Radioactive decay provides an internal source of heat for the earth. This helps in the formation of which type of rocks?

The correct answer is Option 1: Igneous.

Internal heat generated by the radioactive decay of isotopes (such as Uranium, Thorium, and Potassium) in the Earth's mantle and crust is the primary driver of mantle convection. This heat causes the melting of rocks into magma or lava. When this molten material cools and solidifies, it forms Igneous rocks. Since these rocks are formed directly from the cooling of molten mass fueled by internal heat, they are the most direct product of radioactive decay processes.

• Sedimentary rocks are formed by exogenous processes like weathering and erosion, driven primarily by solar energy, not internal heat.
• Metamorphic rocks require heat and pressure to transform existing rocks, but they do not involve the melting-solidification cycle that defines the primary rock formation from internal heat sources.

Therefore, while internal heat influences the rock cycle, it is the fundamental catalyst for the creation of Igneous rocks.