Weathering and Erosion

Learning Objectives Explain the differences in chemical and physical weathering Explain how weathering rates are affected by a variety of factors including climate, topography and rock composition Compare erosion by water, wind, ice, and gravity and the effect on various landforms

Weathering Weathering is the process by which rocks on or near Earth’s surface break down and change There are two types of weathering: Mechanical Weathering Chemical Weathering

Mechanical Weathering Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition Definite change in size and shape Occurs mostly where it is cool and dry

Factors that Affect Mechanical Weathering There are processes in nature that play a vital role in mechanical weathering: Temperature / Frost Wedging Unloading / Exfoliation Biological Activity

Frost Wedging When water seeps into existing cracks in rock it will freeze if the temperature is low enough Once the water freezes, it expands beyond the space provided by the crack The expansion causes the rock to break

Unloading / Exfoliation Unloading reduces pressure on igneous rocks, causing it to expand, which allows slabs of outer rock to break off in layers in a process called exfoliation Exfoliation is the process by which outer rock layers are stripped away

Biological Activity The activity of organisms, including plants, burrowing animals, and humans, can also cause mechanical weathering For example, tree and plant roots wedge into cracks of rocks and cause them to split

Weathered Rock: Talus As sections of rock are wedged loose they may tumble into large piles called talus, which typically form at the base of steep, rocky cliffs

Chemical Weathering Chemical weathering is the transformation of rock into one or more new compounds (new chemical composition) Process by which rocks and minerals undergo changes in their composition as the result of chemical reactions Reactions result in the formation of new minerals and release of dissolved substances Occurs greatest where it is warm and wet

Factors that Affect Chemical Weathering Water – the reaction of water with other substances is called hydrolysis Oxygen – The chemical reaction of oxygen and other substances is called oxidation Carbon Dioxide – combines with water in the atmosphere to form weak carbonic acid which dissolves calcite found in limestone and other minerals Acids – caused mainly by the oxidation of sulfur dioxide and nitrogen oxides emitted by cars. (Many times this leads to acid rain.)

What Affects the Rate of Weathering? Climate Rock Type and Composition Surface Area Topography and Other Variables What Affects the Rate of Weathering?

Rock Type and Composition Climate Rock Type and Composition Climate affects areas precipitation, temperature, and evaporation Chemical Weathering occurs greatest in warm/wet climates Mechanical Weathering occurs greatest in cool/dry climates Characteristics of rocks, including how hard or resistant they are, depends on their type and composition Sedimentary rock are more easily weathered than igneous or metamorphic rocks Due to differential Weathering, which is caused by variations in composition, unusual and spectacular rock formations and landforms are created

Spheroidal Weathering Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock The spherical layers of highly decayed rock are referred to as saprolite Ultimately, the process causes the corners and edges of rock to be more rounded

Surface Area Mechanical weathering breaks rocks into smaller pieces As these pieces get smaller, their surface area increases As result, more surface area is available for chemical weathering So, the greater the surface area… the more weathering occurs

Topography / Other Variables Materials on level surfaces remain in place as they undergo changes Materials on sloped surfaces tend to move as result of gravity As materials move downward, they expose underlying rock surfaces and provide more opportunities for weathering to occur Animals also increase weathering through the release of carbon dioxide during decay Living plants release carbon dioxide as well, which combines with water to produce acid

Erosion and Deposition Erosion is the process that transports the Earth’s materials from one place to another At some point, the movement of materials slows and the minerals are “dropped” in a new location (deposition) Gravity is a huge factor in many erosional agents, such as streams and glaciers, because the force of gravity pulls materials downslope

What is Erosion? Erosion Video: What is Erosion

Erosional Agents Water More power to move large particles than most winds Greater the volume of water, the more material that can be eroded The steeper the slope, the increase of erosional ability Small channels (rill erosion) of water run downward to meet larger channels (gully erosion) draining into an area called a watershed

How Does the Force of Water Shape the Earth? Water Erosion

Erosional Agents Glacial Erosion Although glaciers cover less than 10% of the Earth’s surface, their erosional effects are large-scale and dramatic They scrape out and gouge large sections of landscape They are so powerful, they have the ability to pick up large rocks and move them over great distances

How Do Glaciers Shape the Earth? Glacial Erosion

Erosional Agents Wind Major erosional agent in areas on Earth that experience limited precipitation, high temperatures, and therefore minimal vegetation to hold soil in place As result, wind can easily pick up and transport fine, dry particles The effects of wind erosion can be damaging and dramatic, although wind erosion is relatively insignificant to erosion by water and glaciers Wind barriers can be created as a farming method to reduce soil erosion

Erosional Agents Living Variables Plants, animals and humans all have activities in their daily lives that affect and move Earth’s materials from one place to another For example, plants move surface materials as they carry on their life process (roots grow) Animals burrow into soil – relocating the Earth’s materials Humans relocate soil every time they plant a garden or build a new development

How do Human Forces Shape the Earth? Erosion Video: Human Impact