Introduction to Physical Geology

Introduction to Physical Geology

• Earth is composed of rocks.
• Rocks are made of minerals.
• Earth is constantly changing, leading to changes in rocks.

Weathering

• Weathering: the physical disintegration or chemical alteration of rocks on or near Earth's surface.
• Erosion: the physical removal and transport of weathered material by water, wind, ice, or gravity.

Controlling Factors of Weathering

• Composition and structure of the rock
• Nature of the ground slope
• Climatic variations
• Floral effects

Agents of Weathering

• Wind
• Glaciers
• Running water
• Sea waves
• Underground water

Types of Weathering

• Physical weathering
• Chemical weathering
• Biological weathering

Physical Weathering

• Also known as mechanical weathering.
• Physical disintegration and reduction in the size of rocks without changing their chemical composition.
• Examples: exfoliation, frost wedging, salt wedging, temperature changes, and abrasion.
• Causes of physical weathering: changing temperature, freezing water, plant roots.

Mechanical Weathering Processes

• Exfoliation: Layers of rock peel off.
• Frost Wedging: Water expands when freezing inside cracks, forcing rocks apart.
• Salt Wedging: Dissolved salts crystallize in cracks, pushing rocks apart.
• Temperature Changes: Repeated heating and cooling cause expansion and contraction, weakening rocks.
• Abrasion: Rocks collide with each other, wearing them down.

Mechanical Weathering: Frost Wedging

• Water trapped in cracks freezes and expands, widening the cracks over time.

Mechanical Weathering: Temperature Changes

• Daily and seasonal temperature changes cause some minerals to expand and contract, creating stress and eventually breaking rocks into smaller fragments.

Mechanical Weathering: Salt Wedging

• Dissolved salts crystallize in cracks of rocks and exert pressure to break the rocks apart.
• Common in arid climates.

Mechanical Weathering: Abrasion

• This occurs when rocks collide with each other.
• Rocks are transported by wind or water and rub against each other weakening and breaking them down.

Chemical Weathering

• Decomposition and disintegration of rocks due to chemical reactions.
• Water causes a change in the chemical composition of rocks.
• Processes: carbonation, hydrolysis, hydration, oxidation, and solution.

Types of Chemical Weathering

• Solution: Dissolution of soluble minerals in water.
• Oxidation: Reaction of atmospheric oxygen with minerals, often causing color changes.
• Carbonation: Reaction of carbonate or bicarbonate ions with minerals, dissolving minerals.
• Hydration: Minerals absorb water, causing expansion leading to weakening of rocks.

Chemical Weathering: Carbonation

• Carbon dioxide and rainwater combine to form carbonic acid.
• This weak acid reacts with carbonate minerals, dissolving them.

Chemical Weathering: Hydrolysis

• Chemical reaction between H+ and OH- ions in water and minerals in rocks.
• Produces new compounds that are often softer and weaker than the original rock.

Chemical Weathering: Hydration

• Minerals absorb water, resulting in expansion and stress on the rock, leading to disintegration.

Chemical Weathering: Hydration Examples

• Anhydrite absorbs water to form gypsum.

Biological Weathering

• Disintegration or decay of rocks and minerals by chemical or physical agents of organisms.
• Processes: organic activity from algae, rock disintegration by plant growth / roots, burrowing/tunneling organisms, secretion of acids.

Plant Roots in Biological Weathering

• Plant roots penetrate into cracks or rock surfaces, grow and exert pressure, causing rocks to split apart or break into smaller pieces.

Other Biological Agents

• Burrowing animals creating tunnels or cracks
• Animals (snails) producing acids
• Lichen, algae, and decaying plants leaching minerals and dissolving rocks.
• Chemicals excrete from human activity.

Differential Weathering

• Weathering rates vary based on rock type, texture, and climate.
• Harder rocks weather more slowly.
• Wet climates generally speed up weathering processes like that of limestone.