Geology And Geophysics

A Guide to Backyard Geology



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Geologists love field trips. They'll travel thousands of miles to explore rich geological sites such as the Grand Canyon of the United States or the Great Rift Valley of Africa. In these locales, earth and rocks from many different eras lie exposed, laid down in strata, like pages in a book.

It's unlikely that you'll find anything so geologically rich in your own backyard, but the typical backyard is by no means barren of significance. It will reveal to you much about the forces and processes which have created the plot of land upon which you have built your home.

A pile of rocks and "soil" samples gathered from one small plot of land became the treasured return cargo of the Apollo 11 moon mission in 1969. The area over which the astronauts collected these samples was very restricted - not much larger than a good sized backyard. Yet the samples answered many fundamental questions about the moon's origin and history. The rocks were overwhelmingly of volcanic origin, had no significant organic content, and lacked the water content found in Earth's rocks. The public was somewhat disappointed that the rocks looked so ordinary, yet they answered many questions that had excited the curiosities of scientists for centuries.

Similarly, you're unlikely to come across rocks of great rarity in your backyard, but if you're willing to do a little digging (both physically and through research), you'll discover much about what's been going on in your backyard over the centuries, millennia, and even millions of years before you moved there.

Digging is usually necessary because the surface of your back yard may have been affected by the original site preparation work when your house was built or by subsequent landscaping. Many of the native rocks and boulders may have been removed, and new soil may have been trucked in to support gardening. If you dig down a foot or more, you'll more likely find naturally deposited rocks and soil. By no means is this foolproof, however. While at college, I was surprised to learn that the entire area surrounding my campus was originally built atop sand dunes. I didn't have a clue until I read about it.

Keep samples of the dirt and the rocks that you find beneath your yard's surface. Both rocks and dirt can be classified into three basic types, so it's easy to get started. But there's plenty of room to advance your studies, because each classification system, with its sub-classifications and combinations, gets more complicated the closer you look and the more accurate you try to be.

For soil, the three basic types are clay, silt and sand. All three types are a mixture of organic matter and rock or mineral content. Silt and sand are both created from rock, or shells, by identical physical processes, but they got to their present location by different means. A rock surface has been broken and ground down by the erosive forces of wind, rain, streams or ocean currents and tides, or glaciers. Expansion and contraction caused by heating and freezing can also cause rocks to crack and break apart.

Less common forces have also turned rock into soil. On the moon, erosion is virtually absent, but the moon is coated with soil, mainly from volcanic activity and meteorites that have smashed and pulverized the rock on its surface. The Earth has also been subjected to meteorite bombardment, so your backyard has, no doubt, a modicum of soil produced extra terrestrially. Chemical erosion also occurs, particularly from acids in the atmosphere that can accumulate in rain. Ancient stone monuments, such as the sphinxes and pyramids, have shown increased rates of weathering when acid rain became noticeable. Plant roots also break rocks apart.

The difference between sand and silt is determined by grain size. Sand has coarser grains, and you'll recognize it as sand because it won't hold water as well. If you dampen it and try to mold it, the soil will readily fall apart. In addition to being made from hard rock such as quartz, sand might contain fragments of shells.

Silt has been ground down to finer grains. It will hold its shape better when molded, but not as well as clay. It will also feel less gritty to the touch. Sand would have been deposited mainly by fast moving water in oceans and streams, or moved by wind as is the case of desert sand dunes. Silt is carried further downstream and accumulates where the water has slowed or has been left behind after floods.

Clay has the finest grains. Pure clay soil is powdery, and when dampened can be molded and will retain its shape. Its particles are so fine and flake-shaped that water cannot drain easily between them. If water puddles easily on your soil and cakes when dried then clay is a dominant constituent. Long, long ago the soil was part of volcanic rocks. Water and chemical weathering dissolved minerals from these rocks, and the dried minerals became the clay soil.

The dirt sample that you've dug up is very unlikely to be pure clay, sand or silt. Real life is messier. A soil's characteristics are affected by the proportion of each of the three basic types, the type of rock from which it came, and the amount and type of organic matter it contains. Soil classification systems vary by country and organization. A United States Department of Agriculture's soil survey has identified over 20,000 soil types.

A similar situation exists with rocks. The three basic rock types are igneous, sedimentary and metamorphic.

Igneous rocks were once part of the magma layer miles beneath the Earth's surface. The moon rocks are nearly exclusively igneous. The rocks formed either by volcanic forces, or they intruded into the Earth's crust and later migrated to the surface by tectonic forces. Granite is formed in this way. Basalt is the most abundant igneous rock. If the rock is porous, it most likely was formed by an explosive eruption. If it is dense, it probably flowed as lava.

Sedimentary rocks, as its name implies, was once sediment, most often transported by water, and compressed and cemented together by pressure and dissolved minerals. Shale and sandstone are common sedimentary rocks, as is chalk.

Metamorphic rocks were once igneous or sedimentary rocks, but they have been transformed by great pressure and/or heat. They may have once been buried deep beneath the Earth's surface or been near a concentration of hot lava or subjected to horizontal, tectonic pressures. The result is a harder, denser rock. Limestone becomes marble; shale becomes slate. These rocks will be more common in mountainous or hilly regions. The hard rock will typically have bands or layers running through it. If you find one in your backyard, you can be assured it has quite a history.

Pay particular attention to any varieties of soil and rocks. Test soil samples for texture and interaction with water. Rocks can be sorted by general shape, density, color, and size. Check how smooth they are and if they are uniform or contain composite material. How easily do they break? Are they numerous? All are clues to why the earth in your yard took is in its present form.

If there is nearby public land, borrow some samples from there as well to check for variations. Inquire whether your property is in a flood plain. Make note of any dominant nearby geological features, such as rivers, mountains, or outcrops of rock. These will add context to the samples and make them more meaningful.

To gain insight into the processes that create distinct geological characteristics, consult a book about geomorphology. "Earth's Changing Surface: An Introduction to Geomorphology" by M. J. Selby is one of several university-level textbook on the subject, but you can find more casual books by using the keywords, "changing earth", at an online bookstore, or look for online articles on the subject. Popular writer, John McPhee, has also written a series of books on the topic.

Finally, you can use your favorite search engine to explore topics that I've touched upon in this brief guide. For example, search "rock identification" and "soil identification" - also "classification". You'll find the many detailed classification systems that are in use. Search the word "geology" in combination with the name of your town, state or province, or county. See if you can locate geological maps for your area on the internet. The website, "About.com", has links to geological maps of many varied areas, most color-coded by rock type or age. Type the words, "geological map", into their search box.

In no time you'll be "Earth literate" and begin to appreciate the true history of this planet that's been evolving thousands of times longer than the lifespan of mankind, not to mention the lifespan of your backyard.

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