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 in a hot, wet region; basalt weathers faster than granite.

Step-by-step solution

Understand What Weathering Is

Weathering is the process of breaking down rocks and minerals through contact with Earth's atmosphere, waters, and biological organisms. There are two main types: mechanical (or physical) weathering and chemical weathering.

Differentiate Mechanical and Chemical Weathering

Mechanical weathering involves breaking rocks into smaller pieces without changing their chemical composition. This can be due to factors like temperature changes, frost, etc. Chemical weathering involves the alteration of the rock's chemical composition, often driven by water and chemicals present in the environment.

Analyze the Environmental Conditions

The region is described as hot and wet. These conditions enhance chemical weathering because water and warmth accelerate chemical reactions that break down rocks.

Determine the Rock Composition

Granite is a coarse-grained igneous rock containing mainly quartz and feldspar. Basalt is a fine-grained igneous rock that is mostly composed of pyroxene and plagioclase.

Evaluate Which Type of Weathering Will Predominate

In a hot, wet environment, chemical weathering will predominate over mechanical weathering because the conditions favor chemical changes in the rock's minerals.

Assess Which Rock Will Weather More Rapidly

Basalt will weather more rapidly than granite in a hot, wet environment. This is because basalt's minerals (like pyroxene) are more susceptible to chemical breakdown than the minerals in granite (like quartz).

Summarize the Findings

Chemical weathering will predominate in a hot, wet region, and basalt will weather more rapidly than granite due to its mineral composition.

Key concepts

Chemical Weathering

Chemical weathering is a natural process that alters the chemical structure of rocks. This type of weathering is prominent in environments where conditions are hot and wet, allowing for faster chemical reactions due to the presence of water and warmth. When rocks undergo chemical weathering, their minerals are changed into different substances. For example, feldspar in granite can transform into clay minerals, while dissolving minerals like calcite found in other rocks can be removed by water.

Chemical weathering plays a crucial role in soil formation as it helps break down larger rock pieces into smaller particles, enriching the soil with minerals. Over time, these processes not only alter landscapes but also regulate the composition of nutrients in an ecosystem. Remember, water is the main driver of chemical weathering as it facilitates reactions like hydrolysis, oxidation, and carbonation.

Mechanical Weathering

Mechanical weathering, also known as physical weathering, is a process where rocks are broken down into smaller pieces without any change in their chemical makeup. It is primarily driven by physical forces such as temperature variations, ice expansion, and even the activity of plants or animals.

Common examples include freeze-thaw cycles, where water seeps into cracks, freezes, and expands, widening the cracks and leading to the eventual breaking of the rock. Another example is exfoliation, where layers peel off due to temperature-induced expansion and contraction.

This form of weathering is prevalent in climates with significant temperature fluctuations, unlike tropical regions where chemical weathering is more dominant. Despite not changing the rock’s chemical structure, mechanical weathering contributes significantly to the landscape as it makes rocks more susceptible to other forms of weathering, including chemical processes.

Igneous Rocks

Igneous rocks are fascinating features of the Earth's crust, formed through the cooling and solidifying of magma or lava. They are primarily categorized into two types: intrusive (or plutonic) rocks, which form beneath the Earth's surface, and extrusive (or volcanic) rocks, which solidify at the surface.

The texture and composition of igneous rocks depend on their cooling history. For instance, slow cooling allows for large crystals to form, as seen in granite, while rapid cooling leads to tiny crystals, characteristic of basalt. Igneous rocks can be further distinguished based on their mineral content. They are enriched with minerals such as quartz, feldspar, pyroxene, and plagioclase, which contribute to their durability and weathering characteristics.

Understanding the formation and characteristics of igneous rocks is key to studying geological features and processes, as they are the primary source of information about the Earth's mantle and tectonics.

Granite

Granite is one of the most well-known types of igneous rock, often recognized by its coarse-grained texture. It primarily contains quartz, feldspar, and mica, giving it its characteristic hardness and resistance to weathering. The quartz component is notably resistant to both mechanical and chemical breakdown, which is partly why granite is durable and not easily weathered.

Commonly found in continental crusts, granite is used extensively in construction due to its strength and aesthetic appeal. Its formation involves slow cooling of magma beneath the Earth's surface, allowing large distinct crystals to develop.

In weathering contexts, granite's mineral composition makes it less susceptible to rapid chemical weathering compared to other rocks like basalt. This inherent resistance plays a crucial role in shaping landscapes where granite formations remain relatively intact over long geological periods.

Basalt

Basalt is a type of igneous rock that forms from the rapid cooling of lava at or near the Earth's surface. Known for its fine-grained texture, basalt consists mainly of pyroxene and plagioclase, which makes it denser and generally darker compared to granite.

Because basalt cools quickly, it generally has smaller crystals compared to granite, making it more vulnerable to chemical weathering. In hot, wet climates, chemical weathering processes such as hydrolysis and oxidation can easily break down basalt's mineral components, leading to rapid alteration and decomposition.

Despite being less resistant to weathering, basalt plays a crucial role in Earth's geological processes. It is one of the most common rock types found on Earth's surfaces, particularly ocean floors, and has significant implications for volcanic and tectonic activities.