4.3: Rocks

Rocks, in the form of what is called bedrock, are exposed at the Earth’s surface over wide areas, and they everywhere underlie the unconsolidated surficial materials, called regolith, you will learn about later in this chapter. Moreover, owing to their physical and chemical breakdown under the conditions of the Earth’s surface environment they are the source of that surficial material. There’s no room in this course for a systematic and detailed treatment of rocks—that’s the province of an introductory course in geology—but you should know at least some general things about them.

Rocks are naturally occurring aggregates of minerals. Again, that succinct definition is not very revealing of the nature and variety of rocks. I suppose that it verges on common knowledge that there are three kinds of rocks: igneous, sedimentary, and metamorphic. These three categories are for the most part distinct, although there is some fuzziness to the boundaries.

Igneous rocks are those that form by cooling and solidification of magma. Magma is the term for melted rock in the Earth’s interior. At certain times and in certain places in the Earth’s shallow interior, down to a hundred or so kilometers, the rocks of the upper mantle or lower continental crust melt, to form magma. The melting is only partial, commonly up to fifteen to twenty percent of the rock, but the magma collects and then moves upward owing to its buoyancy (it’s slightly less dense than the surrounding rock). It either becomes parked in subsurface spaces, called magma chambers, there to cool to form coarse-grained igneous rocks like granite or gabbro, or it rises all the way to surface to be ejected from volcanoes, either as flowing liquids or as already solidified particles.

Most of the igneous rocks you are likely to encounter as bedrock outcrops on the continents are of three general kinds (although this is a great oversimplification of the wide range of igneous rocks):

• coarse-grained and light-colored rocks like granite, which consist mainly of feldspar with some quartz;
• fine-grained and dark-colored rocks like basalt, which consist mainly of ferromagnesian minerals and feldspars; and
• rocks, called tuff, that form by lithification of volcanic ash, the solid particles that are ejected explosively from certain kinds of volcanoes.

Sedimentary rocks are those that form from sediments that are deposited at the Earth’s surface and become lithified (turned into solid rock, by a variety of processes) as they are buried more and more deeply in the earth’s crust. Sedimentary rocks vary widely in composition and origin. There are two major realms of sediments and sedimentary rocks: siliciclastic and chemical. Siliciclastic sediments and rocks are those that consist of particles of minerals or rock that are derived from weathering of bedrock on the continents. Chemical sediments and rocks are those that are precipitated from natural surface waters, mainly in the ocean. By far the most important chemical sediments are carbonate sediments. Most carbonate sediment is produced by carbonate-secreting organisms, like clams or corals, although some is precipitated inorganically, in somewhat the same way that rock candy grows in a beaker on your windowsill.

Most of the sedimentary rocks you are likely to encounter are of the following kinds. (Again, this is a highly simplified list, in large part because these major sedimentary rock types grade imperceptibly into one another.)

• sandstones: Sandstones consist of sand-size particles of various minerals, mainly quartz but also potassium feldspar, as well as sand-size fragments of fine-grained rocks.
• shales: Shales consist mostly of very fine-grained mineral particles of quartz and clay minerals.
• limestones: Limestones consist mainly of the calcium carbonate mineral calcite. Also abundant are dolostones, which consist mainly of the carbonate mineral dolomite.

Metamorphism refers to changes in mineral composition or rock geometry that occur in solid rocks with increasing temperature and pressure. These changes produce a rock called metamorphic rock. It is important to remember that changes producing metamorphic rocks take place while the rock is solid. Any part of the rock that is melted eventually cools to form an igneous rock.

The basic idea behind changes in mineral composition during metamorphism is that each mineral has associated with it a range of temperature and pressure in which it is stable, in the sense that it remains in existence indefinitely. When a mineral is out of its stability range, it is liable to be converted into, or replaced by, one or more other minerals that are stable at those temperatures and pressures. A simple way of phrasing this is that new minerals grow at the expense of the original minerals. These changes in mineral composition act very slowly, because the rock remains solid and atoms can be exchanged between adjacent minerals grains only by atomic diffusion—although very small percentages of pore solutions along grain boundaries can expedite the transport of atoms.

Deformation (change in shape) is an important factor in the development of many metamorphic rocks. For most kinds of metamorphic rocks, change in mineral composition is accompanied by strong shearing deformation. Strong shearing tends to obscure or even obliterate original geometrical features like stratification in a sedimentary or volcanic rock. Also, most metamorphic rocks show some degree of what is called foliation: development of planar features in the rock, like layering or a tendency for the rock to split along parallel planes.

Below are brief descriptions of the most common metamorphic rocks. As with the sedimentary rocks, there are all gradations among these types.

• slate: Slate is produced by low-intensity metamorphism of shale. Individual crystals are too small to see, but the rock has a strong tendency to split along parallel planes, as a result of growth of new mica crystals of sheet-silicate minerals all parallel to one another.
• phyllite: Phyllite is produced when slate undergoes further metamorphism. Phyllite typically has a silky sheen.
• schist: Schist is formed by metamorphism of many igneous and sedimentary rocks. It has clearly visibly flakes of sheet-silicate minerals (muscovite, biotite, and/or chlorite), and it tends to split along parallel planes in the same way as slate and phyllite.
• gneiss: Gneiss consists mainly of granular minerals like feldspar, quartz, and ferromagnesian minerals. Gneiss is often prominently layered.
• quartzite: As the name implies, quartzite consists mainly of quartz. It forms mostly by metamorphism of quartz-rich sedimentary rocks.
• marble: Marble consists mainly of calcite. The calcite crystals tend to be large enough to see with the unaided eye, having grown larger during metamorphism without change in overall mineral composition.