Volume 2 - No. 3
ROCKS TELL STORIES
By Dr. Ross A. Maxwell,
How many of us really are observant, when we are hiking? Of course we take notice of some of the flowers and bugs; even the sand, if it impedes our progress, but few of us pay any heed to such commonplace things as rocks. The next time you are on an outing, examine the rocks and fossils, collect a few specimens and try to read the story they tell. This may be the beginning of an interesting hobby that will lead you over thousands of miles of beautiful country, and open your eyes to a new and exciting world that is waiting to be explored.
The rocks that make up the earth's crusts are all around us; each rock tells its own story, and their combined stories are the history of the earth. Scientists have told us that a long time ago the earth was very hot and that perhaps the surface was molten. As time elapsed, the surface cooled and molten material, if such existed, became hardened to form a rigid shell. In time the rain fell and the runoff collected in depressions to form the oceans, seas, and lakes. Rainfall and melting snow on the land surfaces provided water for streams that flowed gently or surged with rapid currents onward toward the sea, and each of these streams carried a load of gravel, sand, and silt that was eventually deposited on the floor of the ocean.
The stream water, as well as the water in the ocean, contains invisible mineral matter in solution, just as limited amounts of sugar become invisible in a cup of steaming coffee. The muds and sands that lie on the bottom of the ocean are saturated with water containing these invisible minerals. When certain conditions are brought about, the water gives up the minerals as a mineral cement or glue that binds the sand and mud particles together to form solid rock. In this manner the coarser materials are bound into a rock that in general appearance is much like slabs of concrete. This type of rock is called conglomerate, and rightly so, for it consists of a conglomeration of all kinds of rock fragments.
Then there is the origin of sandstone which consists of sand grains like those seen on a bathing beach, that are bound together with mineral glue. The mineral glue is precipitated in the opening around the individual sand grains; the openings become smaller until the masses of individual grains are bound together into a resistant rock.
The fine material carried by the streams and dumped on the ocean floor may become solid rock. Frequently, this material can be in part consolidated into rock by the weight of the overlying layers of sand and mud. This weight squeezes the small particles together, but this force alone does not always change mud into rock, for there is usually a good supply of Nature's mineral glue present to aid in holding the silt particles together. These types of rock, called shale, or clay, frequently disintegrate rapidly and when wet, form mud and often cause the motorist plenty of trouble.
What is this peculiar mineral glue that Mother Nature uses to bind small insignificant rock fragments together to make massive ledges of solid and resistant rock? Probably one of the most common mineral cements is lime. Most stream water, as does the water in the lakes, the seas, and the oceans, contains varying amounts of lime. It is probably more common in water obtained from wells and springs that have their source in the rocks below the earth's surface. This mineral is invisible in solution, but the drinking glass that stands near the water faucet may have a thin coating of gray or yellowish gray mineral matter that is likely to be lime. The teakettle that used to sing as it stood on the back of the cook stove soon had a thick coating of lime scale. Many of us who are accustomed to drinking hard water are familiar with the taste of lime. Lime isn't the only kind of mineral cement. Iron, silica, and some other minerals have all had a part in this cementing process, but lime is probably the most common.
There is a fourth kind of rock - limestone - that is common in many parts of our continent. It consists of lime plus various kinds of impurities. The impurities may be sand grains, mud particles, volcanic ash, small masses of iron, humus, and various other things. Limestone is likewise deposited in the ocean or in other bodies of water. Sometimes its deposition is due to chemical reactions that cause the lime to go out of solution and settle as a scum or ooze on the bottom of the ocean. Perhaps more frequently the lime is given up as a precipitate from the water by organic action. Corals are important limestone builders and large coral reefs and islands are frequently built in this manner. Microscopic plants and animals, and some of the varieties of shell fish play an important part in the precipitation of the lime scum on the ocean floor that in turn becomes consolidated to form limestone.
So far we have been considering only the sedimentary rocks, so named because they are formed by the accumulation of sediment on the floor of the ocean. Another important characteristic of the entire class is that they are stratified or occur in layers. Many of the rocks of this class contain fossils, and they are usually reddish-brown, yellowish-brown, gray, bluish-gray, and less frequently almost white or almost black. Impurities give them their variable colors. Pure lime is white, but oxidized iron in the limestone usually produces the various shades of yellow, brown, and red. Organic material frequently produces blue, green, or blackish tints. These same substances give similar colors to the shales, clays, and sandstones. White sand grains cemented with relatively pure lime will form a white or light colored sandstone, but the addition of iron stains will produce various colors.
