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 will predominate, and basalt will weather more rapidly than granite.

Step-by-step solution

Understand Weathering Types

Weathering is the process of breaking down rocks. There are two primary types of weathering: mechanical (physical) and chemical. Mechanical weathering involves the physical breakdown of rocks into smaller pieces without changing their chemical composition. Chemical weathering involves the chemical alteration of minerals within the rock, often resulting in new minerals and soluble salts.

Analyze Conditions

In a hot, wet region, the environmental conditions are conducive to chemical reactions. The presence of heat accelerates chemical reactions, while water acts as a solvent, promoting chemical weathering. Thus, chemical weathering is more likely to predominate in such an environment.

Examine Rock Types

Granite is a coarse-grained igneous rock composed mostly of quartz, feldspar, and mica. Basalt is a fine-grained igneous rock primarily composed of plagioclase and pyroxene. Basalt's minerals weather more easily than those in granite, especially under chemical weathering conditions.

Determine Weathering Rate

Given the susceptibility of basalt's minerals to chemical weathering, basalt will weather more rapidly than granite in a hot, wet climate. The stability of granite's main minerals (like quartz) under these weathering conditions makes it more resistant compared to basalt.

Key concepts

Mechanical Weathering

Mechanical weathering, also known as physical weathering, happens when rocks are broken down into smaller pieces without any change in their chemical structure. Imagine ice wedging into cracks in the rock, expanding as it freezes, and slowly prying the rock apart. Wind can also play a part, blowing tiny rock particles against larger rocks and wearing them away over time. In some cases, plant roots can grow into rock crevices and force the rocks to crack as the roots expand. This process can happen anywhere in the world, but it’s most effective in climates where temperature changes cause freeze-thaw cycles.

Mechanical weathering does not rely on water or chemical reactions.

• Involves physical force – like ice, water, or wind.
• Doesn't change the chemical nature of rocks.
• Often increases the surface area of rocks, making them more susceptible to further weathering.

Chemical Weathering

Chemical weathering changes the actual mineral composition of rocks through chemical reactions. When water, oxygen, carbon dioxide, or acids contact minerals, they can alter them and result in new minerals and dissolved ions. This process transforms the rock to a softer or more brittle state, enhancing its breakdown. A key factor in chemical weathering is the presence of water, which acts both as a solvent and a medium for these reactions.

Targets specific minerals that are more susceptible to breakdown in acidic or moist environments.

• More prominent in hot and wet climates.
• Examples include oxidation (rusting) of minerals containing iron, and the dissolution of limestone in acidic rainwater.
• Can completely transform minerals and contribute to the formation of soil.

Igneous Rocks

Igneous rocks form from the cooling and solidification of magma or lava. These rocks are categorized based on their texture and mineral composition. Texture varies depending on where the rock cools and crystallizes: coarse-grained rocks like granite form slowly beneath the Earth's surface, while fine-grained rocks like basalt form quickly at or above the surface.

Igneous rocks are considered primary rocks from which others, such as sedimentary and metamorphic rocks, can form.

• Granite and basalt are two common types.
• Granite is usually lighter in color with larger crystals.
• Basalt is darker with a finer texture.

Granite

Granite is a well-known igneous rock characterized by its durability and coarse texture. Formed deep underground, granite slowly cools, allowing crystals of quartz, feldspar, and mica to develop. These minerals make it quite resilient to both mechanical and chemical weathering. Quartz is notably resistant to chemical attacks, which gives granite enduring qualities.

Despite its rough texture, granite is widely used in construction due to its strength.

• Often appears in shades of pink, gray, or white.
• Used in countertops, buildings, monuments, and statues.
• Resists weathering better than many other rock types, especially in chemically active environments.

Basalt

Basalt is another common type of igneous rock, typically formed at the surface from lava flows. It is dense and fine-grained, composed mostly of plagioclase and pyroxene minerals. Unlike granite, basalt weathers relatively quickly, especially in environments that promote chemical weathering, such as humid and hot regions.

This quick weathering is due to its mineral composition, which reacts more readily to chemical changes in the environment.

• Commonly found in oceanic crust and volcanic regions.
• Tends to appear in dark colors such as black, dark gray, or green.
• Used in construction, particularly as an aggregate for road-base and concrete.