Part 2

By examining and studying the different rocks and rock layers, geologists try to discover in which unit of geologic time these rocks formed. Those rocks that formed during a period of geologic time are called a system of rocks; those that formed during an epoch are called a series. For example, the Cambrian System of rocks formed during the Cambrian Period; the Cretaceous System of rocks formed during the Cretaceous Period; the Tertiary System of rocks formed during the Tertiary Period. We are now in the younger epoch (called Recent) of the Quaternary Period of the Cenozoic Era. The rocks that are forming now are the Recent Series of rocks.

ERA PERIOD EPOCH CENOZOIC QUATERNARY (lasted 0-1 million years) Recent Pleistocene TERTIARY (lasted 62 million years) Pliocene Miocene Oligocene Eocene Paleocene —63 million years ago— MESOZOIC CRETACEOUS (lasted 72 million years) JURASSIC (lasted 46 million years) TRIASSIC (lasted 49 million years) —230 million years ago— PALEOZOIC PERMIAN (lasted 50 million years) PENNSYLVANIAN (lasted 30 million years) MISSISSIPPIAN (lasted 35 million years) DEVONIAN (lasted 60 million years) SILURIAN (lasted 20 million years) ORDOVICIAN (lasted 75 million years) CAMBRIAN (lasted 100? million years) —600? million years ago— LATE PRECAMBRIAN EARLY PRECAMBRIAN

These time estimates are from the paper, Geologic Time Scale, by J. Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961. (The time divisions are not drawn to scale)

EXPLANATION CENOZOIC 1 Quaternary 2 Tertiary (Oligocene, Miocene, and Pliocene) 3 Tertiary (Eocene) 4 Volcanic (extrusive) igneous rocks MESOZOIC 5 Upper Cretaceous (Gulf series) 6 Lower Cretaceous (Comanche series) 7 Jurassic 8 Triassic PALEOZOIC 9 Permian 10 Mississippian and Pennsylvanian 11 Cambrian, Ordovician, Silurian, Devonian and undivided Paleozoic 12 Rocks (Precambrian) older than Paleozoic 13 Intrusive igneous rocks (Precambrian, Mesozoic or Cenozoic)

These rocks are found either at the surface or directly beneath the soils and subsoils which cover most of Texas.

Geologists also subdivide rocks into lesser units. One of these, called a group, is made up of two or more formations. A formation comprises rocks or strata (layers of rock) that are recognized and mapped as a unit. Some formations consist of layers of one particular type of rock, such as limestone or shale. Formations are named after a nearby geographic locality, and in some formation names, the type of rock is included. For example, three of the Texas geologic formations are called Buda Limestone, Del Rio Clay, and Eagle Ford Shale.

Geologic Map

The geologic map (pp. 4-5) shows the rocks that are found at the surface in Texas. Some of these are extremely old. Some, geologically speaking, are very young.

WHAT ARE ROCKS AND MINERALS?

Although rocks and minerals are often mentioned together, and to some people they have similar meanings, geologists make a distinction between the two words. In general, rocks are made up of minerals, and minerals are made up of chemical elements.

Chemical Elements

The chemical elements include oxygen, silicon, calcium, sulfur, carbon, gold, silver, and many others. There are 90 naturally occurring elements. Each is made up of molecules that consist of only one kind of atom. Chemical elements may either be combined with each other or occur alone. They are the building blocks of our world for they make up all the gases, all the liquids, all the minerals, all the plant and animal life, and all the other physical matter. Some of the chemical elements that occur in the rocks and minerals mentioned in this book are listed below.

Aluminum Al Barium Ba Beryllium Be Boron B Calcium Ca Carbon C Cerium Ce Chlorine Cl Copper Cu Fluorine F Gold Au Hydrogen H Iron Fe Lead Pb Magnesium Mg Manganese Mn Mercury Hg Molybdenum Mo Oxygen O Potassium K Silicon Si Silver Ag Sodium Na Strontium Sr Sulfur S Thorium Th Tin Sn Uranium U Vanadium V Yttrium Y Zinc Zn Zirconium Zr

We can compare the chemical elements to the letters of our alphabet. The letters, like the chemical elements, are fundamental building blocks, and they can be brought together in various combinations to form words.

Minerals

A mineral can be compared to a word of our language. We combine letters to form a word, and nature combines certain chemical elements to form each particular mineral. For example, calcite, a mineral that is abundant in Texas, is always made up of the same proportions of the same three elements: 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.

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.

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.)