Petrology basics — rocks formed from magma at the Earth’s surface Rocks that are formed when magma erupts and cools at or very near the Earth’s surface are called:

Introduction / Context:
Classifying igneous rocks by their cooling environment helps predict grain size, texture, and engineering behaviour. Surface cooling typically yields fine grains and sometimes vesicular textures relevant to construction use.

Given Data / Assumptions:

• Magma that reaches the surface is termed lava.
• Cooling rate at the surface is rapid compared with deep crustal settings.
• Rock categories: plutonic (intrusive, deep), hypabyssal (shallow intrusive), volcanic (extrusive).

Concept / Approach:
Volcanic (extrusive) rocks such as basalt and rhyolite cool quickly, resulting in fine-grained or glassy matrices. Plutonic rocks (e.g., granite) form at depth with coarse grains; hypabyssal rocks are intermediate.

Step-by-Step Solution:

Verification / Alternative check:
Field mapping correlates flow structures, pyroclastics, and glassy textures with extrusive volcanics.

Why Other Options Are Wrong:

• Plutonic (A): deep intrusive; coarse-grained.
• Hypabyssal (B): shallow intrusive; medium grain.
• Igneous (D): umbrella term includes both intrusive and extrusive; not specific to surface formation.

Common Pitfalls:
Using “igneous” when the question asks for the specific surface-formed subclass; ensure the extrusive/intrusive distinction is clear.

Final Answer: