Rocks are classified as sedimentary, metamorphic, or igneous on the basis of the a. age of the rocks. b. way the rocks were formed. c. types of fossils the rocks contain. d. number of minerals found in the rocks.
The Correct Answer and Explanation is:
Correct Answer: b. way the rocks were formed.
Rocks are classified into three major types—sedimentary, metamorphic, and igneous—based on the processes by which they form. This classification system helps geologists understand the history of Earth’s crust and the conditions that existed when the rocks were formed.
Igneous rocks are formed from the solidification of molten material called magma or lava. If the molten rock cools and solidifies beneath the Earth’s surface, it forms intrusive igneous rocks like granite. If it erupts onto the surface and cools quickly, it forms extrusive igneous rocks like basalt. The texture and mineral composition of igneous rocks depend on how quickly the magma or lava cools and where it cools.
Sedimentary rocks form through the accumulation and compaction of sediments. These sediments are produced by the weathering and erosion of existing rocks. Over time, layers of sediment build up and are compressed into rock. Sedimentary rocks often contain fossils, which are preserved remains of ancient organisms. Common examples include sandstone, limestone, and shale. These rocks typically form in layers and are found in areas that were once underwater, such as riverbeds, lakes, and oceans.
Metamorphic rocks originate when existing rocks are subjected to intense heat, pressure, or chemically active fluids, causing them to change physically or chemically without melting. This process, known as metamorphism, alters the mineral structure and texture of the rock. Common metamorphic rocks include slate (from shale), marble (from limestone), and schist.
Therefore, the primary criterion for classifying rocks is how they are formed, not their age, fossil content, or mineral count. Each rock type represents a stage in the rock cycle, illustrating how Earth materials are continuously recycled over geological time through different physical and chemical processes.
