Rocks of the Western US
Approximately 200 million years ago, the western edge of what is now North America contained only Nevada and a small sliver of Washington State. As the supercontinent Pangaea split up, the western edge of the continent became an active plate boundary. The Western States were built by violent eruptions, subductions, collisions, and accretions as well as by the processes of erosion, deposition, and lithification. As a result, there are a wide variety of rocks to be found throughout the Western US (Figure 2.1).
Figure 2.1: The rock cycle.
Igneous Rock Classification
Igneous rocks differ not only in their cooling rates and subsequent crystal sizes, but also in their chemical compositions. Rocks found in continental crust, such as granite, have high silica content and low iron and magnesium content. They are light in color and are called felsic. Rocks found in oceanic crust, like basalt, are low in silica and high in iron and magnesium. They are dark in color and are called mafic.
| Crystal size | Felsic | Intermediate | Mafic | Ultramafic |
|---|---|---|---|---|
| large (plutonic) | granite | diorite | gabbro | peridotite |
| small (volcanic) | rhyolite | andesite | basalt | -- |
| none | (glassy)obsidian, tuff, pumice | obsidian | obsidian | -- |
As rocks at the Earth’s surface erode and weather, the sediment that forms can be compacted and cemented into sedimentary rock. Generally, these sediments are transported by wind, water, or ice to a depositional environment such as a lakebed or ocean floor; here they build up, burying and compacting lower layers. As water permeates the sediment, dissolved minerals may precipitate out, filling the spaces between particles and cementing them together. Sedimentary rocks can also accrete from fragments of the shells or skeletal material of marine organisms such as clams and coral.
Metamorphic Rock Classification
Metamorphic rocks are classified differently depending on the protolith (parent rock) they are made from. The following chart shows common rocks and the metamorphic rocks that they can become.
| Parent rock | Metamorphic rocks |
|---|---|
| shale | slate, phyllite, schist, gneiss (in order of increasing heat and pressure) |
| granite | gneiss |
| sandstone | quartzite |
| limestone | marble |
| peridotite | serpentinite |
Sedimentary Rock Classification
Sedimentary rocks are classified by their sediment size or their mineral content, and each one reveals the story of the depositional environment where its sediments accumulated and were eventually lithified.
| Sediment size (decreasing size) | Sedimentary rock | Environment of deposition |
|---|---|---|
| gravel | conglomerate | river beds, mountains |
| sand | sandstone | beaches, river sand bars, sand dunes |
| sand, silt, clay | graywacke | continental shelf |
| silt | siltstone | quiet water |
| clay | shale | very quiet water, lakes, swamps, shallow oceans |
| Mineral content | Sedimentary rock | Environment of deposition |
|---|---|---|
| calcium carbonate skeletons of marine organisms | limestone | tropical reefs, beaches, warm shallow seas |
| precipitated calcium carbonatelake | travertine, tufa | hot springs, playas (dry lake beds), drying seas |
| gypsum | rock gypsum | playas, drying seas |
| halite | rock salt | playas, drying seas |
Every rock is capable of being melted, weathered, or changed by heat and pressure. Any rock that has been subjected to intense heat and pressure can recrystallize into a metamorphic rock. This process destroys features in the rock that would have revealed its previous history, transforming it into an entirely new form as the minerals within recrystallize and realign. The pressure to transform a rock may come from burial by sediment, or from compression due to plate movements, while the heat may be from very deep burial or from contact with magma.
Why do we see different kinds of rocks at the surface?
As you walk across the surface of the Earth, you will observe an amazing variety of rock types. If all rocks were fl at-lying layers and there was no erosion, then we would only see one type of rock exposed on the surface. Often, however, rocks have been worn away (eroded), and the underlying layers are now exposed at the surface. Layers of rock may also be tilted, folded, or faulted to reveal the underlying rocks at the surface.
When rocks are flat-lying layers and there is no erosion, folding, or faulting, the person walking across the surface sees only one rock type.
When rocks are worn away (often by streams), the person walking across the surface sees the underlying layers of rock exposed.
When rocks are folded or tilted, the person walking across the surface sees several layers of rock exposed.
