They are formed by compression and hardening of loose sediments.
Explanation
Rocks are classified into three major groups based on their mode of formation: Igneous, Sedimentary, and Metamorphic. Sedimentary rocks are formed through the process of lithification, which involves the accumulation, compaction, and cementation of sediments over geological time.Option Analysis:
- They are formed by cooling of magma. is incorrect: Rocks formed by the cooling and solidification of magma (beneath the surface) or lava (on the surface) are known as Igneous rocks (e.g., Granite, Basalt). They are often referred to as primary rocks.
- They are formed by heat and pressure on existing rocks. is incorrect: Rocks formed when existing rocks undergo recrystallization and reorganization due to intense heat and pressure are called Metamorphic rocks (e.g., Marble, Slate).
- They are formed by compression and hardening of loose sediments. is correct: Sedimentary rocks are formed from the accumulation of fragments (sediments) derived from the weathering and erosion of pre-existing rocks. These sediments are deposited in layers and undergo lithification-the process of compression and hardening-to form solid rock (e.g., Sandstone, Limestone, Shale).
- They are formed directly from the Earth's core. is incorrect: Rocks are not formed directly from the Earth's core. The core consists primarily of iron and nickel under extreme pressure and temperature. Rock formation occurs within the lithosphere (crust and upper mantle).
Key Takeaway: Sedimentary rocks are characterized by their formation through the lithification of denuded sediments and typically exhibit distinct stratification (layering) and the potential presence of fossils.