Weathering, Erosion, and Soils Mandy Meeks, Baldwin Arts and Academics Magnet, Earth Science

Weathering

What is weathering?  Weathering refers to surface processes that break down rock.  Weathering breaks rock into small loose pieces called sediment.  Examples: sand, silt, and clay  Over millions of years weathering has changed Earth’s surface.

Mechanical Weathering  Mechanical weathering occurs when rocks are broken apart by physical processes.  The overall chemical makeup of the rock stays the same- each fragment of rock has the same characteristics as the original rock.

Mechanical Weathering- Plants and Animals  When plants grow in cracks in rocks, the roots enlarge the cracks.  Burrowing Animals loosen sediment and bring it to the surface where other weathering processes act on it.

Mechanical Weathering- Ice -Ice Wedging occurs in cold climates where water enters cracks in rocks and freezes. -Water expands as it turns to ice, extending the cracks and breaking rocks apart. -Ice wedging is a problem for roads in highways in the mountains, causing potholes to form.

Mechanical Weathering: Surface Area  Mechanical weathering reduces rocks into smaller pieces, which increases their surface area.  This exposes more of the rock to more water and oxygen, which speeds up the process of chemical weathering.

Chemical Weathering  Chemical Weathering occurs when chemical reactions dissolve the minerals in rocks or change them into different minerals.  This changes the chemical composition of the rock and often makes the rock weaker.

Chemical Weathering- Natural Acids  Carbonic Acid is formed when carbon dioxide and water react in the air.  Carbonic acid causes minerals like calcite to dissolve.  This causes stones like limestone to weather away, forming caves.  Chemical weathering causes the mineral feldspar, which is found in granite and sandstone, to be broken down into kaolonite, forming clay.

Chemical Weathering- Plant Acids  Plant roots and decaying plants give off acids that dissolve minerals in rocks.  As the rock breaks apart, nutrients become available to the plants.

Chemical Weathering- Oxygen  Oxidation is when some materials chemically react when exposed to oxygen and water.  Example: rusting of iron  Magnatite, a mineral that contains iron, oxidizes to form limonite.  Many oxidized minerals have a reddish color.

Weathering and Climate  Climate is the pattern of weather that occurs in a particular area over many years.  In cold climates, freezing and thawing causes many rocks to break down due to ice wedging.  Chemical weathering occurs faster in warm, wet climates.

Weathering and rock type  Some rocks weather more quickly than others.  Example: Marble and granite

Soils

Formation of Soil  Soil is a mixture of weathered rock, decayed organic matter, mineral fragments, water, and air.  Rock fragments do not become high quality soil without organic matter.  Soil can take thousands of years to form.

Factors Affecting Soil Formation 1.Climate 2.Slope of Land 3.Types of Rock 4.Types of vegetation 5.Amount of time rock has been weathering

Composition of Soil  Rock and mineral fragments  Organic matter  Humus- decayed plant and animal material in soil

Soil Profiles  Soil is divided up into layers called horizons.  All horizons of soil form the soil profile.  Soils have three horizons- A (top layer, lots of humus), B(middle layer, less humus), and C (bottom layer, consists of partially weathered rock).

Erosion

Soil Erosion  Soil erodes when it is moved from the place where it formed.  In normal conditions there is a balance between soil erosion and soil production.  Agents of erosion include gravity, water, wind, and glaciers.  Soil erosion is harmful because plants do not grow well when topsoil has been removed.

Human Causes of Erosion  Agricultural cultivation  Plowing, leaving fields bare after harvest  Forest Harvesting  Overgrazing

Erosion by gravity  The greater an object’s mass is, the greater is gravitational force is.  Mass movement is any type of erosion that happens as gravity moves materials downslope.  Material can move downhill in a large mass (slump) or when sediments slowly move downhill (creep).  Other kinds of mass movement include rockslides and mudslides

Glacier Erosion  A large mass of ice and snow moving on land under its own weight is a glacier  Continental glaciers cover 10% of Earth; they once covered as much as 28% of Earth.  Glaciers erode land as they pass over it and deposit eroded material somewhere else

Wind Erosion  Wind erodes the Earth via deflation (blowing small loose sediment around) and abrasion (windblown sediment striking rock).  Wind erosion can lead to sandstorms and dust storms.  Dunes form when sediment is deposited by the wind.  Many dunes migrate away from the wind unless they are planted with grass.  Planting vegetation is one of the best ways to slow wind erosion.

Water Erosion  Water that does not soak into the ground or evaporate but instead flows across Earth’s surface is called runoff.  The amount of rain and the length of time rain is falling affect runoff.  Slope of the land and vegetation also affects runoff.

Types of Water Erosion  Water can travel in channels, carrying away soil and causing rills and gullies.  Water can travel in sheets, picking up and carrying away sediments.  Water can form streams  Sediment carried by a stream is called the load.

Groundwater  Water that soaks into the ground is called groundwater.  The water table is the upper surface of groundwater.  Carbon dioxide dissolved in groundwater can form carbonic acid, causing caves and sink holes as it weathers away limestone.

Consequences of Erosion  Building on steep slopes  Lots of time and $$ spent trying to slow down erosion  Beachfront developments are affected by shoreline erosion.  Loss of Soil Productivity  Sediment pollution  Runoff from pesticides and fertilizers