Rocks
We have already compared the chemical [elements] to the alphabet and the minerals to words. We can now go a step further and compare rocks to sentences. We put words together to make sentences; nature puts minerals together to make rocks. A sentence does not have to be made up of a definite number of words, nor does a rock have to be made up of a definite number of minerals. Some rocks, such as [granite], may be composed of several minerals. Others, such as [dolomite] and rock [gypsum], consist of only one mineral.
Minerals do not lose their identities when they make up a rock. Instead, they are merely associated together in varying proportions. Some rocks, as we will find later, instead of being composed of the minerals themselves, are made up of fragments of earlier-formed rocks.
Ordinarily, we think of rocks as hard and solid substances, such as [limestone] and [granite], but some geologists consider loose and uncemented materials, such as [sand], [gravel], or [volcanic ash], to be rocks also. The words [sediments] or deposits are often used to describe this uncemented or loose material.
Rocks are commonly grouped, according to how they formed, into three great classes known as [igneous], [metamorphic], and [sedimentary].
A rock is made up of minerals. The [igneous] rock [granite], for example, consists chiefly of [quartz] and [feldspar]; other minerals such as [mica] and hornblende are commonly present.
[IGNEOUS ROCKS]
[Igneous rocks] result from the cooling of hot, molten rock material or [magma]. Magma that reaches the surface through volcanoes is called [lava]. Magma comes from deep within the earth and is made up of a mixture of molten mineral materials. Igneous rocks have been forming throughout the geologic past and are still forming today. We can understand how they form when we look at pictures of hot, molten lava flowing from volcanoes, such as Mauna Loa in Hawaii. As lava cools, it hardens into rock.
[Extrusive] or [Volcanic] [Igneous] Rocks
The [igneous rocks] that form on the earth’s surface are called [extrusive] or [volcanic] igneous rocks. When [magma] flows to the surface, it cools and hardens quickly. The mineral grains that form during this fast cooling may be too small to be distinguished from each other. Some [lava] cools too quickly for minerals to crystallize—then the rock is volcanic glass.
[Extrusive] [igneous rocks] form at the earth’s surface from [lava] that cools and hardens relatively quickly.
No [volcanic] [igneous rocks] are forming in Texas now. However, during [Tertiary] time, in the Big Bend area and in other parts of the Trans-Pecos country of west Texas, [lava] came to the surface and hardened. (The [physiographic outline map], [p. 42], shows where these areas are located.)
[Intrusive] [Igneous] Rocks
The cooling and hardening of hot, molten [magma] also takes place below the earth’s surface. Here, the magma cools slowly to form rocks made up of mineral grains that are large enough to be readily visible. These rocks are known as [intrusive] [igneous] rocks. We know that they are present below the surface in Texas because of wells drilled in many areas of the State. In Pecos County, a well reached [granite], an intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas have reached the granite basement rocks at much shallower depths. But not all intrusive [igneous rocks] in Texas are found underground. In the Trans-Pecos country of west Texas, in the [Balcones fault zone], and in the [Llano uplift] of central Texas, some are now seen at the surface. They, like all [intrusive rocks], were formed below the ground, but earth’s processes of uplift and erosion have gradually uncovered them.
[Intrusive] [igneous rocks] form from molten rock material ([magma]) that cools and hardens beneath the earth’s surface.
[SEDIMENTARY ROCKS]
[Sedimentary rocks] are made up of [sediments], which are rock and mineral grains that have come from weathered rocks of all kinds. Rocks are weathered when water, ice, snow, wind, and other agents cause them either to dissolve, as table [salt] does when put in water, or to break apart, as old pavement commonly does.
Soils
Some of the broken-down rocks, along with associated plant and animal matter, develop into soils. When you examine soil with a magnifying glass, you may be able to see some of the small rock and mineral grains that still remain in it. Some soils have formed on top of the rocks from which they came, and some have been moved in from another place.
Soils develop from weathered rock and associated organic material.
SOIL SUBSOIL WEATHERED ROCK BEDROCK
[Sedimentary] Rock Materials in Broken Fragments
Water and wind not only weather the rocks and soils but also move the weathered materials (the [sediments]) and deposit them in other places. Whenever you see a dust or [sand] storm, or a muddy creek or river, you are observing the movement of sediments by wind and water to other land areas or to the sea. The combination of weathering and movement is called erosion.
