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; basalt will weather faster than granite.

Step-by-step solution

Understand Weathering Types

Mechanical weathering refers to the physical breakup of rocks without changing their chemical composition; this can happen due to wind, water, or temperature changes. Chemical weathering involves the breakdown of rocks through chemical reactions, often involving water, leading to mineral alterations.

Analyze Specific Conditions

In hot, wet regions, chemical weathering is more dominant due to the abundance of water and warmth, which facilitate chemical reactions. Water acts as a solvent and a reactant, promoting chemical changes.

Compare Granite and Basalt Weathering Rates

Granite is a felsic rock composed primarily of quartz and feldspar, both relatively resistant to chemical weathering. Basalt is a mafic rock rich in minerals like olivine and pyroxene, which weather more easily in chemical processes due to their susceptibility to hydrolysis and oxidation.

Determine Faster Weathering Rock

Basalt will weather more rapidly than granite in a hot, wet environment. Its minerals are less stable chemically and more prone to reactions with water and oxygen, while granite's minerals resist chemical change.

Key concepts

Mechanical Weathering

Mechanical weathering refers to the process where rocks are broken into smaller pieces by physical processes. Unlike chemical weathering, it doesn’t involve any chemical change in the composition of the rock. This type of weathering can occur due to several natural phenomena.

Examples include:

• Frost wedging, where water enters cracks in a rock, freezes, and expands, causing the rock to break apart.
• Thermal expansion, where repeated heating and cooling cause the rock to crack.
• Abrasion, where particles carried by wind, water, or ice scrape against rocks, leading to physical wear.

Mechanical weathering is particularly effective in areas with extreme temperature fluctuations or where water is abundant and subject to seasonal freezing.

Chemical Weathering

Chemical weathering involves the chemical breakdown of minerals in a rock, leading to the alteration of the rock’s internal structure. This process is aided by factors such as water, air, and biological activity.

Here's how it typically works:

• Hydrolysis: Water reacts with minerals like feldspar to form clay.
• Oxidation: Oxygen in the air reacts with minerals, especially iron-rich ones, causing rust-like effects.
• Carbonation: Carbon dioxide dissolved in water forms carbonic acid, which can dissolve softer rocks like limestone.

Chemical weathering is most intense in hot, wet climates where water facilitates and speeds up these chemical reactions.

Granite and Basalt Weathering

Granite and basalt, both common at Earth's surface, respond differently to weathering processes due to their varied mineral compositions. Granite, enriched with quartz and feldspar, tends to resist chemical weathering. Quartz is particularly sturdy and remains largely unchanged. Feldspar, on the other hand, can undergo hydrolysis but at a slower rate compared to minerals in basalt. Basalt, composed predominantly of minerals like olivine and pyroxene, weathers more quickly. These minerals are more chemically unstable and react readily with water and acids in the environment. In hot and wet conditions, basalt's susceptibility means it will degrade faster than granite.

Mineral Composition and Weathering Rates

The weathering rate of a rock is largely determined by its mineral composition. More reactive minerals on the chemical spectrum contribute to faster weathering. Factors influencing these rates include:

• Solubility: Rocks containing minerals with high solubility, like halite, dissolve quickly in the presence of water.
• Mineral stability: Minerals like quartz are extremely stable, slowing weathering processes, whereas olivine and pyroxene are less stable and faster to react.
• Climate conditions: Despite a rock’s mineral composition, environmental factors like temperature and humidity can alter rates of weathering significantly.

Understanding these factors in tandem helps geologists predict how different rocks will weather over time and under varying climatic conditions.