Question

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

Short answer

Chemical weathering will predominate, and basalt will weather faster due to its mineral composition.

Step-by-step solution

Understand Weathering Types

Weathering is the breakdown of rocks and minerals at Earth's surface. There are two main types of weathering: mechanical (physical) and chemical. Mechanical weathering occurs through processes like freeze-thaw cycles or root growth, while chemical weathering involves chemical reactions altering the rock's minerals.

Consider Climate Conditions

In a hot, wet region, the climate conditions are conducive to chemical weathering. High temperatures increase the rate of chemical reactions, and moisture is necessary to facilitate many chemical weathering processes like hydrolysis and oxidation.

Analyze Rock Composition

Granite is an igneous rock composed mainly of quartz, feldspar, and mica. Basalt is also igneous but consists predominantly of minerals like pyroxene and plagioclase, which are more susceptible to chemical weathering compared to the minerals in granite.

Determine Predominant Weathering Type

Given the hot, wet conditions, chemical weathering will predominate because these conditions accelerate chemical reactions more than mechanical ones.

Identify Which Rock Weathers Faster

Between granite and basalt, basalt will weather more rapidly in these conditions. This is because the minerals in basalt, such as plagioclase, are less resistant to chemical weathering compared to the quartz and feldspar in granite.

Key concepts

Chemical Weathering

Chemical weathering involves the breakdown and alteration of rocks through chemical reactions. This process changes the minerals within the rock, often forming new, more stable minerals. Some key processes of chemical weathering include:

• Hydrolysis: This is where water reacts with minerals, such as feldspar in granite, turning them into clay minerals.
• Oxidation: Similar to rusting, this process affects rocks containing iron, turning them into iron oxides.
• Dissolution: Minerals dissolve in water, often affecting calcite in limestone.

Chemical weathering is highly dependent on the presence of water and heat. In hot, wet climates, the rates of chemical weathering significantly increase due to the abundance of water to act as a solvent and high temperatures to speed up reactions.

Mechanical Weathering

Mechanical weathering, also known as physical weathering, is the process of breaking down rocks into smaller pieces without changing their chemical composition. This process is mainly driven by:

• Freeze-thaw cycles: Water seeps into cracks in rocks, freezes, expands, and eventually breaks the rock apart.
• Thermal expansion: Repeated heating and cooling can cause rocks to expand and contract, leading to fractures.
• Biological activity: Roots may grow into cracks in rocks, causing them to split further.

Unlike chemical weathering, mechanical weathering is typically more effective in climates with frequent temperature variations. However, in regions with consistent temperatures, mechanical weathering plays a smaller role compared to chemical processes.

Igneous Rocks

Igneous rocks are formed from the solidification of molten magma. There are two main types: intrusive, which cool slowly beneath the Earth's surface, like granite, and extrusive, which cool quickly on the surface, such as basalt. The mineral composition of these rocks influences their susceptibility to weathering.

• Granite: It is composed primarily of quartz, feldspar, and mica. These minerals are generally more resistant to chemical weathering, making granite a durable rock.
• Basalt: It contains minerals like pyroxene and plagioclase, which are less resistant to chemical weathering compared to the quartz and feldspar in granite.

The composition of igneous rocks determines their response to weathering, with minerals in basalt weathering faster than those in granite, especially under similar environmental conditions.

Climate Effects on Weathering

Climate plays a crucial role in determining the rate and type of weathering that predominates in a region. In hot, humid environments:

• Chemical weathering dominates: High temperatures and moisture levels facilitate rapid chemical reactions, such as hydrolysis and oxidation, leading to faster breakdown of minerals.
• Increased biological activity: Wet climates support more vegetation, which can enhance both chemical (through organic acids) and physical weathering (root growth).

In contrast, cold or dry climates see more mechanical weathering due to:

• Freeze-thaw cycles: Common in cold environments, these cycles can cause significant physical breakdown of rocks.
• Limited water availability: Reduces the chance for chemical reactions, making mechanical processes more significant.

Overall, climate conditions are key to understanding which weathering processes will be most effective in any given area.