TeXes Science Practice Test

Which term describes the insertion of magma through underground cracks that cools to become rocks?

1. Extrusion

2. Intrusion

3. Metamorphosis

4. Recrystallization

The correct answer is: Intrusion

The term that describes the insertion of magma through underground cracks, which then cools to become solid rock, is "intrusion." This process occurs when magma from beneath the Earth's surface forces its way into pre-existing rock layers, typically due to tectonic activity. As the magma cools slowly within the Earth, it forms what are known as igneous intrusions or plutonic rocks, such as granite. This is distinct from extrusion, which pertains to magma that escapes the Earth's surface and cools quickly, forming volcanic rock. Metamorphosis relates to the transformation of rocks under heat and pressure, leading to changes in mineral composition and texture, rather than the cooling of magma. Recrystallization involves the reforming of mineral structures in existing rocks but does not describe the actual process of magma cooling and solidifying. Hence, the correct identification of "intrusion" precisely captures the geological phenomenon of magma cooling underground to form rocks.