TYPES OF ROCK
Crustal rocks and minerals appear in a bewildering array of types, since chemical elements can combine in myriad ways. But in broad terms, geologists recognize three kinds of rock.
Rocks that form in a molten state are called igneous. These include basalt, which erupts as molten lava, and granite, which forms deep within the Earth and is exposed by erosion or geological activity.
Sedimentary rocks are formed by the slow accumulation of layers of particles, such as sand, mud, or organic debris. Shale, sandstone, and limestone are common sedimentary rocks.
Metamorphic rocks are those that have been altered by heat, pressure, or both. Metamorphism makes a rock harder and denser, but traces of the rock's original nature are usually preserved. The process can turn shale into slate, sandstone into quartzite, limestone into marble, and so on.
Earth is a geologically active planet, and these three types of rock are involved in a continual process of recycling, driven by weathering, heat, and pressure. Although Earth formed some 4.6 billion years ago, the oldest known rocks are about 3.9 billion years old, and these are rare exceptions; most of the surface is only about 100 million years old. This explains why we do not find the number of craters here that we do on the Moon or Mars. While Earth experienced just as many impacts, most craters have been eroded by the weather or erased by geological activity.