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 more rapidly? Why?

Short answer

Chemical weathering predominates; basalt weathers faster due to its more easily altered minerals.

Step-by-step solution

Analyze Weathering Types

Mechanical weathering involves the physical breakdown of rocks into smaller pieces without changing their chemical composition. Chemical weathering involves the alteration of the rock's chemical composition, often facilitated by water, acids, and natural chemicals.

Understand Environmental Impact

In hot and wet regions, conditions are ideal for chemical weathering. The presence of heat and moisture enhances chemical reactions that break down rocks chemically rather than purely mechanically.

Compare Granite and Basalt

Granite is composed mainly of quartz and feldspar, which are more resistant to chemical weathering. Basalt, on the other hand, is composed of minerals like pyroxene and olivine, which are less resistant to chemical change and break down more easily.

Determine the Predominant Weathering Process

Given the hot and wet conditions of the environment, chemical weathering will predominate over mechanical weathering as moisture and temperature accelerate chemical processes.

Identify Which Rock Weathers Faster

Basalt will weather more rapidly than granite due to its mineral composition being more susceptible to chemical changes in the conditions provided.

Key concepts

Chemical Weathering

Chemical weathering is a process that changes the mineral composition of rocks through chemical reactions. Unlike mechanical weathering, where rocks are physically broken down, chemical weathering involves the transformation or dissolution of minerals. This transformation is often facilitated by components in the environment like water, oxygen, acids, and even biological organisms.

Water plays a crucial role as it can carry dissolved substances that enhance these chemical reactions. For example, rainwater, which is slightly acidic due to dissolved carbon dioxide, can react with minerals to form new, softer minerals that can be more easily washed away.

• Common processes include oxidation, where minerals react with oxygen to form oxides, often seen as rusting in iron-rich rocks.
• Hydrolysis, where water reacts with minerals to form new minerals and soluble salts, is another key process.

Thus, in regions that are hot and wet, like tropical climates, chemical weathering is very effective. The warm temperatures accelerate chemical reactions, while the abundant moisture increases the availability of water to facilitate these changes.

Mechanical Weathering

Mechanical weathering refers to the physical breakdown of rocks into smaller pieces without any change to their chemical composition. It is primarily driven by physical forces that exert pressure on rocks and often prepares them for further chemical weathering.

There are several mechanisms at play in mechanical weathering:

• Frost wedging, where water seeps into cracks in a rock, freezes and expands, causing the rock to break apart.
• Thermal expansion, where rocks expand and contract due to temperature changes, eventually leading to cracking.
• Biological activity, such as tree roots growing into cracks, can also physically break rocks apart.

However, unlike chemical weathering that thrives in hot and wet conditions, mechanical weathering is more common in environments where there are frequent temperature changes and less moisture. Therefore, in the context of the original exercise, chemical weathering would be favored due to the hot and wet environment.

Granite vs. Basalt Weathering

Understanding how granite and basalt respond to weathering processes is crucial in geology. Both are igneous rocks, but they have different compositions and structures, which affect how they weather.

Granite is primarily made up of quartz and feldspar. These minerals are more resistant to chemical weathering due to their stable chemical structures. It means granite tends to be more durable and weathers slowly over time.

• Quartz, a major component of granite, is very resistant to chemical weathering.
• Feldspar, though less resistant than quartz, still puts granite at an advantage compared to basalt.

On the other hand, basalt is composed mainly of minerals like pyroxene and olivine, which are less resistant to chemical weathering. These minerals are more susceptible to chemical alterations because of their reactive nature. In environments that favor chemical weathering due to heat and moisture, basalt tends to weather more rapidly than granite.
In summary, granite holds up better against chemical weathering than basalt, but both will eventually break down if subjected to strong enough environmental factors.