THE ROCK CYCLE
When the Earth formed there were no rocks. About 4.5 billion years ago, the great bombardment, the process that built the Earth from the solar nebula, released prodigious amounts of energy as swarms of meteorites crashed into the growing planet, converting gravitational potential energy into heat. That heat produced a molten ball orbiting the Sun. There was no land, no oceans and no atmosphere. Only when the bombardment subsided and the Earth began to cool did rocks appear. First, as the temperature drooped below the melting point of surface of surface rocks, the outer crust of the Earth gradually solidified, like the first layer of ice on a pond in winter. Then, when surface temperatures dropped below the boiling point of water, then first rains fell to Earth. Together, these two events began the rock cycle, a cycle of internal and external Earth process by which rock evolved, destroyed, and altered.
Igneous Rocks
Igneous rocks, which solidify from a hot liquid and thus were the first to appear on the Earth, come in two principle types. Volcanic or extrusive rocks solidify on the Earth's surface in what are by far the most spectacular of all rock–forming events, volcanic eruptions. Red–hot fountains and flows of lava ooze down the slopes of the growing volcanic cone. The most common variety of rock basalt, a dark, even–textured rock rich in oxides of silicon. magnesium, iron, calcium, and aluminum. Basalt makes up most of the rock in Hawaii, as well as most of the new materials formed at mid–ocean ridges. Other volcanoes feature rocks richer in silicon; if these magmas mix with a significant amount of water or other volatile (easily boiled) substance, the volcanic rock can become the frothy rock pumice.
Igneous rocks that harden underground are intrusive rocks. Dark–colored basalt often exploits underground cracks near volcanoes to form layers or sheets of igneous rock. The Palisades on the Hudson River near New York City formed this way. Lighter in color and density, granite is perhaps the most common intrusive rock in the Earth's crust. Hard, durable granite, with its attractive pink or gray colors and speckled array of light and dark minerals, makes an ideal ornamental building stone. New England is particular famous for its many fine granite quarries.
Igneous rocks are still being formed on Earth—for example, when new plate material is formed at diverging boundaries or in active volcanoes. In other places, such as Yellowstone Park region, hot springs and geysers revel hidden sources of underground heat and may indicated places where intrusive rocks are forming today.
Sedimentary Rocks:
When the first rains began to fall on the first igneous rock, the process of weathering began. Small grains washed off the recently harden volcanic rocks, flowed down through streams and rivers into the seas, and they deposited on the sea floors. There fast moving waters of the rivers met the slower currents of the oceans. Over time, layers of sediment accumulated, especially at the mouth of rivers near these layers became thicker and thicker. In many places on the Earth right now–the Mississippi River delta that extends into the Gulf of Mexico, for example–layers of sediment may reach several kilometers (miles) in thickness.
As the first sediments became buried deeper, temperatures and pressure on them increased. In addition, water flowed through the layers of sediment, dissolving and redeposting glue-like chemicals–sometimes like the crusty deposits that build up on an ordinary faucet when water drips continuously. The net result of all of these process–pressure, heat and the effects of mineral–layden water–was to weld the bits of sediment together into new layered rocks. This kind of rock, appropriately called sedimentary rock, contains grains of material worn off previous rocks. Other common sedimentary rocks, include salt deposits, may form from layers of chemical precipitates.
While sedimentary rocks can form at the base of a single mountain or cliff, the collection of grains often comes together from many different places, The grains in a single fragment of sedimentary rock being formed in the Mississippi delta, for example may have come from a cliff in Minnesota, a valley in Pennsylvania, and a mountain in Colorado. Similarly, sediments deposited near the mouth of the Colorado River carry bits of history from much of the North American West. Deltas inevitably contain particles from all rocks in their rivers' drainage system.
As you travel across the United States you will encounter many common varieties of sedimentary rocks. They are easy to spot in road cuts and outcrops because of their characteristic layered appearance, like the pages of a book or a many–layered cake.(444) Sandstone forms mostly from sand–sized grains of quarts (silicon dioxide or SiO2), the commonest mineral at the beach, and from hard material and rock fragments. Many sandstone's represent ancient beaches, deserts, or steam beds–places where concentrations of sand are found today. Sandstone's usually feel rough to your touch, and you can just barely see the individual grains that are cemented together.
