A. INTRODUCTION

Overview

Do you know where the Ural Mountains are? the Atacama Desert? the East Pacific Rise? Your tour of Planet Earth will take you to many of its lesser-known reaches, and you will need a guide to help find your way. This unit can serve as such, and you should develop the habit of referring to it frequently throughout the course. It begins with a description of the major surface features of Earth: the continents and ocean basins. It then describes some of the rock types that you will encounter again and again in later units. Finally, a map of the world is keyed to every place and region name used throughout the book. A handy table enables you to look up any place name and find out where it is.

B. THE FACE OF THE EARTH

The picture beginning this chapter shows the familiar view of our planet from space: the deep blue of the oceans contrasting with the tan of the continents, spangled with white clouds and softened by the haze of the atmosphere. We are a very different kind of planet from the others in our solar system, and the strong contrast between continents and oceans is one of our most striking features.

The contrast is not only one of rock and water, though the visible presence of liquid water is unique in itself. The waters are gathered into oceans because these are the low points of the Earth's surface. Even if there were no water at all on Earth, the ocean basins would be remarkable features. In fact, the elevations of the rocky surface are dominated by two elevation ranges: that of sea level to 1,000 meters (3,300 feet) above it and that of 4,000 to 5,000 meters (13,000 to 16,400 feet) below sea level. Figure 2-1 shows graphically the distribution of elevations on Earth. The curve starts at the left at the highest elevations (8848 m or 29,030 ft at Mt. Everest) and proceeds to the lowest (11,000 m or 36,100 ft below sea level in the Mariana Trench). Each elevation is shown in direct proportion to the area of the Earth that stands at that elevation. Note how the two elevation ranges just cited mark the continental and oceanic platforms, which together account for the lion's share of the surface. The continental slope bridges these two elevations, while the extremes -- mountains and oceanic trenches -- take up only a tiny fraction of the available acreage.

If we look at a map of the world today, we can see it divided into two parts, land and water. The world's oceans make up about 71% or the earth's surface, compared with 29% made up by continents. On a world map the topography and bathymetry (underwater topography) is surprisingly consistent. On land most elevations are less than one kilometer above the sea surface. Of the total world's surface 20.8% is between 0 and 1 kilometer in elevation. The oceans also display surprising consistency. Most of the world's sea bottom is between 3 and 4 kilometers in depth. Of the total worlds surface 22.6% is between 3 and 4 kilometers below sea level. We have different terms for describing the continental geography. We call the region of continent, that is large land masses like North America or Australia, etc, that are above sea level the continental platform. That's because they are a platform above the surrounding ocean floor. Generally sea level goes up and down, so that the present day shore isn't a good indicator of where the continents begin and end. A better indicator is the continental shelf. This is defined as the sharp break between the relatively flat continental slope and the more steeply dipping continental slope.

Earth is indeed a split-level planet. The liquid oceans are where they are because that is where there is room for their waters. In places, the oceanic waters lap onto the edges of the continents, forming shallow seas floored by continental, not oceanic, rocks. These are the continental shelves which in places like the Grand Banks off Newfoundland or the Falkland Banks near southern Argentina can extend hundreds of kilometers beyond the present-day shorelines.

As it happens, the rocks flooring the oceans and continents differ from one another, as we shall see in the Plate Tectonics unit. There, also, we shall investigate the reasons for this dichotomy of elevations. But before we can do that, we need to learn something about the rocky materials of the Earth's surface.

C. ROCK TYPES

Rocks are divided into three varieties, based on their mode of origin: igneous, sedimentary, and metamorphic. Igneous rocks are produced by freezing of molten rock, which is called magma within the crust of the Earth, or lava when it erupts onto the surface.

Sedimentary rocks are formed when grains of sediment (mud, clay, sand, gravel, skeletal remains of plants and animals) are deposited by water, wind, or ice, or from low-temperature chemical precipitation, such as the precipitation of the mineral salt from evaporating water. Sedimentary rocks are classified on the basis of their texture, that is the size and shape of their mineral grains, and composition, both of the grains and the cement holding the sedimentary rock together. In the field, we generally identify sedimentary rocks on the basis of their grain size. Very fine grained sedimentary rocks are called siltstone or mudstones. Often these grains are so small they can only be seen with a hand magnifying lense. Sandstones have intermediate grain size, and the largest sedimentary rocks are called conglomerates or breccias depending on whether their large, pebble to block-sized, grains are rounded or angular. There is another class of sedimentary rock that is not mechanically deposited; instead it is chemically deposited. These are called limestones and evaporites. Evaporites are formed when salt-rich water dries, leaving behind a layer of salt and other crystals. Limestones are formed by the precipitation of calcium carbonate from sea water to form thick sediments of only this precipitated material.

All clastic sediment is derived from previously existing rocks. With time, these sediment grains may become lithified (made into rock) by burial, compaction, and cementation, often by precipitation of minerals that bind together the sediment grains into solid rock. Sandstones are formed from sandy deposits, shales from mud, and limestones from calcium-rich sediments -- often made up largely of the shells and skeletal remains of marine creatures large and small. Sedimentary rocks form a relatively thin veneer covering some three-quarters of Earth's surface but making up only about one-twentieth of the crust by volume.

Metamorphic rocks are any rocks that have been heated and/or squeezed to such an extent that their chemical and physical makeup are altered. Geologists noticed in the middle of the 1700s that in some regions sedimentary rocks would slowly change character over a region or near an igneous pluton or basalt dike. Sedimentary rocks where the original bedding was clear would become harder, and have a different orientation of fractures. This lead to the naming of a new class of rocks. Metamorphic in Greek simply means to change form. Today we define metamorphism as mineralogical changes that can be correlated with changes in heat or pressure. Metamorphic rocks are identified on the basis of their mineralogy and structure. The most common structure observed in these rocks is foliation. Foliation is a closely spaced layering observed in metamorphic rocks. If this layering is 1 mm or more thick and often very sinuous, this rock is called a gneiss. If the layering is finer, and generally more planer, this rock is a schist. From experimental studies it is known that different phases of some minerals exist under different heat and temperature conditions. Earth scientists have found that some minerals can only be made under specific conditions of heat and temperature. For example, the following reaction only occurs at temperatures of 500° C or slightly above.

