The Three Main Types of Rocks
In geology, rocks are classified into three main groups based on how they are formed: igneous, sedimentary, and metamorphic. Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). Sedimentary rocks are formed from the accumulation and compaction of sediments. Metamorphic rocks are formed when existing rocks are changed by intense heat and pressure.
Section 2: Formation and Characteristics
Igneous rocks, like granite and basalt, typically have a crystalline structure from the cooling process. Sedimentary rocks, such as sandstone and limestone, are formed in layers and are the only type that can contain fossils. Metamorphic rocks, like marble and slate, are existing rocks that have been physically or chemically altered deep within the Earth, often resulting in a banded or foliated appearance.
Section 3: A Practical Example
Imagine a volcano erupts. The lava that cools on the surface forms igneous rock (like basalt). Over time, wind and water break this rock into small pieces (sediment). These pieces wash into a river and settle at the bottom of a lake, eventually being compacted into sedimentary rock (sandstone). If this sandstone is later buried deep underground and subjected to immense pressure and heat, it will transform into a metamorphic rock (quartzite).
Section 4: Importance in the Rock Cycle
Understanding these three types is fundamental to understanding the rock cycle, a continuous process where rocks are created, changed, and destroyed. Any rock type can be transformed into another. For example, a metamorphic rock can melt to become magma and form an igneous rock, or an igneous rock can be weathered to become a sedimentary rock. This cycle illustrates the dynamic nature of Earth's crust.