Bellringer What processes form igneous rocks? Sedimentary? Metamorphic?

Weathering

Review: Weathering – when rocks on or near Earth’s surface break down and change Erosion – the removal and transport of weathered material from one location to another Deposition – when the erosion movement slows and drops the weathered material E W D

Weathering Weathering, erosion, and deposition have been happening for millions of years, constantly changing Earth’s surface

Weathering Two types of weathering: Mechanical (Physical) Chemical

Mechanical Weathering Mechanical weathering – breaking down rocks and minerals into smaller pieces Also known as physical weathering because it changes the physical appearance of rocks Changes the rock’s size and shape, but not its chemical composition The most common type of weathering in cool and dry climates

Mechanical Weathering Since weathering constantly wears away at rocks, the more a rock has been weathered, the smoother and rounder it will be

Mechanical Weathering Agents 1. Freezing temperatures Frost wedging – when water in rock cracks freezes and expands, making the rocks crack open or break

Mechanical Weathering Agents 2. Pressure Rocks buried underneath other rocks are under a lot of pressure When that extra weight and pressure are suddenly removed (construction, mining, etc), the relief in pressure can cause cracks that are parallel to the rock surface

Mechanical Weathering Agents Example: exfoliation – outer rock layers are stripped away

Mechanical Weathering Agents 3. Tree and Plant Roots – wedge into cracks of rocks and cause them to split

Chemical Weathering Chemical weathering – breaks down rocks into new substances through chemical reactions Changes the rock’s chemical composition Most common type of weathering in warm and wet climates

Chemical Weathering Agents Water – can dissolve many minerals in rocks and acts as a medium for chemical reactions to occur Hydrolysis – the chemical reaction of water with other substances

Chemical Weathering Agents 2. Oxygen – combines with minerals and causes chemical reactions Oxidation – chemical reaction of oxygen with other substances Example: the oxidation of iron produces rust (Fe3O4 + O2  Fe2O3)

Chemical Weathering Agents 3. Carbon Dioxide – mixes with water or water vapor to form carbonic acid

Chemical Weathering Agents Carbonic acid dissolves calcite, a mineral found in limestone, which can create caves where the rock has been eaten away

Chemical Weathering Agents 4. Acid rain – rain with a pH < 5.6 Formed when sulfur dioxide (from burning fossil fuels) or nitrogen oxides (car exhaust) are released into the atmosphere and react with water vapor to create sulfuric and nitric acids, respectively

Chemical Weathering Agents Eats away at rocks/minerals Can kill plants and animals (especially aquatic animals)

What Affects Weathering? 1. Climate Mechanical weathering occurs more quickly in colder places (more ice = more frost wedging) Chemical weathering occurs more quickly in warm, wet climates because chemical reactions occur more quickly in higher temperatures

Mechanical or Chemical? Montana Florida Cool and dry = mechanical warm and wet = chemical

What Affects Weathering? 2. Rock Type and Composition Sedimentary rocks are more likely to weather than igneous or metamorphic rocks Why might that be? Igneous and metamorphic rocks are forged together by high heat and/or pressure Sedimentary rocks are loosely layered and cemented together

What Affects Weathering? 3. Surface Area – how much of the rock is exposed to wind and water As a rock breaks down into smaller and smaller pieces, the MORE surface area it has The more area exposed, the more weathering can take place As rocks are broken down into smaller and smaller pieces, their surface area (SA) and rate of weathering increase. (low SA) (high SA)

What dissolves faster? Sugar Cubes Granulated Sugar or

Surface area increases as rocks are weathered into smaller pieces

What Can Affect Weathering? 4. Topography – materials on sloped surfaces are more likely to weather and erode due to gravity As rocks roll downslope, they expose more underlying rock, which are then subject to weathering

Quiz I’ll give you 5-10 minutes to look over your notes (Chemistry – Rock Cycle) Make sure you place your quiz in the correct box

Weathering Analysis – look at the images and answer the corresponding questions. DO NOT WRITE ON THE COLORED PAGES!