Chapter 13: Problem 16

What weathering processes produce a granite dome? Describe the sequence of events.

Short Answer

Expert verified

The formation of a granite dome occurs through a sequence of physical, chemical and biological weathering processes. The sequence begins with physical weathering such as ground freezing, thawing and tree root growth which break up the granite into smaller pieces. This is followed by exfoliation where pressure release causes the granite to expand, crack and eventually peel off in sheets from the main rock body. Chemical weathering like oxidation changes the mineral composition of the granite, weakening and further breaking down the rock. Biological weathering by organisms such as lichen that release acids also contributes to the degradation of the rock.

Step by step solution

Identification of weathering processes

Granite domes are formed primarily through three types of weathering processes. These are physical or mechanical weathering, chemical weathering, and biological weathering. The physical weathering involves the breaking up of rocks by mechanical forces such as water, wind, ice, and changes in temperature. The chemical weathering involves the alteration or decomposition of rocks by chemical reactions. The biological weathering involves the actions of plants and animals.

Elaboration on the sequence of weathering processes

The sequence of weathering processes that results in the formation of a granite dome usually starts with physical weathering. As granite is a hard, crystalline type of rock, it is highly resistant to weathering. Weathering begins when the granite is exposed to the elements, usually through uplift and erosion of the overlying materials. Physical weathering such as ground freezing and thawing, and tree roots growing into the cracks, break up the granite into smaller pieces.

Explanation of Exfoliation process

Subsequent to physical weathering, an important process in granite dome formation is exfoliation. This involves the peeling away of outer layers of the rock, much like the way skin peels. This happens because of the pressure release as the overlying material is removed, causing the granite to expand and crack parallel to its surface. Over time, these cracks get larger and eventually lead to sheets of rock separating from the main body of granite.

Contribution of Chemical & Biological Weathering

Along with physical weathering and exfoliation, chemical and biological weathering also contribute to the formation of a granite dome. Chemical weathering, for example through oxidation, can change the mineral composition of the granite, leading to weakening and further breaking down of the rock. Biological weathering also contributes by organisms like lichen that release acids that break down the rock.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Weathering Processes

Weathering is a natural and continuous process that breaks down rocks and minerals on Earth's surface. It is essential in the formation of landforms like granite domes, with different types of weathering playing distinct roles. The processes include physical, chemical, and biological weathering.

These weathering processes work together to disintegrate the rock material. Over time, the combined effects of weathering can significantly alter the landscape, setting the stage for new geological formations to develop.

Physical Weathering

Physical weathering, also known as mechanical weathering, involves the breakdown of rocks into smaller pieces without changing the rock's mineral composition.

Examples of Physical Weathering

Thermal expansion and contraction due to temperature changes
Frost wedging where water seeps into cracks, freezes, and expands
Crystallization of salts from evaporating water
Abrasion from wind, water, or ice carrying particles that grind against rock

These actions can create cracks and fissures in granite that ultimately contribute to the formation of a dome-shaped structure as pieces of rock gradually break away.

Chemical Weathering

Chemical weathering changes the chemical structure of rocks through various reactions. Water, acids, and gases in the atmosphere can chemically interact with rock minerals, which often leads to their dissolution or transformation into other minerals.

Key Chemical Weathering Processes

Hydrolysis, where water reacts with minerals to form new compounds
Oxidation, the reaction of minerals with oxygen, resulting in ‘rusting’
Carbonation, where carbon dioxide in water forms carbonic acid that dissolves certain minerals

As minerals within granite are affected, the rock becomes weaker and more susceptible to other weathering processes.

Biological Weathering

Biological weathering is the breakdown of rocks by living organisms. Animals, plants, fungi, and bacteria all contribute to this process.

Biological Influences on Weathering

Roots of trees and plants growing into cracks, exerting pressure on the rock
Organisms like lichens and mosses, which produce chemicals that dissolve rock minerals
Burrowing animals disrupting the rock structure

This type of weathering can be considerable over time, as even microscopic life forms can produce acids that weaken rock.

Exfoliation

Exfoliation is a type of physical weathering specific to rock formations such as granite domes. It occurs when overlying rock layers and soil are removed, often by erosion, relieving pressure on the underlying rock. The rock below expands slightly and fractures parallel to the surface, creating sheet-like sections.

This 'unloading' often leads to the peeling away of rock layers, much like an onion being peeled. Exfoliation is a critical step in the creation of granite domes, contributing to their distinctive rounded shapes as layers are stripped away.

Geological Erosion

Geological erosion is the wearing away of Earth's surface by natural agents such as water, wind, ice, and gravity. It is a powerful force in shaping the landscape and is responsible for transporting weathered rock and soil from one location to another.

Erosion can expose deeper rock layers, as occurs with granite domes, and result in the transportation of sediments that can form new geological features elsewhere. While often slow, over geological time scales, erosion can dramatically reshape entire regions.

Rock Decomposition

Rock decomposition is the end result of combined weathering and erosion processes. It involves the breakdown of bedrock into soil and sediment.

Three main types of weathering—physical, chemical, and biological—contribute to rock decomposition. As these processes continue, rocks weaken and eventually disintegrate into smaller fragments or soil constituents. Rock decomposition is a fundamental aspect of soil formation and plays an important role in the nutrient cycle and the preparation of the Earth's surface for vegetation.

What weathering processes produce a granite dome? Describe the sequence of events.

Short Answer

Step by step solution

Identification of weathering processes

Elaboration on the sequence of weathering processes

Explanation of Exfoliation process

Contribution of Chemical & Biological Weathering

Key Concepts

Weathering Processes

Physical Weathering

Examples of Physical Weathering

Chemical Weathering

Key Chemical Weathering Processes

Biological Weathering

Biological Influences on Weathering

Exfoliation

Geological Erosion

Rock Decomposition

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