Broadly, metamorphism involves __. a. the settling of crystals in a melt as it cools b. the sorting of grains by size, as accomplished by rivers and beach waves c. the cementation of loose grains and the precipitation of new minerals into pore spaces d. changes in mineralogy and texture in response to heat and stress
The Correct Answer is Explanation
Correct Answer: d. changes in mineralogy and texture in response to heat and stress
Explanation:
Metamorphism is the process by which existing rocks (igneous, sedimentary, or older metamorphic rocks) undergo changes in mineral composition and texture due to exposure to elevated temperatures, pressures, and chemically active fluids, without melting into liquid magma. This transformation happens in the solid state, meaning the rock does not melt but instead recrystallizes under extreme conditions.
The correct answer, choice (d), highlights the key aspects of metamorphism—changes in mineralogy and texture due to heat and stress. As rocks are buried deeper within the Earth’s crust or are subjected to tectonic pressures (such as during continental collisions), they are exposed to higher temperatures and directed pressure (stress). These conditions cause the minerals within the rock to become unstable and rearrange themselves into new mineral structures that are more stable under the new conditions.
For example, shale, a sedimentary rock, can undergo metamorphism to become slate, and with further metamorphism, it can become phyllite, schist, and eventually gneiss. In this progression, the mineral grains grow larger and align more distinctly, showing clear changes in texture. Additionally, the mineral makeup changes as new minerals form—such as garnet, staurolite, and kyanite—depending on the pressure and temperature conditions.
In contrast:
- (a) describes a process involved in igneous differentiation, not metamorphism.
- (b) refers to sedimentary processes, particularly grain sorting by agents like water or wind.
- (c) describes lithification, the process that turns loose sediments into sedimentary rock.
Thus, the essence of metamorphism lies in the recrystallization and realignment of minerals, driven by heat, pressure, and chemical changes—all occurring without the rock melting. This process significantly alters the rock’s original form while still maintaining a solid state.