Facts of the Matter Richard Brill

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Studying rocks can unlock history of the earth

THE HISTORY of our planet is long and complex. Geologic processes have shaped and reshaped its surface and crust and have destroyed much of the direct evidence of its history over the past 4 1/2 billion years.

Nevertheless, geologists use a variety of methods to not only determine the age of rocks, but also unravel their history from the information they contain.

Rocks remember the conditions under which they were formed and store the secrets in their chemical composition and physical features.

Geologists reconstruct the conditions at the time and place that the rock was formed millions or even billions of years ago using those clues. Information about the rocks of a given area can be stitched together like a patchwork quilt with that of other areas to reconstruct past conditions over larger areas and over time.

Everything we know about the ancient Earth comes from rocks, which are nothing more than collections of minerals, which are themselves chemical compounds in crystalline form. Despite the great variety of minerals and rocks that are known, more than 99 percent of Earth's crust is formed from only eight chemical elements and thus from only eight kinds of atoms.

There are three major rock groups: igneous, sedimentary and metamorphic. Each of them forms uniquely, and each leaves evidence of its formation in different ways that a geologist can interpret like reading the pages of a book.

Igneous rocks are those that cooled from a molten state, either above or below ground. The particular type of rock that forms depends upon the composition of the molten rock, how fast it cooled and what happened to it while it was cooling.

The atoms that comprise the molten rock are in an agitated state at high temperatures due to their thermal energy, and they are moving too fast to stick together to form crystals.

As the molten rock cools below 1,800 degrees Fahrenheit, certain atoms could be moving slowly enough to form chemical bonds and begin to form crystals. These high-temperature minerals exist as crystals in the hot liquid rock as it continues to cool and other minerals begin to form.

All igneous minerals do not form at the same temperature, but rather they form in a well-known sequence as the molten rock cools. If cooling takes place slowly enough to maintain chemical equilibrium between the mineral crystals and the liquid magma, the higher-temperature minerals are resorbed into the melt and release their atoms when the next mineral in the crystallization sequence begins to form.

In nature there are few circumstances where this equilibrium is maintained. This could be due to convection currents in the molten mass, or different cooling rates between its center and outside, leading to igneous rocks with a variety of mineral compositions that reflect the cooling history of the original magma. The size of the crystals in the igneous rock depends on the cooling rate.

Lava on the surface cools rapidly compared with magma that cools underground. Volcanic activity produces a range from glassy obsidian that cooled too fast for crystals to form, to crystals large enough to see with the naked eye.

Magma can take millions of years to cool miles below the surface before erosion and uplift eventually brings the solidified rocks to the surface. These plutonic rocks contain crystals that are anywhere from a fraction of an inch to several feet in size, and they often contain the less common chemical elements in rare minerals.

Sedimentary rocks are the result of solidification (lithification) of sediments by chemical and physical processes. They typically consist of resistant mineral grains that have weathered out of pre-existing igneous, metamorphic or other sedimentary rocks, then transported by water, ice or wind, deposited and buried, where they are compacted, heated to temperatures up to 400 degrees Fahrenheit and cemented together by chemically deposited minerals.

Sedimentation in different geological environments leaves a distinct set of features in the resulting sedimentary rock. (Examples of sedimentary environments are lakes, nearshore or deep sea, deltas, stream beds, sand dunes, flood plains, beaches, glaciers, river channels, lagoons and barrier islands.)

Each of these environments leaves clues of various kinds: composition, size, sorting and rounding of the mineral grains; sedimentary structures such as mud cracks and ripple marks; type of fossils; the shape, size and vertical sequence of sedimentary layers; and the overall shape and extent of the rock formation.

Metamorphic rocks are the result of a change in mineral composition by heat and temperature without melting and addition or loss of atoms. The change occurs by rearrangements of atoms into new crystal structures.

Metamorphism occurs when rocks are heated anywhere from a few hundred degrees to just below their melting point as they are buried 25 miles or more below the surface. The variety of atoms in the original rock is small because most rocks contain only those eight chemical elements, but the number of different minerals those eight kinds of atoms can form is amazingly large.

When the atoms in crystals of pre-existing rocks gain enough thermal energy, they can break free from their crystal lattices, migrate through the solid rock and link up with different atoms in new crystal structures, forming new minerals that are stable at the higher temperatures and pressures at depth.

Metamorphism is a slow process that takes place in its most extreme form deep in the core of folded mountain ranges such as the Rockies or Himalayas. The altered rocks might be brought to the surface over millions of years as the mountain ranges weather and their weathered fragments are eroded, transported and deposited to become sediments somewhere else.

Interpreting the history of metamorphic rocks requires knowledge of what kinds of minerals are stable under different conditions. Certain minerals, such as corundum (rubies and sapphires) and garnet, form within a restricted range of temperature and pressure. The stability ranges of these minerals come from laboratory measurements. The presence of these minerals and others are indicators of the depth of burial of the rock and can give much information about their history and the history of their surroundings.

The three rock types are the products of the "rock cycle" of geologic processes that recycle them within Earth's crust, allowing us to piece together the history and environments of the distant past of our miraculous planet.

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