Who hasn’t stopped to pick up a pebble, to admire or wonder about its color or texture, or just to toss it in your hand? Rocks speak to us, especially as children, and for some people, like geologists, sculptors, stone masons, and rock hounds, the fascination with rocks lasts a lifetime. Of course rocks can’t really talk, but they can tell us about the history of our rocky planet and its living creatures, if we know where and how to look.
Every pebble we find is part of the geosphere, the rocky covering of our planet. The Earth’s crust is a layer of rock from three to thirty miles thick that underlies the land and oceans. Beneath the crust is the mantle, a much deeper, denser, and hotter region of semi-solid rock from 150 to 190 miles thick that surrounds the Earth’s core. The geosphere includes all the solid rock, sediment, sand, soil, and molten rock in the crust and upper mantle. This is an active region where rocks are formed and changed, where they get buried or pushed up to the surface, worn down by erosion or compacted and cemented, changed into new rocks or melted into magma.
Collecting rocks is fun for there is such variety to be found. Rocks can form in three different ways, and over the eons, they can change from one kind into another. Igneous rocks form from the cooling and hardening of molten rock (magma). They can be intrusive, like granite, formed when magma cools slowly underground, or extrusive, formed from the fast cooling of lava above ground. Obsidian is a black-colored natural glass that forms from cooling lava, and pumice, a rock so light it can float, comes from the frothy foam on lava.
Sedimentary rock forms from sediments that collect in places like oceans, deserts, or lakes and over time get buried and compacted by the weight of sediments above. Chemicals dissolved in the water fill the spaces, cementing the grains together. Sandstone feels like sandpaper and is composed of tiny grains of sand that eroded from other rocks. Limestone is a sedimentary rock formed from the shells of ancient sea animals, while shale comes from clay deposits.
Metamorphic rocks are made from the transformation of other rocks by intense pressure or heat or both. Without melting, one kind of rock changes into a new kind of rock. The great stresses and heat during earthquakes or plate movements cause physical changes by folding and contorting the rock, as well as chemical changes in its composition. Marble is metamorphosed limestone, and shale becomes slate under heat and pressure. Gneiss, pronounced “nice,” a rock with a striped or banded appearance, is metamorphosed from granite, a rock with a speckled look.
What can we learn from rocks? In granite we can see clues to its chemical composition. Granite forms from magma trapped in seams and spaces in the rocks, deep underground, where it cools slowly over millions of years. A piece of granite that you find might be a billion years old – formed hundreds of millions of years before there were dinosaurs! As magma cools, crystals grow from the chemicals in it, a bit like rock candy made from sugar water. A close look at granite shows speckles or flecks of different colors – black, gray, and white or pink. These are the mineral crystals in granite – mica, quartz, and feldspar.
Minerals are naturally occurring solids with crystal form and a definite chemical composition. There are more than 3000 known minerals, though only a few are very common. All rocks contain minerals, some just one kind, and others have several. Depending on the composition of the magma, different minerals will form as it cools, each kind growing at a specific temperature. In the case of granite, mica crystallizes first, forming crystal facets that we see as tiny reflective plates. Feldspar comes next, growing among the mica crystals. The last mineral to form is quartz, filling in the spaces in the rock, with little room to form regular crystal shapes. The end result is a rock with interlocking minerals easy to distinguish by eye. Growing crystals takes time, so a magma that cools slowly underground can form larger crystals than a magma that cools quickly above ground as lava from a volcano.
The word crystal conjures up pictures of diamonds and emeralds – and rightly so, for all gemstones are minerals. But so are all metals, including gold and silver. Most minerals that we find in rocks look nothing like they do when they’ve been cut and polished. Even pure mineral samples can be hard to tell apart, but a few simple tests can help with identification. Luster is a mineral’s sheen. It can be shiny like metal, glassy, pearly, opaque, dull, or greasy looking. The colors of minerals differ, though this can be misleading, as some minerals can vary. Quartz can be clear or white, rosy pink, smoky gray, yellow and other colors. Amethyst is the name for purple quartz. Color variations come from small substitutions in the chemicals within the mineral. Though color itself is not a reliable clue, the color of the mark or streak a mineral leaves on a rough ceramic surface is definitive. Talc leaves a white streak while hematite makes a reddish brown mark. The hardness of minerals is rated on a hardness scale, which ranges from talc, the softest mineral, with a value of one, to diamond, the hardest, with a value of ten. A simple way to test hardness is by scratching the mineral with your thumbnail. It will make a mark on talc but not on quartz, which is too hard to be scratched. A few minerals, such as magnetite, are magnetic. These are just a few of the characteristics that can be tested to identify minerals.
Minerals have many uses in our lives. Glass is made from quartz, and a quartz crystal keeps time in our watches. Talc is a component of many plastics as well as baby powder, and gypsum is a mineral in wallboard and cements. We brush our teeth with abrasive minerals in toothpaste and eat the mineral halite, or salt, in our food. We may not be able to recognize these minerals in the rocks we pick up, but from ancient times people have found ways to extract and make use of them.
Sedimentary rocks provide us with some of the most important clues about the history of life on Earth, because here is where we find fossils, traces or remnants of organisms from past geologic ages. Fossils can be impressions of shells, skeletons, leaves, or footprints, preserved in the rock as it hardens. Or the shells and bones may be gradually replaced by minerals, leaving a perfect replica in stone of the creature’s skeleton or shell, or even of a whole tree trunk. Fossils are remarkable in preserving details even down to the cell structure in some cases, allowing us to trace the ancestry of many present-day animals and contributing to our understanding of evolution. The layers of sedimentary rock are laid down year by year, with the oldest at the bottom. Layers containing fossils of sea animals tell us where there were once oceans. We can read the layers like chapters in a book, learning about changes in the environment and about the kinds of plants and animals that lived at different times in Earth’s history.
Rock that is exposed at the Earth’s surface is subjected to erosion. Fragments break off and get washed or blown away, carried along by glaciers or rivers, becoming smaller and rounder as they get bumped and scraped. Geologists classify rock fragments into size categories that range from boulders (greater than ten inches across) to cobbles, pebbles, sand, silt, and, lastly, microscopic particles of clay. Rocks are generally sharp-edged when they first split off from the parent rock, and it takes a lot of abrasion to round them off. The size and the roundness of rocks can be clues to the distance a rock has traveled and its journey along the way. A smooth, round pebble would have had to travel a long way in a stream, glacier, or ocean, while a pebble that is still sharp and angular must be close to its source.
Next time you stop to pick up a pebble, will you look for clues about how it formed or what minerals it contains? Will you examine it for fossils and consider how old it must be? Will you feel how rounded its edges are and wonder where it’s been? Or will you just give it a good toss and see how many times you can make it skip across a pond? It doesn’t matter. There are many ways, all rewarding, to appreciate the rocks around us.
Dixon, Dougal. Geology. Franklin Watts Science World. New York, NY: Franklin Watts, 1982.
Plummer, Charles, C., David McGeary and Diane H. Carlson. Physical Geology. Boston, MA: McGraw-Hill, 1999.