Texas Rocks and Minerals (eBook)

An Amateur's Guide

Roselle M. Girard (Autor)

eBook Download: EPUB

2016 | 1. Auflage
244 Seiten
anboco (Verlag)
978-3-7364-1649-9 (ISBN)

Infos

Earth's outer crust Geologists Time and rock units Geologic map What are rocks and minerals? Chemical elements Minerals Rocks Igneous rocks Extrusive or volcanic igneous rocks Intrusive igneous rocks Sedimentary rocks Soils Sedimentary rock materials in broken fragments Sedimentary rock materials in solution Cementing materials and chemical sediments Sedimentary rocks formed by plants and animals Metamorphic rocks Static metamorphism Contact metamorphism Dynamic metamorphism Occurrence and properties of minerals How minerals occur Crystalline minerals Crystals Imperfect crystals Amorphous minerals Some distinguishing properties of minerals Color Luster Transmission of light Hardness Streak or powder Cleavage Parting Fracture Specific gravity Effervescence in acid Some special occurrences of minerals Cave deposits Concretions Geodes Petrified wood Collecting rocks and minerals Rock and mineral identification charts How to use the mineral identification charts Key to mineral identification charts Mineral identification charts How to use the rock identification charts Rock identification charts Descriptions of some Texas rocks and minerals Anhydrite Asbestos Barite Basalt Calcite Cassiterite Celestite Cinnabar Clay Copper minerals (chalcocite, chalcopyrite, malachite, azurite) Dolomite Feldspar Fluorite Galena Garnet Gneiss Gold Granite Graphite Gypsum Halite Hematite Limestone Limonite Llanite Magnetite Manganese minerals (braunite, hollandite, pyrolusite) Marble Mica Obsidian and vitrophyre Opal Pegmatite Pyrite Quartz Quartzite Rhyolite Sand and sandstone Schist Serpentine Shale Silver minerals (argentite, cerargyrite, native silver) Sulfur Talc and soapstone Topaz Tourmaline Uranium minerals (carnotite, uranophane, pitchblende) Volcanic ash (pumicite) Composition, hardness, and specific gravity of some Texas minerals Books about rocks and minerals Nontechnical books for beginners Textbooks and other reference books Selected references on Texas rocks and minerals

Textauszug

A mineral is made up of chemical elements. The mineral calcite, for example, always consists of the same proportions of calcium, carbon, and oxygen.

Each mineral has its own characteristic internal structure and other properties. At ordinary temperatures, nearly all the minerals are solids rather than gases or liquids. (Water and mercury are the principal exceptions.) In addition, minerals are inorganic rather than being composed of plant or animal matter.

 

When a single chemical element is found alone in nature as a solid, it is considered to be a mineral, too. Gold, silver, copper, lead, and sulfur are some of the chemical elements that can occur alone as solid minerals. When they occur this way, we refer to them as native silver, native copper, or native sulfur. Although the element mercury is a liquid rather than a solid at ordinary temperatures, it too is a mineral when it occurs alone in nature. It is then called native mercury.

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...

Produkt-Details

• Erscheint lt. Verlag: 25.9.2016
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geologie
• Themenwelt: Technik
• ISBN-10: 3-7364-1649-0 / 3736416490
• ISBN-13: 978-3-7364-1649-9 / 9783736416499