Science- Chapter 4 Study Notes
Section 1:Weathering and Soil Formation
● Weathering - a mechanical or chemical surface process that breaks rocks into smaller pieces.
○ Freezing and thawing, oxygen in the air, and even plants and animals can affect the
stability of rock.
● Mechanical Weathering - breaks rocks into smaller pieces without changing them chemically.
○ The broken-down pieces are identical in composition to the original rock.
○ Two main causes of mechanical weathering are ice wedging and living organisms.
○ Ice Wedging
■ In some areas of the world, air temperature drops low enough to freeze
water. After the temperature rises, the ice thaws. The freezing and thawing
breaks up the rock.
● Ice wedging works best in places where it is wet and the temperature
goes above and below freezing point
■ When it rains or snows on rock and melts, the water seeps into cracks in the
rock. When temperature reaches below freezing, crystals form. As crystals
form, they take up more space. When water freezes, its molecules move
apart, which make it expand more. From this expansion, pressure is exerted
onto the rock which causes it to break and crack.
■ Ex: Potholes form from ice wedging.
○ Plants and Animals
■ Plants and animals also cause mechanical weathering. Plants can grow in the
most inconvenient places. Their roots grow deep in cracks and absorb water.
As they grow, they get thicker and longer. Then, they release pressure and
wedge rocks apart.
■ Gophers, prairie dogs, and others that burrow in the ground break rock apart
when they dig through soft rock or sediment. They can also push rocks and
sediments to the surface where chemical weathering is rapid.
Chemical Weathering
● Occurs when the chemical composition of a rock changes.
● Most common in tropical(warm and humid) regions.
○ Natural Acids
■ Certain rocks have a unique chemical reaction with natural acids.
● For example, carbonic acid (water mixed with carbon dioxide in air or
soil) can change the chemical composition of some of the minerals
found in rocks.
● Vinegar reacts with carbonic acid found in chalk, which dissolves it.
Similarly, carbonic acid dissolves some rocks, like limestone, dolomite,
and marble.
○ Plant Acids
■ Sometimes, acid is produced from plant roots.
● Ex: Moss
● A large amount of plants produce a substance called tannin which
forms tannic acid. Like carbonic acid, tannic acid can dissolve minerals
in rock. This weakens the rock, making it possible to break into
smaller pieces.
● You can tell when plant acids are doing their job if you peel back on
the plant and you see discoloration on the rock.
○ Effect of Oxygen
■ Oxidation - effects of chemical changes by oxygen.
■ Rust - iron-containing material that has been oxidized.
● Can weaken rock and break apart. Some rocks that have been oxidized
can turn red or orange.
Soil
● A mixture of weathered rock, organic matter (decomposed leaves, twigs, roots, etc.), water,
and air that supports the growth of plants.
● Parent Rock
○ The type of soil is dependent on the parent rock, also called bedrock.
○ Ex) Where limestone is chemically weathered, clay soil is common because clay comes
from dissolved limestone.
● The Slope of the Land
○ Topography (surface features) also influences the type of soil.
○ In steeper surfaces, there is less soil development because rock fragments constantly
qmove downhill.
○ However, in lowlands, wind and water deposit fine sediments that thicken soil.
● Climate
○ In tropical regions(warm and moist), rock weathers quicker so deep soil can develop
faster.
○ Tropical regions have soil with more lush vegetation and more organic material.
○ Humus: dark-colored material that forms when fungi/bacteria decompose dead plants
or animals
■ helps soil absorb water and provides nutrients
● Time
○ Takes thousands of years to form
○ As weathering continues, the soil looks like its parent rock less and less
● Organisms
○ Lichens are small organisms that can grow directly on rock. As they grow, they take
nutrients from the rock, forming a thin soil.
○ Plant roots can break down the bedrock.
○ Dead plant material adds organic matter to the soil.
○ Some plants contribute more organic matter than others.
Section 2: Erosion of Earth’s Surface
Erosion- the wearing away and removal of rock and sediment
The four agents of erosion are ice, wind, water, and gravity.
Gravity
● Gravity is a natural force that pulls every object toward the core of Earth.
● Mass Movement: The erosion that occurs when gravity alone causes rock or sediment to
move down a slope
○ Can occur anywhere there are hills or mountains.
