Backyard geology using everyday logic
What is awaiting discovery outside your back door? We all dream of discovering that hidden treasure just waiting for us to find it. When we were young anything could have been that treasure- so why allow ourselves to outgrow it? I say- dig in the dirt!
If you find something that sparkles back at the sun we must determine if it is metal or mica. What is mica? A soft chip of a mineral that scratches very easily, where as metal won't scratch with such little effort. Disappointed that you didn't strike it rich? Perhaps it won't add excitement into your pocketbook, but stretch your mind and imagine how it got here. If you find this equally as thrilling, you are one of us- a backyard geologist.
With modern man's ability to remove and replace soil and stone with trains, trucks and bulldozers it may be exceptionally difficult for most backyards to be truly native. However, the rocks themselves can tell a story of how they were created. Although research is always good, we must first find a starting point. First lets employ logic to guess at how our new finds came to be.
Rocks that came from deep within the earth are kind of "born into being" onto the surface. Think of these are more "pure" stones. Quartz crystals fall into this category as a pure mineral formation as well as obsidian. Crystals are the other thing in the earth that dreams are made of, and quartz crystals are the most prevalent in various colors. If you are lucky enough to find these you know that they formed around - underneath actually- a volcano. Obsidian is natural glass that forms on the surface or a volcano. It cools so rapidly that no crystals have time to form, so how do you identify it? Think of the drinking glasses in the back of your cupboard with chips around the rim. The are curved or concoidal fractures. Natural glass is the same in that is it very brittle and won't break in straight lines. If you find shards of it, it even makes the same sound which is why many people use them in wind chimes.
The pumice that we use around the house is also composed of little shards of this volcanic glass with one more ingredient: gas. The gas bubbles made the spaces between the shards. This could likely be sitting around your yard. To see if what you found could be pumice, throw it in still water and see if it floats. All of the gas pockets maintain this rock's buoyancy.
Granite is also very common, and also formed under volcanoes.That stuff has to be stored SOME place before it build up enough pressure to come out! It is most easily identified as a heavy, hard rock with various colored flecks all over it. The slower the magma cooled to create the stone, the larger the flecks- or crystals- will be. An example for comparison: Some of the granite in Yosemite in CA is almost pure white and very resistant to erosion showing that it has very tiny crystals. Pike's Peak in CO has big crystals and erodes, falling apart rather easily. Think of a nut muffin. If the nuts are ground up fine, the muffin will slice and hold together whereas the same muffin with large chunks of nuts falls to bits when you try and cut it. Following this logic, these bits crumble to the ground and slowly accumulate, layer after layer. These crumbs of granite have broken down and are well on their way to evolving into sedimentary stone.
If a rock has any kind of grit, it was likely ground down through weathering and erosion, which makes it sedimentary. From large glued together river pebbles in conglomerates to finer-than-sandpaper siltstone, they all feel gritty on some level, and were washed away and redeposited to make something new. If a rock has shell fossils you really lucked out on easily identifying it as marine sediment. However, if you aren't so fortunate to have a give-away clue, try spilling some of your soda on your rock. Calcium left from layers of shell and other marine debris will react and fizz. Fun and easy, your inner explorer is ready to continue.
Having applied our every day logic to how sedimentary and volcanic (technically plutonic) rocks came to be, lets spice it up and throw in a couple more variables: heat and pressure. When I think heat and pressure I think of a grilled cheese sandwich. Layer after layer of goodness: butter, bread, cheese, bread, butter. Nice and orderly, right? If you find a rock with such order to it, it is possibly metamorphic where heat and pressure have "re-squished" it. A piece of granite after being reheated turned into gneiss. The difference? the little specks have now turned into nice orderly stripes through the rock. That sparkly piece of shale that fell off of your grandmother's shed would turn into a very shiny piece of schist, with one big layer of pretty shiny rock, with kind of a fish scale texture to it.
Sometimes we can tell something has had pressure added to it by the external attributes. Something that has moved along a fault line underground would have definitely been put through some pressure. Slicken sides are a sign that a stone has been metamorphosed. Rocks that don't have any seeming reason to have a smooth, shiny side to them. There is no real surprise that the CA sate rock is serpentine, a metamorphic rock that is a step before jade.
When you get a general idea of what the rock might be and want to know what is actually in it, invest in a loupe and magnify those little minerals to study their individual hardness and shape, and start your collection of mineral books. If you look around you and think, "I wonder how that hill got there" you are looking into geomorphology: the study of landforms. It is all an application of logic to make the best educated guess into the earth's past. If you are able to do site specific studies, where you know what you see around you is actually from there and not terribly altered by man, some exciting stories in stone are waiting to be discovered.