Rocks are classified as igneous, sedimentary, or metamorphic, based primarily on their __. a. crystal or grain size b. method of formation c. mineral composition d. texture
The Correct Answer and Explanation is:
Correct Answer: b. method of formation
Rocks are classified into three main types—igneous, sedimentary, and metamorphic—primarily based on their method of formation. This classification system reflects the processes that create and transform rocks within the rock cycle.
Igneous rocks form from the solidification of molten rock material called magma or lava. When magma cools and solidifies beneath the Earth’s surface, it forms intrusive (plutonic) igneous rocks like granite. When lava cools quickly on the surface, it forms extrusive (volcanic) igneous rocks like basalt. The speed of cooling affects the crystal size; slower cooling results in larger crystals, while rapid cooling produces finer grains.
Sedimentary rocks form through the accumulation and compaction of sediments, which may include fragments of other rocks, minerals, and organic material. These sediments are often deposited by water, wind, or ice. Over time, layers of sediment build up and are compacted and cemented into solid rock. Examples include sandstone, formed from sand-sized particles, and limestone, often formed from the remains of marine organisms. Sedimentary rocks typically show visible layers and may contain fossils.
Metamorphic rocks are formed when existing rocks are subjected to high temperatures and pressures, typically deep within the Earth’s crust. These conditions cause physical and chemical changes without melting the rock. This process, called metamorphism, alters the mineral composition and texture of the rock. Common metamorphic rocks include schist and marble. Metamorphic rocks often show foliation—layered or banded textures caused by pressure.
While characteristics like grain size, texture, and mineral composition provide useful clues, the primary basis for classification is the rock’s origin or method of formation. Understanding how a rock formed gives geologists insights into Earth’s history, including past environments, geological events, and tectonic activity. This foundational principle helps organize and interpret the diverse types of rocks found on Earth.
