Weathering-a natural process that causes rocks to change, breaks them down, and causes them to crumble.
Causes of Weathering:
Thawing
Oxygen in the air
Plants
Mechanical weathering-breaks rocks into smaller pieces without changing them chemically
Two Causes of Mechanical Weathering
1. Ice wedging-freezing and thawing of rocks (can cause pot holes to develop
Rain and melted snow seeps into cracks, then freezes, when the water freezes, crystals form and grow taking up more space than the water did, this expansion exerts pressure on the rocks. Rocks can crack and eventually break apart.
2. Living organisms
A. Plants-plants grow in inconvenient places, the roots can grow in cracks where water collects, slowly exerting pressure and wedging rocks apart
B. Animals-burrow in the ground, the burrowing through sedimentary rock breaks rock apart
Chemical Weathering: occurs when chemical composition of rock changes
Chemical weathering takes place fastest in hot and wet climate.
Three Causes of Chemical Weathering:
1. Natural Acids: when rocks react chemically with natural acids in the environment.
Ex. Carbonic acid-water mixes with carbon dioxide to dissolve rocks such as marble and limestone.
2. Plant Acids-plant roots produce acid that reacts with rocks.
Ex. Tannic acid-dissolves some minerals in rocks and is produced by some plant roots.
If you see moss or other plants growing on a rock, peel it back you’ll likely see discoloration on the rock where plant acids are reacting chemically.
Growing plants can cause mechanical and chemical weathering.
3. Oxygen-when minerals in rocks combine with oxygen, chemical weathering takes place.
When you see rusty cars, reddish soil, or reddish stains on rock, you witness oxidation.
When some iron containing minerals are exposed to oxygen, they can weather to minerals that are like rust.
Oxygen is a major cause of chemical weathering.
Soil-mixture of weathered rock, organic matter, water, and air that supports the growth of plant life.
Factors that affect soil formation:
1. Parent rock: they can weather and the soil will be very similar to the parent rock.
Ex. Limestone is chemically weathered clayey soil is common.
EX. Sandstone is weathered sandy soil forms.
2. The slope of the land:
topography-surface land features
The topography of an area also influences the types of soils that develop.
EX. Steep hillsides, soil has little chance of developing because rock fragments move downhill, but in lowlands, where land is flat, wind and water deposit fine sediments that help form thick soil.
3. Climate:
If rock weathers quickly, deep soils can develop rapidly.
Humus: organic matter in soil found in tropical climate.
Humus helps soil hold water and provides nutrients plants need to grow.
4. Time: it takes time for rocks to weather.
In young soil the parent rock determines the soil characteristics.
As weathering continues soil resembles the parent rock less and less.
Thicker, well-developed soils are where erosion hasn’t taken place or new sediment has not been deposited.
5. Organisms:
Lichens-small organisms that consist of algae and fungus that live together for mutual benefit.
Lichens can row directly on rock.
Some plants contribute more organic matter to soil than others.
Section 2 Notes:
Agents of Erosion:
Erosion-the wearing away and removal of rocks or sediment
1. GRAVITY-pulls everything toward the earth’s center.
As a result water flows down hill and rocks tumble down slopes.
Mass movement-when gravity alone causes rocks or sediment to move down the slope-happens around mountains.
Four types of mass movement:
1. creep-the name for a process in which sediments move slowly downhill (common where freezing and thawing occur)
2. slump-occurs when a mass of rock or sediment moves downhill along a curved surface (frequently occur on slopes that have been undercut by erosion)
3. rock slides-layers of rock break loose from slopes and slide to the bottom
4. mudflows- a mass of wet sediment that flows downhill over the ground surface
2. ICE-agent of erosion
glaciers-large deep masses of ice
When the ice in a glacier becomes thick enough, it’s own weight causes it to flow downhill
As glaciers move over Earth’s surface they erode areas.
Continental glaciers: located near poles in Antarctica and Greenland.
Valley glaciers: are found at high elevations on many continents.
Glacial erosion: when this type of erosion occurs you can see long striations on the surface of rocks.
A glacier slides over rock that has cracks and ice can pull out pieces of rock.
Also the pieces of rock freeze to the bottom of the glacier and the rock scratches the rocks like a giant sheet of sandpaper.
Valley glaciers form large bowls and remove rocks from mountain tops-cirques
Sediment deposited directly from glacier ice is called till.
After the glacier melts, usually in the summer time this water produces rivers that carry and deposit sediment. Sand and gravel deposits laid down by these rivers are called outwash.
3. WIND
Erosion of the land by wind is called deflation.
The wearing away of rocks by blowing sand is called abrasion. It is like a sand blaster.
Common in deserts and cold regions with strong winds.
Sand dunes-when wind blows sand around a rock or clump of vegetation.
Silt-fine sediment
Loess-accumulation of silt can blanket earth’s surface and is as fine as talcum powder.
4. WATER
runoff-water that flows over Earth’s surface.
Runoff is an important agent of erosion-especially if the water is moving fast.
Water can flow in different ways:
1. sheet flow-when water flows downhill as a thin sheet
like when you wash a car and the water runs off the hood in a sheet
2. rills-when a sheet of water flows around obstacles and becomes deeper-small channels cut into the sediment at Earth’s surface.
3. gullies-runoff flows through the rills and becomes deeper and the channels get to be .5m across.
4. streams-most important agent of erosion on Earth.
Water flowing can be small or large-streams that flow down steep slopes can have lots of energy. Some streams snake back and forth eroding and depositing sediments.
When rivers enter oceans or lakes, the water slows and sediment is deposited
Large accumulation of sediment is called a delta.
New Orleans was built on a delta formed by the Mississippi River.