Question

Is it possible for two igneous rocks to have the same mineral composition but be different rocks? Support your answer with an example.

Short answer

Yes, granite and rhyolite have the same composition but different textures.

Step-by-step solution

Understand Mineral Composition

Mineral composition refers to the minerals present in a rock and their relative proportions. It determines many of the rock's properties, like color, texture, and density.

Define Igneous Rock

Igneous rocks are formed from the solidification of molten magma or lava. They can be classified based on where they form (intrusive or extrusive) and their mineral composition.

Consider Texture of Igneous Rocks

Two igneous rocks can have the same mineral composition but differ in texture. Texture is affected by the rate of cooling; slow cooling forms large crystals (coarse-grained) while fast cooling forms small crystals (fine-grained).

Provide an Example

Granite and rhyolite are examples of igneous rocks with the same mineral composition. Both are rich in quartz and feldspar but differ in texture; granite is coarse-grained while rhyolite is fine-grained due to differences in cooling rates.

Key concepts

Mineral Composition

Understanding mineral composition is crucial for identifying and characterizing igneous rocks. Mineral composition refers to the specific minerals that make up a rock and their proportions relative to each other.
These minerals dictate many of the rock's physical properties such as:

• Color: Different minerals impart different colors to the rock. For instance, rocks rich in iron-bearing minerals tend to be darker.
• Density: The specific gravity of the minerals contributes to the rock's overall density.
• Durability: The hardness of the minerals affects how resistant the rock is to weathering and erosion.

Granite is an example of an igneous rock with a mineral composition rich in quartz, feldspar, and mica, which determines its light color and higher durability.
Being able to analyze and understand these compositions helps geologists compare rocks and infer their formation history.

Texture

Texture is an important aspect that describes the appearance and feel of a rock's surface, particularly with regard to the size and arrangement of its mineral grains.
In igneous rocks, texture varies widely and provides insights into the rock's formation process.

Here's how texture plays a role:

• Coarse-grained texture: Formed when magma cools slowly beneath the Earth’s surface, allowing large crystals to grow, as seen in granite.
• Fine-grained texture: Resulting from rapid cooling of lava on the surface, leading to small crystals that are hard to see without magnification, typical of rhyolite.
• Glassy texture: Occurs when lava cools so quickly that no crystals have time to form, creating rocks like obsidian.

These textures help geologists distinguish between different types of igneous rocks even when they have similar mineral compositions.

Cooling Rates

The rate at which molten rock cools directly impacts the formation and characteristics of igneous rocks, particularly their texture.
Cooling rates are influenced by the environment in which the rock forms.

Key points about cooling rates include:

• Slow cooling: Occurs deep within the Earth (intrusive), where magma cools over thousands to millions of years, resulting in large mineral crystals, as seen in rocks like granite.
• Rapid cooling: Happens at or near the Earth’s surface (extrusive), where lava is exposed to air or water, leading to quick solidification and small crystals, creating rocks such as basalt and rhyolite.
• Instantaneous cooling: Results in a glassy texture when lava cools almost instantly, as with obsidian, due to contact with water or air.

Understanding cooling rates not only aids in classifying igneous rocks but also provides insight into the geological processes and environment where the rock formed.