Understanding Granite Properties and Rock Types

This question asks us to identify the incorrect statement about granite. To answer this, we need to understand what granite is and its key characteristics, particularly its rock type and composition.

What is Granite?

Granite is a common type of felsic intrusive igneous rock. This means it formed from the slow crystallization of magma below the Earth's surface. Its composition primarily consists of quartz and feldspar, with minor amounts of mica and other minerals. The slow cooling process underground allows large crystals to form, which is why granite has a coarse-grained texture.

Analyzing Each Statement about Granite

Let's examine each statement provided in the options:

1. Statement 1: Granite is the most abundant rock in the continental crust.

   Analysis: This statement is generally considered true. Granite and related rocks (like granodiorite) make up a significant portion of the Earth's continental crust.

2. Statement 2: It is the most common sedimentary rock.

   Analysis: This statement is incorrect. Granite is an igneous rock, not a sedimentary rock. Sedimentary rocks are formed from the accumulation and cementation of mineral or organic particles (sediments), or from the precipitation of dissolved minerals. Examples of sedimentary rocks include sandstone, shale, and limestone. Granite does not form through these processes.

3. Statement 3: These are igneous rocks that form when hot, molten rock crystallises and solidifies.

   Analysis: This statement is correct. As discussed earlier, granite forms from the cooling and solidification of magma (molten rock) beneath the Earth's surface. This process defines it as an igneous rock.

4. Statement 4: Granite is composed mainly of quartz and feldspar with minor amounts of mica.

   Analysis: This statement is correct. The primary minerals found in granite are quartz and feldspar (typically orthoclase and plagioclase), along with smaller amounts of minerals like mica (biotite or muscovite) and amphibole.

Identifying the Incorrect Statement

Based on our analysis, Statement 2 incorrectly identifies granite as a sedimentary rock. Granite is fundamentally an igneous rock, formed from the cooling of magma.

Revision Table: Rock Types

Understanding the main categories of rocks is crucial. Here's a quick overview:

Additional Information about Granite and Rock Cycle

Granite is a key component of the Earth's rock cycle. As an intrusive igneous rock, it forms deep underground. Over geological time, uplift and erosion can expose granite at the surface, forming mountains and vast rock formations. Surface processes like weathering and erosion break down granite into sediments, which can then be transported and deposited to form sedimentary rocks. If granite is subjected to intense heat and pressure, it can be transformed into a metamorphic rock like gneiss.

The chemical composition of granite is predominantly silica and alumina, classifying it as a felsic rock. This high silica content contributes to its relatively low density compared to mafic rocks like basalt, which are more common in oceanic crust.