Rock Types and What they Mean

• Wave-cut platform – this is a flattened area of rock that has become eroded by tide and waves, usually existing between high and low tidal areas.

• Weathering – this is rock breakdown for any reason, such as mechanical, biological, and chemical forces.

Wow, that's a bunch of terms! Don't worry if you haven't gotten them all because you will gradually have a deeper appreciation for what they mean.

ROCK TYPES AND WHAT THEY MEAN

While every rock is unique and different, this doesn't mean that you can't categorize them in some way. There are actually only three major types of rocks. These are igneous, sedimentary, and metamorphic rocks.

The term igneous refers to "fire" or molten lava. Rocks of this type of form only when hot lava cools into rock. Particles can settle as this happens or minerals can precipitate to create something more interesting than a black rock. Look for different mineral grain sizes and textures. The texture depends on how slowly or quickly the lava cools. Larger crystals come from slower-cooling lava and vice versa.

Some interesting options among these:

• Granite pegmatite – this is igneous rock formed near the top of a magma chamber. It has many possible colors, including quartz gray, white, pink, and dark-colored mica.

• Diabase – this is rock solidifying just below the earth's surface. It cools rapidly, giving rise to a salt and pepper appearance. Basalt and gabbro are identical but differ in the size of the crystals. Diabase is in-between the two. Gabbro has the largest inclusions. Figure 1 shows a boulder with some diabase and gabbro in it:

Figure 1.

• Diorite – this is rock that crystalized in a mountainous area where black hornblende and white plagioclase come together. Like diabase, it is black and white.

Sedimentary rocks are layered in some way. They are made whenever rock particles or other minerals settle out from within the water or air. Minerals can also simply precipitate out of solution at varying times or concentrations, giving rise to a layered look. Lithification is when these layers become rock. Here are some interesting choices:

• Limestone – limestone is extremely common. It comes from organisms that have shells or other calcium carbonate-containing structures. These settle out as organisms die off to make layered limestone in shallow waters. Figure 2 is a good image of limestone:

Figure 2.

• Shale – shale is made from fine silt and clay that gets deposited in slow moving waters, often outside of river deltas where water cannot keep particles within it.

Shale settles out as a grayish material in these waters. The layers flake off each other easily.

• Sandstone – this is made from sand that builds up in waters that is quickly moving. It will also layer out in desert areas, eventually forming fine-grained sandstone.

Metamorphic rocks are either igneous or sedimentary or both at some point but get changed due to pressure, heat, or chemical reactions. Minerals and texture help define these rocks. These are rocks with different grain sizes and layering orientations compared to what they were. Foliation is seen in these rocks, which is a deformation of the shape of intrusions/grains due to pressure forces.

Some interesting examples include these:

• Manhattan schist – this is rock that was once shale and now has fine grains of biotite along with gray quartz or white orthoclase. The layering comes from compression on the shale.

• Gneiss – this used to be granite but became deformed by applied pressure and heat to make layers that are even-grained but more layered than the original rocks. Figure 3 shows what gneiss might look like:

Figure 3.