Question

Consider the following statements regarding the formation of metamorphic rocks:Which of the statements is/are correct?

1. Tectonic processes can push rocks to lower layers to initiate metamorphism.
2. Metamorphism can occur when magma comes into contact with mantle rocks.
3. Metamorphic rocks are formed when lower layers are pressed against the crust above

• A. All
• B. 1, 2
• C. 2 only
• D. 1 only

Answer 1

Answer:

B. 1, 2

Formation of Metamorphic Rocks

Types of Metamorphism:

• Contact Metamorphism: Occurs when rocks are heated by contact with magma. This is often localized, affecting rocks surrounding an igneous intrusion.
• Regional Metamorphism: Occurs over large areas, typically associated with tectonic plate collisions. The heat and pressure involved are immense, transforming large bodies of rock.
• Dynamic Metamorphism: Occurs along fault zones where rocks are subjected to intense shearing and grinding.
• Burial Metamorphism: Occurs when rocks are buried deep within the Earth, subjecting them to increased pressure and moderate temperatures.

Key Processes:

• Heat: Provides the energy to drive chemical reactions, causing minerals to recrystallize or new minerals to form. Sources of heat include proximity to magma chambers and the geothermal gradient (increasing temperature with depth).
• Pressure: Can be lithostatic (uniform pressure from overlying rocks) or directed (differential pressure from tectonic forces). Pressure causes rocks to compact, deform, and sometimes develop foliation (parallel alignment of mineral grains).
• Chemically Active Fluids: Hot fluids circulating through rocks can transport ions, altering the rock's chemical composition and promoting recrystallization.

Statement Analysis:

• Statement 1: Tectonic processes can push rocks to lower layers to initiate metamorphism.
  This is correct. Plate tectonics involves the collision and subduction of crustal plates. During these processes, rocks can be pushed to greater depths where temperatures and pressures are higher, leading to regional metamorphism.
• Statement 2: Metamorphism can occur when magma comes into contact with mantle rocks.
  This is correct. When magma intrudes into the Earth's crust or upper mantle, it heats the surrounding rocks. This high-temperature environment causes contact metamorphism in the rocks adjacent to the magma body, including mantle rocks if the intrusion is deep enough or originates from the mantle.
• Statement 3: Metamorphic rocks are formed when lower layers are pressed against the crust above.
  This statement is phrased imprecisely and is generally incorrect as a primary mechanism. While pressure is a key factor, metamorphism is driven by increased heat and pressure due to burial, tectonic forces, or magmatic activity, not simply by lower layers being 'pressed against the crust above.' The pressure is typically from overlying rocks (lithostatic) or from tectonic forces.