○ Creep: sediments slowly move downhill
■ Common where freezing and thawing occur
■ As ice expands in soil, it pushes sediments up. As soil thaws, the sediments
move farther down the slope
■ Over time, it can possibly cause damage to structures
○ Slump: occurs when a mass of rock or sediment moves downhill, leaving a curved scar
■ Most common in thick layers of loose sediment but can also form in
sedimentary rock
■ Slumps frequently occur on slopes that have been undercut by erosion
■ Can threaten and destroy buildings
○ Rock Slides: Layers of rock break loose from slopes and slide to the bottom, often
bounce and break apart during movement causing jumbled pile of rocks at the
bottom of the slope
■ Can occur at speeds greater than 250 km/h
■ Can be destructive, sometimes destroying entire villages or causing hazards on
roads in mountainous areas
○ Mudflows:
■ Develop where heavy rains or melting snow and ice saturate sediments,
mudflows.
■ A mass of wet sediment that flows downhill over the ground surface; Can be
thick and flows very slowly, but the ones common on volcanoes can go at
speeds of 160 km/h
Ice
● Ice in the cold regions, snow might fall more than melt
● Glaciers: snow accumulated over many years that form large, deep masses of ice
● When the ice is thick enough, the glacier’s weight causes it to flow downhill (gravitational pull)
● When glaciers move across Earth’s surface, they erode materials from some areas and deposit
sediments in other areas.
● There are two kinds of glaciers: continental glaciers and valley glaciers
○ Continental glaciers - large and thick glaciers that can bury mountain ranges
○ Valley glaciers - Smaller, located in high mountains where the average temperature
isn’t high enough to melt the ice sheets.
■ Average flow is 0.01 to 2 meters a day, but can reach up to 100 meters per day
during a surge.
Glacial Erosion
● Glaciers erode in two different ways:
● One way is when the underlying rock has cracks in it. The ice can pull out pieces of rock
(erosion)
● Scratching: loose pieces of sediment freeze and stick to the bottom of the glacier and drags
along as the sediments scratch the rock underneath
○ Produces large grooves or smaller striations
○ Also creates a fine powder called rock flour
Effects of Glacial Erosion
● Glacial erosion of a rock can be a powerful force that shapes Earth’s surface.
● In mountains, they can remove rock from the mountaintops to form large bowls called
cirques and steep peaks.
● When a glaciers moves into a stream valley, it erodes rock along the valley sides producing a
U-shaped valley. Continental glaciers also shape Earth’s surface.
Glacial Deposition
● Glaciers can deposit sediments.
● When glacier ice melts, the sediment that the ice was carrying gets left behind. Once it is
directly deposited from the glaciers it is made into till.
● Till: A mixture of different sized particles, ranging from clay to large boulders
● In summer, glaciers melt so much that streams often flow from them.
○ These streams carry and deposit sediment.
○ Sand and gravel laid down by these deposits are called outwash.
Wind
● When wind blows across loose sediments like silt and sand, it lifts and carries it.
● Deflation- Erosion of land by wind that leaves particles too heavy to move behind
○ Can lower the land’s surface by several meters.
● Abrasion- A form of erosion that can make pits in rocks and produce smooth, polished
surfaces. It is common in some deserts and cold regions with strong winds.
○ Wind that is carrying sediment can abrade other rocks just as a sandblasting machine
would do.
○ For an easier depiction, think of sandpaper and how it erodes against the wood and
makes it smoother.
● Sand dunes are formed by sand-deposited wind that slows down when it blows on irregular
features of Earth.
○ These dunes move when wind carries sand up one side of the dune and it avalanches
down the other.
○ Sometimes wind carries only fine sediment called silt.
○ When sediment is deposited, an accumulation of silt called loess can blanket Earth’s
surface.
■ Loess is as fine as talcum powder.
■ Often deposited downwind of large deserts and deflated glacial outwash
deposits
WATER
Runoff- an important agent of erosion, especially if water is moving fast.
● The more speed water has, the more material it can carry with it.
Sheet Flow- a thin sheet of water that might begin to move downhill
● Can carry loose sediment grain with it causing erosion of the land :sheet erosion.
● Ex) Rain falling over a car hood.
Rills- form where a sheet of water flows around obstacles and becomes deeper.
● Are small channels cut into sediment at Earth’s surface that carry more sediment that can be
moved by sheet flow.
● Large amounts of sediment can be picked up and carried away.
● If a runoff continues to flow through the rills, more sediment erodes and the channel widens
and deepens. If it gets to be about 0.5 across, they are known as gullies.
Streams- the most important agent of erosion on Earth.
○ Shape more of Earth’s surface than ice, wind, or gravity.
○ Over long periods of time, water moving in a stream can have enough power to cut
large canyons into solid rock.
○ Can sculpt the land over a wide region, forming valleys, and leaving some rock as hills.
○ Often connect to gullies to form stream channels.
○ All streams eventually flow into an ocean or a large lake, the level of water determines
how deep a rock can erode.
Deltas- the part of a stream’s mouth where sediment is deposited and then the stream divides into
the ocean.
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