Question

Which of the following rocks is a metamorphic rock? A. shale C. slate B. granite D. pumice

Short answer

Slate is a metamorphic rock.

Step-by-step solution

Understanding Rock Types

Rocks are typically classified into three main categories: igneous, sedimentary, and metamorphic. Each type forms in different environments and has distinct characteristics.

Identify Each Rock's Type

- **Shale** (A): This is a sedimentary rock, formed from compacted mud, clay, or silt. - **Granite** (B): This is an igneous rock, formed from cooled molten magma beneath the Earth's surface. - **Pumice** (D): This is also an igneous rock, typically formed when lava cools quickly and traps gases, creating a frothy appearance. - **Slate** (C): This is a metamorphic rock, formed from shale through the process of metamorphism, which involves heat and pressure.

Drawing the Conclusion

Based on the definitions, the only rock in the list that is metamorphic is slate, as it forms from shale through heat and pressure changes.

Key concepts

Rock Classification

Rocks are naturally occurring solid aggregates composed of one or more minerals. The classification of rocks helps us determine their origins, characteristics, and uses. Typically, rocks are divided into three primary categories based on their formation processes: igneous, sedimentary, and metamorphic.

• Igneous Rocks: These rocks form from the cooling and solidification of magma or lava. They are often characterized by their crystalline textures and are further divided into intrusive (plutonic) and extrusive (volcanic) rocks.
• Sedimentary Rocks: Formed from the accumulation and compaction of mineral and organic particles. They are often layered and may contain fossils, which makes them particularly valuable in understanding Earth's history.
• Metamorphic Rocks: These result from the transformation of existing rock types due to heat, pressure, and chemically active fluids, without the rock reaching a molten state.

Understanding these classifications not only aids in identifying rocks in nature but also reveals the geological processes that shape our planet.

Igneous Rocks

Igneous rocks originate from the solidification of magma or lava. The word "igneous" comes from the Latin word "ignis," meaning fire, which reflects their fiery origin. They are broadly classified into two types:

• Intrusive (Plutonic) Igneous Rocks: These rocks form beneath the Earth's surface. Magma cools slowly, allowing large crystals to grow. Granite is a common example, known for its coarse-grained texture.
• Extrusive (Volcanic) Igneous Rocks: Formed from lava that cools quickly on the Earth's surface, leading to a fine-grained texture. Pumice, a highly vesicular rock, is an example, often formed during explosive volcanic eruptions.

Igneous rocks are foundational to the understanding of geological processes as they constitute a significant part of Earth's crust.

Sedimentary Rocks

Sedimentary rocks form from sediments derived from pre-existing rocks or organic materials. These sediments are transported, deposited, and then lithified, or converted into rock, through the process of compaction and cementation.

• Clastic Sedimentary Rocks: Made from fragments (clasts) of other rocks, compacted and cemented over time. Shale, a common example, forms from compacted mud and clay.
• Chemical Sedimentary Rocks: Originating from mineral precipitation from solutions. These include rocks like limestone, formed from calcite precipitated from marine waters.
• Organic Sedimentary Rocks: Consist of organic material, such as coal, formed from accumulated plant debris in swamp environments.

These rocks play a vital role in the Earth's geological history, often containing fossils that record past life and environmental conditions.

Heat and Pressure in Rock Formation

The transformation of rocks through heat and pressure is crucial in forming metamorphic rocks. Heat provides energy for recrystallization and influences the minerals present in the rock, while pressure from overlying rocks and tectonic forces alters the rock's physical structure.

• Recrystallization: High temperatures cause minerals to recrystallize into new, stable forms with distinct textures. This process can change the rock's appearance and strength.
• Directed Pressure: Often associated with tectonic activity, it causes realignment of mineral grains, forming foliation or banded textures.
• Contact and Regional Metamorphism: Contact metamorphism occurs when rocks are heated by nearby magma, whereas regional metamorphism is associated with large-scale geological processes, such as mountain building, which apply pressures to broad regions.

This dynamic process not only reshapes existing rocks into metamorphic forms but also showcases the ongoing geological activity within the Earth.