[Conglomerate] from Webb County, Texas, is composed of rounded [gravel] that has been cemented together.
Some of the rock fragments carried by water are still fairly large when they reach their destinations. On the basis of size, they are called [boulders], [cobbles], [pebbles], and [granules]. Loose deposits of these larger-size [sediments] make up what is known as [gravel]. Nature cements gravels together to form rocks such as [conglomerates] (made up of rounded gravel) and [breccias] (made up of sharp-cornered gravel).
The finer [sediments] are called [sand], silt, mud, and [clay]. When cemented, the sand grains become sandstones, the silt particles become siltstones, and the mud and clay particles become [shale]. The [sedimentary] rocks that are made up of these rock fragments are called [clastic] or fragmental rocks.
[Sedimentary] Rock Materials in Solution
As they are weathered, some rocks dissolve and go into solution. For example, a number of the Texas creeks and rivers carry calcium carbonate in solution because they flow through areas where [limestone] rocks, which consist mostly of calcium carbonate, are being weathered. (Water that contains a large amount of dissolved rock material is called hard water.)
Cementing materials and chemical [sediments].—
Some of the waters containing dissolved rock material seep through loose [sediments] where the dissolved material may come out of solution and form a cement, which binds the sediments together. For example, when loose [sand] sediments are cemented, they form [sandstone]. Three of the most common cements are iron oxide, calcium carbonate, and silicon dioxide, although a number of other materials also serve as cements.
Dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some [sedimentary rocks] as well. [Sedimentary] rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers. When the dissolved material comes out of solution, it is said to be precipitated and the mineral [sediments] it forms are the chemical sediments. Some limestones originate this way. You can see examples of precipitated materials by noting the crust-like deposits that form inside some water pipes and teakettles, as dissolved material in the water comes out of solution.
Precipitated [sediments] are commonly observed lining a teakettle.
[Sedimentary rocks] formed by plants and animals.—
The dissolved rock material can come out of solution in another way. Some plants and animals are able to take dissolved calcium carbonate out of the sea water and use it to build their shells and other structures. Some of these organisms, such as corals and algae, can grow upward from the sea floor in large groups to form reefs that later become reef limestones. Other limestones are made up of the remains of plants and animals that collect on the sea floor and become cemented together.
[METAMORPHIC ROCKS]
[Metamorphic rocks] come from earlier-formed rocks that have undergone a change or a metamorphosis. All [igneous] and [sedimentary rocks], and earlier-formed metamorphic rocks too, can be changed, without being moved to some other place, into new and different rocks. As they are changed, they may become harder, new minerals may form, and they may look entirely different. For example, [granite], an igneous rock, can be changed into the [metamorphic rock] known as [gneiss]; [limestone], a [sedimentary] rock, can be changed into [marble]; [shale], a sedimentary rock, can be changed into slate. These changes occur because the earth is a big and complex chemical [system]. The agents that bring about these changes, which always occur below the surface of the earth, are heat, pressure, and [fluids]—both liquids and gases. Several different kinds of change or metamorphism can take place.
Static Metamorphism
Some of the changes occur because the rocks are at great depths. As more and more younger rocks are deposited on top of them, the older rocks become deeply buried. The great thicknesses of younger rocks are heavy, and they squeeze and press down on the rocks beneath them. The deeply buried rocks are also hotter than surface rocks. In general, the temperature increases about 1° Fahrenheit for each 50 feet of depth below the surface. The change of deeply buried rocks into new rocks by pressure and heat is known as static metamorphism.
Contact Metamorphism
Another method of change or metamorphism involves molten [igneous] rock material. When hot [magma] moves up through rocks, it not only heats and pushes them, but it also may soak them with liquids and gases, causing the nearby rocks to change into new rocks, by a process called contact metamorphism.
Some rocks are altered by heat and [fluids] when they are invaded by hot [magma] in a process called contact metamorphism.
UNALTERED ROCK [METAMORPHIC ROCK] [MAGMA]
Dynamic Metamorphism
Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, [fracture], or slide over each other. These forces cause great changes in the rocks in widespread areas. This process of change is known as dynamic metamorphism.