These types of rock are rather easy to identify. A rock that is yellow, brown, or red; has a gritty surface, and the appearance of a mass of sand grains, is probably a sandstone. Shales may be any color; they are usually thin-bedded and remind one of layers of compressed mud. Clays are also variable in color, are normally poorly stratified, and have a distinctive mud-like appearance. Conglomerate looks similar to slabs of concrete, and its color is usually controlled by the color of the pebbles in the rock itself. Limestone is frequently well stratified, of uniform texture, and effervesces freely when a drop of dilute hydrochloric (muriatic) acid is applied.
One of the most interesting secondary features of many sedimentary rocks is their fossil content. These fossils are petrified forms of life from a past geological age. These may be pieces of petrified wood, fossil snails, oysters, or other shell fish, fern leaves, fish, dinosaur bones, and even tracks and trails of animals. Some fossils were preserved quite by accident, but usually they became petrified in the following manner:
A snail, for example, is suddenly buried alive with muds that are being transported by waves. This prevents normal decay and protects the snail's shell and body from the scavengers in the sea. Thus protected, the snail remains intact for some time, and during this interval, the water which contains mineral matter comes in contact with the snail. There may be a chemical reaction produced by the presence of the organic matter or it may be that the water is nearing the saturation point of a certain mineral and about ready to give up that particular mineral. In either case there is an exchange or replacment of organic substances with the inorganic. Thus, the water removes minute particles of the snail's body and replaces`these minute particles of organic matter with minerals. This process does not always take place in the ocean. It may occur in lakes, swamps, marshes, or along streams. During the past geological ages the giant dinosaurs roamed over the North American continent. Sometimes these large fellows ventured too far into a swamp and mired down, becoming buried in the quagmire. The water and mud which covered their bodies prevented normal decay. Similar processes of replacement followed and in turn the bones and hard portions of their anatomy became petrified. In a similar manner huge logs, limbs, twigs, and leaves of trees have been preserved from the ages past.
The igneous rocks represent a second large class. They differ from the sedimentary rocks in that they were originally a molten mass that cooled to become hard and rigid. They are not stratified, and do not contain fossils. The environment in which these molten masses cooled and hardened controlled their physical appearance. Some of the masses were buried thousands of feet below the earth's surface. Naturally, the rate of cooling was slow and as a result such rocks are coarsely crystalline. Other masses cooled near the surface, the rate of cooling was more rapid and the crystals did not have time to grow large; consequently, such rocks are fine grained. In other instances the lava poured out on the surface, and in many rocks of this type, one is unable without the aid of a magnifying lens to see any crystalline structure. More rarely, lavas were chilled immediately to form volcanic glass (obsidian).
By following a few simple rules, igneous rocks can be classified and named. Granites are coarse-grained, and usually flesh-colored, pink, or gray. If the rock is coarse but composed predominantly of dark-colored minerals, it may pass with the trade name "granite" but technically would be called a diorite. All of the coarse-grained rock cooled very slowly at great depths, and it required ages to form the large crystals. If this be true, why do we now find acres and acres of granite exposed at the earth's surface? These masses were uncovered by erosion and although it took ages for the molten lava to cool, it also took ages for the streams to uncover these areas of coarse-grained rocks and expose them to our view. Consequently, when we look at a piece of granite, we should remember that a story of earth history, many chapters in length, has been written since that piece of granite was a molten mass.
The fine-grained rocks are also conveniently classified according to their color. The light-colored ones are generally called rhyolite, or in some cases, felsite. The dark-colored rocks are usually basalts. These types of fine-grained rock cooled more quickly, relatively speaking, than did the granitic types. Also, they may have been uncovered more quickly by the processes of erosion, but we must not get the idea that all granites are older than all rhyolites or basalts. The earth is very old, and there have been numerous periods when igneous rocks were formed. Consequently, some of the fine-grained rocks like basalt are older than some granites, but this is not always the case. Before we can compare the relative age of igneous rocks, we must be trained to read the history as it is told in the rocks themselves.
You will find that rock collecting is interesting. It gives further purpose to short outings and it may lead to an absorbing hobby. Don't seek especially for the rare and showy specimens. The simple ones are more suitable for study. It will not be long before you will have a collection of considerable size. The collecting, too, will lead you into interesting scenic areas, off the beaten path, that you otherwise would probably never see.
Rocks tell stories that contain cold and interesting facts. A little study will develop increased understanding and appreciation of the world in which you live.
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