formed about 1.5 billion years ago, with larger pieces the size of Australia and India and smaller bits the size of Madagascar. These early cratons drifted about on the upper mantle until they cooled and slowed down long enough to stick together in larger and larger masses. Eventually, they grew to become continental landmasses with pushed up mountain peaks and ranges.

The North American continent is made up of seven cratons that fused together millions of years ago. These combined craton landmasses account for about 80% of today’s continental landmass with the ancient rock masses making up only a tiny part of the total landmass.

The Earth’s constant magma recycling melted most of the first rock-forming kernels since their formation millions of years ago and transformed them into new rock over much of the planet’s solid surface. Canada, Africa, and Australia are the only known places that still have rocks unchanged throughout geologic time. In the United States, the oldest Precambrian rock is found in the nearly two billion-year-old Vishnu Schist at the bottom of the Grand Canyon.

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