This is the reaction that takes chert and limestone and turns it into a new mineral called Wollastonite, and carbon dioxide gas. Wollastonite forms in a rock that has been heated under low pressure to at least this temperature. Such "Index Minerals," minerals of distinctive form and composition, are used to define "facies" of metamorphism. Metamorphic facies are defined as, any association of metamorphic rocks within which there is a constant correlation between thermal and chemical composition. The most important metamorphic facies, and the distinctive minerals that define them, are shown below.

There are two main types of metamorphism. Contact metamorphism refers to changes close to an intruding igneous rock such as a pluton or dike. Regional metamorphism refers to metamorphism observed over a wide area. This type of metamorphism probably represents pressures and temperatures occurring in some region of the earth, since been uplifted to the earth's surface at a later date.

Two important pressure and temperature environments of metamorphism are the blueschist and greenschist facies. The blueschist facies is only formed in low temperature, high pressure environments; greenschists only form in more intermediate pressures and temperatures. Blueschists and greenschists are given their respective names because of the distinctive blue and green minerals contained within them.

Igneous rocks are the primary rocks deep within the Earth. To be able to deal with them, you should understand a brief and simplified classification scheme, given in Figure 2-3. This scheme is based on the chemical composition of the rock (represented by the four vertical columns) and the texture of the rock (represented by the two horizontal rows).

The chemical classification is based on the abundance of certain minerals within the rock, especially silica (silicon dioxide, or SiO₂). Rocks with a high silica content are termed felsic; those with a low silica content are termed mafic. Felsic rocks tend to be light in color and have a low density; mafic rocks are usually dark in color and have a higher density.

There is another chemical class of igneous rocks -- the ultramafic rocks. These make up much of the deeper part of the Earth, called the mantle, and in our classification scheme are found to the right of basalt and gabbro. They are dark in color and even denser than the mafic rocks. The rock type listed, peridotite, is only one of several different types of ultramafic rocks.

The physical classification is based on the texture, or grain size within the rock. When a mass of magma cools slowly deep within the Earth, its atoms have time to aggregate into large grains of individual minerals: the rock becomes coarse-grained. On the other hand, lavas on the surface generally cool quickly, resulting in fine-grained rocks.

Granites are familiar rocks, often used for decorative or memorial purposes. Its large grains give it a speckled appearance, and its overall color is often a light gray. Basalt, on the other hand, is a dark, nearly black rock that looks almost uniform in color. Only close inspection with a lens will reveal the small grains of the mostly dark minerals that constitute it. Andesite is a fine-grained rock of intermediate chemical composition. Rhyolite is a fine-grained rock whose chemical composition is very similar to that of granite, but differs from it in grain size.

Study Figure 2-3 and become familiar with the rocks shown in it. We shall refer especially to basalts, granites, andesites, and rhyolites in later units. Several centuries ago, Neptunists believed that all rocks were precipitated from a great ocean. In the later 1700s, however, experiments were done melting and recrystallizing rocks. These experiments showed that basalt could be formed by slowly allowing a liquid melt to cool, and that basalt is an igneous rock, that crystallized from a melt directly. Igneous rocks when they crystallize beneath the earth's surface are called intrusive rocks. This is because they intrude into other geological units. Extrusive rocks are igneous rocks that crystallize from a melt, but above the earth's surface. For example, a lava flow slowly cooling on the earth's surface forms an extrusive igneous rock. The discovery that basalt was an igneous rock solved the "heated basalt controversy," and effectively "sunk" the Neptunists.

Igneous rocks are classified by the relative abundance of certain key minerals, and whether they are of coarse or fine grained in mineral composition. Figure 2-3 shows how igneous rocks may be classified with respect to their mineral composition.

Granite, (density 2.7 g/cc), a common rock often found on continents, is classified as a coarse-grained, igneous, rock that contains the minerals quartz, K-feldspar and a few mafic minerals. Granite differs from a rock like basalt in two key respects. The first is that basalt is a fine-grained, igneous, rock and granite has coarse, or large, crystals. The second difference is that basalt, (density 2.9 g/cc), does not contain the minerals quartz or K-feldspar, but does contain mafic minerals pyroxene and olivine.

D. WHERE IS IT?

Figure 2-5 is a map of the world for your reference use. Marked on it are the features and localities mentioned throughout this text. Refer back to it often, and always be sure that you know where you are during our world travels.

Table 2-1 is the key to finding places on the map. The table lists all the features and localities alphabetically and by number. The location code takes you to the appropriate grid block on the map.

Two other important resources are provided for you in the glossary of geological terms and index at the end of this text. Use both liberally and you will find that you are getting far more out of the course.

Figure 2-5: WORLD INDEX MAP (Click on this image for full-size world map)

TABLE 2-1: WORLD INDEX

For an interesting examination of the earth system check Calder (1991). Dotto (1991) give a wonderfully spectacular collection of views of Planet Earth taken from different Space Shuttle missions.

Every student should have access to a world atlas. One of the best of moderate cost is published by the National Geographic Society, Washington D.C.. Your college bookstore probably carries a small and inexpensive atlas by Rand McNally and Co. that will help you to find major cities and geographic features. Please ask your librarian regarding the location of your college’s map collection and explore these unique references.