In basic geology, which statement best describes an igneous intrusion formed beneath the Earth surface?

Introduction / Context:

This question belongs to earth science and introductory geology. Igneous rocks form either at the surface from lava or beneath the surface from magma. When magma pushes into pre existing rock layers below the Earth surface and solidifies there, it produces structures known as igneous intrusions. Examples include dikes, sills, stocks, and batholiths. Understanding the correct definition of an igneous intrusion helps learners interpret geological maps and cross sections.

Given Data / Assumptions:

• The focus is on the general idea of an igneous intrusion, not on naming a specific type such as dike or sill.
• We distinguish between processes beneath the surface and processes at the surface.
• Magma refers to molten rock below the surface, while lava is molten rock that has erupted at the surface.

Concept / Approach:

An igneous intrusion forms when magma generated in deeper parts of the crust or mantle moves upward and enters surrounding solid rock. It forces its way into cracks, bedding planes, or weak zones and then cools and crystallises in place. Because cooling is relatively slow beneath the surface, intrusive rocks typically have larger, visible crystals. This process is distinct from volcanic eruptions, where lava flows or ash deposits accumulate at the surface.

Step-by-Step Solution:

Verification / Alternative check:

To verify, think of common intrusive features. A vertical dike cuts across rock layers, while a sill intrudes parallel to bedding. Large batholiths such as those beneath the Himalayas or Andes are huge intrusive bodies. All of these examples involve magma moving into pre existing rock and solidifying without reaching the surface as free flowing lava. Therefore, any correct definition must mention magma, surrounding rock, and formation below the surface.

Why Other Options Are Wrong:

• Option A: Refers to ice crystals splashing into lava, which is not a recognised geological process and does not describe an intrusion.
• Option C: Talks about magma exported to an open area forming iron deposits, which resembles ore deposition rather than intrusion into solid rock.
• Option D: Describes an explosive volcanic eruption and smooth outer layer of rock, which relates to extrusive, not intrusive, igneous rocks.

Common Pitfalls:

Learners sometimes confuse intrusive and extrusive processes because both involve molten rock. Another frequent mistake is to focus on the presence of crystals without considering where cooling took place. Remember that the key idea in an intrusion is magma pushing into and solidifying within pre existing rocks below the surface. Keeping the magma versus lava distinction clear helps avoid this confusion.

Final Answer:

An igneous intrusion is best described by the statement that magma is injected into surrounding rock below the surface forming an intrusive igneous body.