In geology, contact metamorphism mainly occurs when rocks are altered by which dominant factor acting near a body of hot magma or lava?

Introduction / Context:
Metamorphism is the process by which existing rocks are changed into new types of rocks through heat, pressure, and chemically active fluids without the rock fully melting. There are different types of metamorphism, including regional and contact metamorphism. Contact metamorphism occurs when rocks are heated by a nearby intrusion of magma or lava. This question asks you to identify the primary agent responsible for contact metamorphism and distinguishes it from other geological processes like weathering and erosion.

Given Data / Assumptions:
• The specific metamorphic process in question is contact metamorphism. • The options mention temperature, pressure, weathering, and flowing water. • We assume standard definitions from introductory geology. • The scenario involves rocks near a hot magma body or newly emplaced lava.

Concept / Approach:
Contact metamorphism occurs when country rock surrounding a hot igneous intrusion is baked by the intense heat coming from the magma. The temperature increases significantly over relatively short distances, forming a metamorphic aureole around the intrusion. Pressure may be present, but it is not the main controlling factor for contact metamorphism; it is much more important in regional metamorphism where large volumes of crust are buried and compressed. Weathering and erosion by wind, rain, and flowing water are surface processes that break down rocks but do not transform them into metamorphic rocks. Therefore, the primary agent of contact metamorphism is high temperature due to proximity to hot magma.

Step-by-Step Solution:
Step 1: Recall that contact metamorphism happens where hot magma comes into contact with cooler surrounding rocks. Step 2: Recognise that heat flows from the hotter magma into the cooler country rock, raising its temperature. Step 3: Understand that this heating causes mineral changes and recrystallisation without full melting of the rock. Step 4: Identify that this process depends primarily on temperature increase rather than large scale pressure. Step 5: Compare this with regional metamorphism, where pressure and deep burial play larger roles. Step 6: Select high temperature from nearby magma as the main agent of contact metamorphism.

Verification / Alternative check:
Geology textbooks describe contact metamorphism as producing rocks like hornfels in zones around igneous intrusions. These rocks show evidence of heating and recrystallisation but often lack the strong foliation associated with high pressure regional metamorphism. Maps and cross sections of intrusive bodies typically show a metamorphic aureole where temperature was highest near the contact. There is no similar relationship between contact metamorphism and surface weathering or flowing rivers, which emphasises that heat from magma is the key factor.

Why Other Options Are Wrong:
Option B, high pressure alone, is more relevant to regional metamorphism in mountain building regions and does not define contact metamorphism. Option C, weathering, and Option D, flowing water, describe processes that break rock into smaller pieces or transport sediments but do not create solid metamorphic rocks. These are part of the external rock cycle but not part of the metamorphic transformation near intrusions. Therefore, they cannot be the primary agents of contact metamorphism.

Common Pitfalls:
Students sometimes memorise that metamorphism involves heat and pressure and then assume both are always equal factors. This leads to confusion between contact and regional metamorphism. Another pitfall is mixing up metamorphism with weathering because both involve changes in rocks. Remember that metamorphism transforms rocks in the solid state within the crust, often at depth or near magma, whereas weathering breaks down rocks at the surface. For contact metamorphism, focusing on heat from nearby magma will guide you to the correct answer.

Final Answer:
The correct choice is High temperature from nearby magma heating the surrounding rocks, because contact metamorphism is mainly driven by intense heat from igneous intrusions acting on neighbouring country rocks.