Rocks of ages

In the past year you've certainly handled objects that are more than 1 million years old. No question. These items are so familiar you barely notice them. Yet they're easy to find. Just pick up a rock: It's one of earth's ancient treasures.

Geologists study rocks to learn how our planet formed, and how it continues to change. "Geology is like a murder mystery," says Dennis Martin, senior geologist at the Minnesota Department of Natural Resources, "and rocks are like fingerprints."

How do you go about investigating these rocky puzzles? As Sherlock Holmes would say, it's "elementary." Start in your backyard!

No matter where you live, interesting rocks abound. Take New Jersey, for example. Although it's small, as states go, it's a great place to collect rocks.

"You can see one-quarter of the earth's history in New Jersey," says Jonathan Husch. He's a geochemistry professor at Rider University in Lawrenceville, N.J. Some rocks found here are a billion years old.

The trick to understanding geology, Dr. Husch emphasizes, is "to listen to what the rocks are saying." Take granite, for example. It feels cold and hard now, but it used to be as hot as 2,000 degrees F. That's why geologists classify granite as an igneous rock, meaning "fire formed."

Igneous rocks began as a molten material called magma. Magma rises from deep inside the earth. It may rocket upward explosively or ooze slowly. As it approaches the surface, it cools and hardens. The cooling process gives geologists important clues.

If magma's temperature drops rapidly, for example, a type of natural glass called obsidian may form.

Granite, on the other hand, has crystals of material you can see. That means it took longer to cool - 10,000 years, perhaps. The more slowly rocks cool, the more - and bigger - crystals can form.

Rocks that travel

Not all rocks were made where you find them now. Some were carried long distances by glaciers. Others had a more spectacular debut. Welded tuff, for instance, arrived with help from a volcano.

Bishop's tuff, a kind of welded tuff, is named for the town of Bishop, Calif., in the eastern Sierra Nevada mountain range. The volcano that created Bishop's tuff erupted some 758,000 years ago. It spewed an estimated 120 cubic miles of glowing, volcanic material high into the atmosphere. When the material fell back to earth, it covered an area of 580 square miles.

Bishop's tuff fell on large regions of California, Wyoming, Kansas, Idaho, Texas, and Mexico. In some places, the material was almost 650 feet thick! Dale Stickney, a geologist at the California Department of Conservation, estimates it may have taken hundreds or even thousands of years for the rock to cool.

How do scientists know which volcano produced the welded tuff? Material from each volcanic eruption "has its own chemical 'fingerprint,' " Mr. Stickney says.

The 'why' of sand

Have you ever wondered where all the sand in the world comes from? The answer has to do with what geologists call the "rock cycle." Think of it as the earth's giant recycling project.

Rocks are hard, but they aren't indestructible. Rocks are broken down by wind, rain, water, natural disturbances, and more.

Sand is produced when rocks erode, so sand is any color that a rock can be: white, red, orange, green, gray, brown, black.... But most sand is quartz (silica), a very common mineral.

Sand is easily carried along by water. It moves down streams and rivers to the shores of lakes and oceans. As sandy layers accumulate, they press together by their own weight. Clay, lime, or iron in the sand may help cement it together. And so the tiny pieces of rock create a new kind of rock: a sedimentary rock called sandstone. (Sedimentary means "settling.")

Some sandstones are more than 450 million years old. New ones are forming all the time. The sand you see in a stream or river today may be sandstone tomorrow. Well, maybe not tomorrow - more like 250 million years from now.

Minerals that 'morph'

Ever watch a TV show in which a person's face slowly changes or "morphs" into someone else's? Certain rock types, called metamorphic (meaning "change") have done the same thing. They started out as sedimentary or igneous rocks and then underwent amazing transformations.

Sandstone, for example, can "morph" into schist. Others rock types become slate, marble, or quartzite. We can't watch this occur, because it happens inside the earth and can take a long time. Basically what happens is that rock is remelted and reformed under high heat and pressure.

The rock's new identity depends on what the original rock is made of, and how those materials respond to heat and pressure. (Minerals in a rock may have different melting points, for example, so they don't all melt at the same time.)

Gneiss (pronounced "nice") is an example of a "morphing" rock. Rock hunters find gneiss throughout New England, in the Adirondacks, and the Rocky Mountains. Look for alternating or irregular gray or pink stripes with medium- and coarse-grains of feldspar and quartz.

According to Vince Matthews, senior science adviser for the Colorado Geological Survey, metamorphic rocks may form when continents collide.

About 1.7 billion years ago, Colorado was the edge of a continent. Wyoming was part of another. As the two continents came together, the layers of granite between them were squeezed and heated. The granite turned into gneiss.

Throughout our planet's history, many portions of its surface have undergone metamorphic changes. Scientists know this because they are always studying the clues that rocks provide.

Space invaders

Earth, it's often said, is a gigantic rock hurtling through outer space. But is everything on this planet really from here? That's easy to answer: Of course not! You can find many objects from outer space - if you know where to look and what to look for.

(c) Copyright 2000. The Christian Science Publishing Society

of 5 stories this month > Get unlimited stories
You've read 5 of 5 free stories

Only $1 for your first month.

Get unlimited Monitor journalism.