Question

Granite and basalt are exposed at Earth's surface in a hot, wet region. Will mechanical weathering or chemical weathering predominate? Which rock will weather most rapidly? Why?

Short answer

Chemical weathering predominates; basalt weathers faster than granite because it contains less stable minerals.

Step-by-step solution

Understanding Weathering Types

Weathering is the process of breaking down rocks. Mechanical weathering involves physical changes, like breaking rocks into smaller pieces. In contrast, chemical weathering involves chemical changes, altering the rock’s chemical composition. Chemical weathering is more prevalent in hot, wet regions because water and heat accelerate chemical reactions.

Analyze the Environment

The environment described is hot and wet. These conditions are ideal for chemical weathering because water facilitates chemical reactions, and higher temperatures increase reaction rates. Thus, chemical weathering is expected to dominate in this environment.

Comparing Granite and Basalt

Granite and basalt are two different types of rock. Granite is a felsic rock, with high silica content and minerals like quartz, which are chemically stable. Basalt, on the other hand, is a mafic rock, rich in olivine and pyroxene, which are less chemically stable and more susceptible to chemical weathering.

Determining Which Rock Weathers Faster

In a hot, wet environment where chemical weathering dominates, basalt will weather more rapidly than granite. This is due to the presence of minerals in basalt that are more prone to chemical alteration compared to those found in granite.

Key concepts

Mechanical Weathering

Mechanical weathering, also known as physical weathering, is the process through which rocks break down into smaller pieces without changing their chemical composition. This type of weathering is often caused by environmental factors, such as temperature changes, water, ice, and wind pressure.
For example, in colder climates, water can seep into cracks of rocks and freeze. As water freezes, it expands, causing the rock to fracture and break apart. This is known as frost wedging.
Other forms of mechanical weathering include:

• Abrasion: Rocks and sediments grinding against each other wearing surfaces away.
• Exfoliation: Layers peeling off rocks due to temperature changes.
• Biological Activity: Roots growing in cracks and breaking rocks apart.

While mechanical weathering significantly impacts the physical appearance of rocks, it does not alter their chemical makeup. This kind of weathering is more prevalent in environments where temperature fluctuates significantly or where physical forces like wind and water movement are dominant.

Chemical Weathering

Chemical weathering involves the breakdown of rocks through chemical changes, altering the minerals that make up the rock. In hot, wet regions, chemical weathering becomes more prevalent because high temperatures and abundant moisture accelerate chemical reactions.
A key process in chemical weathering is oxidation, which is the reaction of oxygen with minerals in the rock, often creating rust-like coatings. Another important process is hydrolysis, where water reacts with minerals to form new compounds, leading to rock decomposition.
Common agents in chemical weathering include:

• Water: Dissolves minerals and facilitates other chemical reactions.
• Oxygen: Promotes oxidation of minerals.
• Carbon Dioxide: Forms weak carbonic acid when dissolved in water, aiding in mineral decay.

In climates that are hot and humid, such as tropical regions, chemical weathering is particularly aggressive. This is because the constant presence of water acts as a catalyst, while the heat speeds up the reactions, leading to faster decomposition of rock materials.

Rock Types

Rocks vary in their makeup and chemical composition, which influences how they weather. Two common rock types are granite and basalt, both formed from volcanic activity but differing in texture and mineral content.
Granite is a type of felsic rock that is rich in silica and composed of minerals such as quartz, feldspar, and mica. These minerals are relatively stable and resistant to chemical change. Therefore, granite is less susceptible to rapid chemical weathering.
In contrast, basalt is a mafic rock containing minerals like olivine and pyroxene. These minerals are less stable and more prone to chemical alterations, making basalt more vulnerable to weathering compared to granite.
In environments where chemical weathering predominates, the mineral composition of a rock is crucial in determining its durability. Basalt, with its easily alterable minerals, will weather more quickly than granite in hot and wet regions. Thus, understanding rock types is crucial for predicting how quickly they might weather in different environmental conditions.