2008-05-30 / Columnists

Drawing On Science For Kids

A Rock You Find On The Beach Is Most Likely An Alien
Commentary By Stephen S. Yaeger

Rocks you find anywhere most likely came from elsewhere. And it wasn't even the same shape or size originally. Either it was transported by someone to the place you found it, or it got there by a force of nature.

In Central Park there's a structure called Cleopatra's Needle. It neither belonged to Cleopatra of Egyptian pharoah fame, nor is it a needle. It's a large, red granite obelisk with Egyptian writing called hieroglyphs. It was brought to New York back in January 22, 1881.

It was doing quite well in Egypt for over 3,000 years, but has been a bit "sick" here in New York. You see, back in Egypt it's quite hot and dry, but here in New York, we have rain, snow and pollution.

As a result the hieroglyphs, which were easily read, are no more. This is due to a process called weathering (the break-up of rock due to atmospheric action).

There are two types of weathering.

Mechanical weathering happens when a rock is broken up into smaller pieces without changing its composition or makeup.

For example when water freezes in a small crack in a rock breaking it apart. This is called ice wedging. Boulders with sharp edges most likely resulted from ice wedging.

You may see a large rock with roots from trees or moss growing into it and breaking it apart. Other types of mechanical weathering involves wetting and drying of rocks and the loss of overlying soil material.

Chemical weathering changes the chemical composition of rock into another substance. This happens when the rock is subjected to rain, oxygen, carbon dioxide, and chemical effects of decaying plants and animals. Some rocks are changed by water. This is called hydrolysis.

Rocks such as feldspar, hornblende, and augite react with water to form clay.

Other rocks such as magnetite and pyrite are changed by oxygen. This is called oxidation and the rocks are changed into hematite more commonly known as rust. When it rains carbon dioxide and sulphurs in the atmosphere dissolve in the rain droplets to form carbonic acid (the stuff that fizzes in your soda) and sulfuric acid. The acids react with certain rocks changing them into different substances.

Less-resistant rocks weather faster than more-resistant rocks. Bryce Canyon National Park in Utah is an example of these rocks. The result is a magnificent display of nature exhibiting beautiful rock structures.

Sand on your beach comes from weathered rocks that were not there to begin with. The sand was deposited on the beach by erosion due to wind and wave action. Erosion is the natural removal and transport of material from one location to another. Wind, water, and glaciers are the primary sources of erosion.

Next time you visit the Bronx Zoo take a look at the large boulders scattered throughout the park. Observe the "scratch" marks and take note of the polished look. If you were a geolo- gist familiar with rocks you would also notice that the boulder is not of the same type as the bedrock on which it rests.

This is a good indication that the boulder came from elsewhere. The problem is: How could such a large rock be transported?

Glaciers are known to be prime movers of material.

In the early 1800's studies were made of glaciers in the Alps of Europe. There was no doubt that the glaciers eroded rocks and carried them downslope; in some cases many miles from the rocks' origin.

Afamous Swiss naturalist, Louis Agassiz (AG-ah-see) concluded that during the ice age glaciers carried rocks of all sizes many miles from their origin.

When the glaciers melted, the rocks were left in place, far from home. The famous Half-Dome in Yosemite National Park, California was once a complete mountain. During the last ice age a glacier carved out a valley taking ½ of the mountain with it.

So when you pick up a nice looking rock you find on the beach, keep in mind that it most likely came from a place far from where you found it.

Questions/comments? E-mail Steve: Drawingonscience@aol.com

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