MAIN: ROCKS AND MINING: ROCKS TRANSFORM

Rocks Transform

Inside their cocoons, caterpillars change into butterflies. This is called metamorphism, from a Latin word that means "to transform." Inside Earth, heat and pressure change a rock's minerals from one set to another. This, too, is called metamorphism. In each case, the basic ingredients stay the same. They're just reassembled into a new form. These three specimens show how the volcanic rock basalt can change both its minerals and its appearance as it is buried deeper and deeper in the Earth's crust.

The basalt is made up of brown glass and the mineral olivine. As it erupted and cooled, escaping gas bubbles created its spongelike texture.

If basalt is buried to 2,000-4,000 atmospheres pressure, equivalent to depths of 7-15 km, and heated to 350°C (660°F), its minerals and glass react to form new minerals-epidote and albite. The result is a metamorphic rock: greenstone.

If greenstone is buried to 18,000 atmospheres, equivalent to a depth of 60 km, and heated to 700°C (1,290°F), its minerals convert to other ones, including garnet and clinopyroxene. The result is a new metamorphic rock: eclogite.

Home Cooked Minerals

Variations on a Metamorphic Theme

Home Cooked Minerals
Andalusite, kyanite, and sillimanite (above) all contain the same elements. In fact, they have exactly the same chemical composition: Al2SiO5. So why on Earth are there three different minerals? Each of these aluminosilicate minerals has a unique internal arrangement of atoms--a result of the combined temperatures and pressures under which they form.

Andalusite is stable at low pressures and temperatures. Kyanite is stable at high pressures and moderate to high temperatures. Sillimanite is stable at a wide range of pressures and high temperatures.

This remarkable diagram enables geologists to estimate the temperatures and pressures at which some rocks form. How was it created? In special laboratories, geologists make synthetic rocks and minerals at conditions similar to those deep within the Earth. They use the results of these experiments to produce temperature-pressure diagrams such as this one.

Minerals Out of Bounds. Each of these two rocks crossed boundaries on the diagram above and contains a coded message. The key is the shapes of the large blocky and rod-shaped crystals, which were once andalusite.

Sillimanite-bearing Schist. Thousands of white, fibrous sillimanite crystals completely replaced the original rod-shaped andalusite crystals when the rock on the left was subjected to a higher temperature.

Kyanite (Pseudomorph after andalusite). The blocklike shapes in the rock on the right were once large andalusite crystals. Now they are occupied by hundreds of white blades of kyanite that formed when the rock was deeply buried and subjected to higher pressures.

Variations on a Metamorphic Theme
Many metamorphic rocks contain large and beautiful crystals. The diverse colors and textures of these rocks resulted from different combinations of temperature and pressure produced by interactions of the gigantic tectonic plates that cover Earth's surface. The rocks formed either where one plate sank below another (subduction zone) or where two plates collided.