Shales and mudstone form from sediments that are much finer grained than sand. These rocks commonly accumulate beneath the calm waters of lakes or in the deep ocean basins, places often teeming with life. There organisms, both large and small, die and are buried in muddy ooze, where they may eventually form into fossils SEE THE CONNECTION!!! that provides archeologists with information about the evolution of life on Earth.
Limestone, another distinctive type of sedimentary rock, forms from the calcium carbonate (CaCO3) skeletons of sea animals. Some limestone's grow from a gradual rain of microscopic debris or broken shells, while others represent a coral reef that spread across the floor of a shallow. Like shales and mudstones, limestone's commonly bear fossils.
Given what we know about plate tectonics, and about the constant movement of materials around the surface of the Earth, it should come as no surprise that just because sedimentary rocks originally formed at the bottom of the ocean, they have not necessarily stayed there since formation. Indeed, it is not all unusually to see sedimentary rocks in mountain passes, thousands of meters above the ocean, or in the middle of continents thousand of kilometers from the nearest open water. Some of the best exposures of limestone's is in Nashville Tennessee, and on the cliffs of Lookout Mountain high above Chattanooga, TN., far removed from the ocean.
One of the best places to get an appreciation of sedimentary rocks is at the beach. If you pick up a handful of sand, you will notice that each grain is different. Some are dark colored, some are light, some have sharp angular edges, some are smooth and worn down. Each of these grains of sand once was part of a rock in a drainage of rivers that feed into the ocean. As the rock weathered away, each grain was chipped off and carried to the sea by wind and water. Eventually, the grains of sand you hold in your hand will form into solid rock and subjected to the forces of plate tectonics. That sandstone may some day be uplifted to an altitude well above sea level, where the grain is weathered again and start the whole cycle all over. Each grain of sand in your hand, then may have made the trip from rock to beach to sandstone many times in its life.
The story of the grain of sand provides a good model for the way materials move about the surface of the Earth. The atoms of rocks, just like those of the air, water, or your body, are always around, but it is always the same matter–the same atoms–recycling.
Metamorphic Rocks:
It may happen that sedimentary rocks are buried deep within the Earth's and thus subjected to intense pressure and heat. There they will be turned into yet another kind of rock, transformed by the Earth's extreme conditions into metamorphic rock. If shales or mudstone are buried like this they may eventually turn into brittle, hard slates, the kind of rock that school blackboards were made of. Even higher temperatures and pressure can transform slates into spectacularly banded rocks called, schists and gneisses, which often boast fine crystals of garnets and other high–pressure minerals. Roadcuts and outcrops of these metamorphic rocks can look like an intensely folded cloth or a giant cross–section of swirled marble cake. Sandstone's, when exposed to high temperature, also metamorphose, recrystallizing to a durable rock in which the original sand grains fuse into a solid mass known as quartzite.
The Story of Marble
Of all the metamorphic rocks, none tells a more astonishing tale than marble, a rock of extraordinary beauty. If you ever travel the roads of Vermont, chances are you will pass an outcrop or roadcut of distinctive greenish–white cast, a rock with intricate bands and swirls. These marble take a high polish and are a prize for any sculptor or architect for centuries. However, no works of humans can match the epic process that formed the stone. Most marbles began as limestone, rocks that originate primarily from the skeletal remains of sea life. Over the ages, these limestone's in the area we now call Vermont were buried deeper and deeper, crushed under the weight of many kilometers' thickness of sands and shales and more limestone's in an ancient sea. However, no ocean or sea can last forever in our dynamic planet.An ancient collision of the Eurasian and North American plates compressed and deformed this ocean basin, crumpling the layer rock into tight folds and subjecting the sedimentary pile to intense temperatures and pressures. The buckled and contorted formations were uplifted to high elevations when the Appalachian Mountains formed. During the intense pressures and high temperatures associated with the converging tectonic plates, the limestone's were metamorphosed to the marble that we used today. Many millions of years of erosion and uplift have exposed these ancient rocks, gradually to weather away and begin the cycle again. Humans, in a futile quest for immortality, quarry the marble for their monuments and tombstones and other transient reminders of Earth's incessant change.
Igneous, sedimentary, and metamorphic rock all participate in the rock cycle. Igneous rock, once formed, can be weathered to form sedimentary rocks, and can themselves undergo metamorphism. Layers of sedimentary rocks also can be transformed into metamorphic rocks. All three kinds of rocks can be subducted into Earth, partially melted, and reformed as new igneous rocks. Thus the rock cycle never ceases.
