Geology is a complex subject, but as an educator I try to reduce the complicated to simple components and then build from that. To make geology simple all you have to remember is there are only three kinds of rocks. Yes, only three: igneous, sedimentary and metamorphic.
Igneous from the Latin "igni" meaning fire. When you turn on your ignition, you fire up your car. Basically any rocks created directly from pools of hot magma in the Earth's mantle up to 70 miles below the surface are igneous. Hot liquid rock that flows up in plumes but does not make it to the surface and cools at depth is called granite. However, if the magma manages to make it to Earth's surface, it is called volcanic rock. Same exact magma, but it cools more quickly and at the surface. Now, wait a minute you say. Granite is all over the Sierra and is not buried at depth. True, but that rock originally formed far below and was later uplifted and exposed to view.
Sedimentary rocks are formed from -- duh -- sediment from rocks that are weathered, eroded and deposited. This cemented together material can vary in composition from small boulders to very fine silt. Sandstone, shale and conglomerate are some common sedimentary rocks. Biological action can also result in sedimentary rocks. Seashells of trillions of ancient marine invertebrates create huge layered beds of calcium carbonate known as limestone. And coal is the accumulation of organic matter, mainly trees, from about 300 million years ago.
The final type is metamorphic: literally to change shape, which is brought about by tremendous heat and pressure driven by Earth's constant tectonic activity, as surface rocks are recycled back into the mantle. The parent material can be igneous, sedimentary or even older metamorphic rock. Sandstone under heat and pressure becomes quartzite. Limestone becomes marble.
See how simple geology is?