Weathering

What is Weathering?

• The physical, chemical, and biological processes that break down rocks and minerals, turning large particles into smaller pieces.

Main Weathering Types

• Mechanical Weathering– Occurs when rocks are

broken down by physical processes, without triggering a chemical change in the parent material.

• Chemical Weathering– Occurs through the

chemical interactions between water, atmospheric gases, and other substances with rocks.

Mechanical Weathering• Wind and rain are the two main

forces that contribute to weathering (abrasion)

• Further affected by daily and seasonal temperature variation causes thermal expansion and contraction of the parent material

• Ice Wedging

• Biological mechanical weathering

• Crystallization of minerals such as salt

Thermal Expansion

• Temperature differences within rock can cause expansion or shrinking

• This results in cracking or fracturing

Abrasion

• Weathering due to the movement of material over a rocks surface

– Water and wind alone can cause abrasion, both can also carry sediments or rock that can cause further abrasion

Fracturing, Unloading or Pressure Release

• Fracturing or jointing occurs due to internal or external forces on the rock along joints within the rock

• Forces of Earth can push rocks deep in the ground to the surface

• Outer layers can also be removed due to weathering and erosion, exposing lower layers

• The pressure within this newly exposed rock is still high, but surface pressure is lower, this causes the rock to shift and expand, resulting in exfoliation

Ice Wedging

Biological Mechanical Weathering

• Root wedging– As sediments build

in rock cracks, plants can grow. As these plants grow, their roots grow larger and can make cracks bigger, wedging rock apart

• Burrowing of animals

Mineral Crystallization

• Elements from dissolved rock, water, and the atmosphere collect in rock fractures or along grain boundaries

• These elements precipitate and crystallize, disintegrating and weathering the rock

Chemical Weathering• Occurs due to chemical

reactions that dissolve minerals in rocks or that change the chemical structure of these minerals resulting in new types of minerals

• Acids that are naturally formed can contribute to chemical weathering

• Oxidation (Rust)

• Minerals dissolve in water creating a solution

• Biological Chemical Weathering

Chemical Weathering by Acids

• Carbon dioxide in the atmosphere combines with water to form a weak acid (carbonic acid)

• This acid reacts with minerals in rocks such as calcite and feldspar, causing weathering and can result in the formation of minerals such as clay

Oxidation• Chemical weathering of materials due to

continued exposure to oxygen and or water

• Specifically affects metallic materials such as iron and copper

Solution in Water• Water is a polar material

that easily moves through cracks and pores

• Due to its polar nature, it forms bonds with ions

• Releases hydrogen ions that can easily penetrate the mineral structure of rock

• Also carries ions that can be bonded with rock minerals

Biological Chemical Weathering

• Weathering due to acids released by plants or animals or due to the decomposition or organic material

• Example: Lichen– Alice Algae and Freddie

Fungus took a “lichen” to each other. Their marriage has been on the rocks ever since.

Surface Area

• Surface area is the measure of how much exposed area a solid object has, expressed in square units.

• As materials are mechanically weathered, surface area increases

• Greater the surface area, greater the ability to be weathered due to greater exposure

Effect of Climate on Rock Type

• Different types of rock and minerals vary in their chemical and physical composition, resulting in some being more susceptible to weathering than others

• Temperature and moisture due to climate can also result in different rates of weathering