What is weathering? What is erosion? Rocks & Weathering What is weathering? What is erosion?

Compare 2 US mountain ranges:

Sierra Nevada Mountains, CA

Appalachian Mountains, NC

What is weathering? The process that breaks down rock and other substances at earth’s surface Caused by different forces

What is erosion? The removal or movement of weathered materials from one place to another Caused by different forces

Weathering and Erosion work together CONTINUOUSLY to wear down and carry away the rocks at earth’s surface. weathering erosion

What causes weathering? Mechanical = physical Rocks and other materials on earth’s surface break down into smaller pieces; no change in the composition of the object itself Chemical Rocks and other materials on earth’s surface break down due to chemical changes in the object; the rock itself is changed; it has a new composition

Weathering acts slowly over time – it is a destructive force that changes the shape of earth’s surface. Both types of weathering work together to break down rock on earth’s surface.

Mechanical Weathering Mechanical (or physical) weathering affects all the rock on earth’s surface. What are the FORCES that cause mechanical weathering??

1. Release of Pressure “stuff” on top of the rock applies pressure on top of the rock, holding it in place. When that stuff erodes away, that pressure is released. This loosens the outer layer causing it to crack and flake off

Freezing and Thawing (ice wedging) Water seeps into cracks and freezes; when ice freezes it expands and pushes cracks open wider; remelts so water seeps further in – repeats over and over, and eventually cracks get larger and pieces break off. Cracks become wider and deeper. **big factor in areas where temperature warms in the day (above freezing), and cools at night (below freezing)

3. Animal Actions Animals burrow in the ground, loosening and breaking apart rocks in the soil Traveling, walking, pacing across rocks can also wear down rocks

4. Plant Growth Roots and plants grow in cracks, widening cracks even further as roots push down and out; can eventually break rocks apart

5. Abrasion Grinding away parts of rock – caused by wind carried sand, rock particles and ice; wears away expoed rock surfaces. Works like sandpaper, reshaping rock over time. Arches National Park, Utah

6. Thermal Expansion repeated daily heating and cooling of rock; heat causes expansion; cooling causes contraction. Similar to release of pressure, layers crack when cooling contracts the rock

Chemical Weathering Chemical weathering also affects all the rock on earth’s surface. What are the FORCES that cause chemical weathering??

1. Action of Water This is the MOST IMPORTANT cause. Water itself can dissolve components that make up the rock itself, which changes the rock’s overall composition

2. Oxygen (oxidation) Metals (iron, copper) in the rock react with oxygen in the air, which creates a new substance Rocks turn reddish, brownish, greenish

3. Carbon Dioxide Carbon dioxide is found in the air, when it dissolves in rainwater, rivers, groundwater, it makes a weak acid called carbonic acid – which weathers soft rock, dissolving it. Most caves formed this way – carbonic acid dissolving limestone.

4. Living Organisms The roots of plants, lichens, fungi and other microorganisms produce weak acids which dissolve rock around the roots.

5. Acid rain Pollutants in the air such as sulfur, carbon, and nitrogen combine with water vapor and rain to make acid rain. The rain then causes weathering.

How fast does weathering occur? This depends on different factors: Type of rock Is the rock soft or hard? Softer rocks are more vulnerable to both kinds of weathering. Permeable rocks (filled with little tiny holes so water can get through) are more vulnerable too.

Climate Warm climates – the heat speeds up chemical weathering Cool climates – speeds up mechanical weathering (ice wedging and heat expansion) Wet climates – speeds up BOTH chemical (all chemical weathering requires water) and mechanical

3. Surface Area Weathering is a SURFACE reaction – occurs wherever the atmosphere (oxygen) or water can get on or in the rock. The more surface area that is exposed, more weathering that will occur – so as a rock breaks down into smaller and smaller pieces, it weathers more and more easily because more surface area is exposed to air and water.

4. Presence of Plants and Animals as both contribute to weathering Topography steep terrains more open to weathering as rock is usually more exposed there; less chance of “stuff” building up to cover and protect rock from weathering