Transcript Weathering and Soil Formation

Weathering and Soil
Formation
What is
weathering?
Weathering
 The breaking down of rocks and other
materials on the Earth’s surface
 2 types:
 Mechanical
 Chemical
Mechanical weathering
 Rocks are broken into smaller pieces and
different shapes
 Chemical makeup of rocks does not
change
 Beginning of process: rocks are sharp
and angular
 As process continues, rocks are smooth
and have rounded edges
Mechanical Weathering
 Caused by several agents:
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Temperature
Frost action
Organic activity
Gravity
Abrasion
Temperature
 During the day, sun heats outside of rock and it
expands
 During the night, the outside of the rock cools
and contracts
 Cycle of heating and cooling continues each
day, and parts of rock crack or peel off
 Causes exfoliation: rocks break off in curved
sheets or slabs
Frost Action: see
demonstration
Frost Action
 Water gets into cracks of rocks, where it
freezes
 Freezing water expands inside crack of
rock, and crack grows until it forces rock
to break
 Can you think of examples of this you
have seen before?
Organic Activity
 Roots of plants loosen rocks
 Plant growing in a rock’s crack can make
crack grow as roots grow and spread
 Called root-pry: breaking apart of rocks
caused by plant roots
Gravity
 Gravity pulls loose rocks down cliffs of
mountains: called a landslide
 Rocks that fall collide with other rocks
and break them into smaller pieces
Abrasion
 Wearing away by solid particles carried in
the wind and water
 wind and water pick up particles that
have been eroded
 Sharp edges of sand and particles cut
into exposed rocks
Chemical Weathering
 Weathering that causes changes in the
chemical makeup of rocks
 Minerals can be added or removed from
rocks
 Substances react chemically with rocks
and break them down
Causes of chemical
weathering
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Water
Oxidation
Carbonation
Sulfuric Acid
Plant Acids
Water
 Can dissolve minerals that hold rocks
together
 Can form acids when it mixes with certain
gases
 These acids speed up rock decomposition
 Can combine with a mineral to form a
new mineral
Oxidation
 Process in which oxygen combines with
another substance
 Forms a new substance
 Example: iron and oxygen combine to
form iron oxide (rust)
 Indicated by its color
Carbonation
 Carbonic acid reacts chemically with
other substances
 Carbonic acid: weak acid formed when
carbon dioxide dissolves in rain
Sulfuric acid
 Sulfur oxides are a byproduct of burning
coal
 When sulfur oxides dissolve in rainwater,
they form sulfuric acid
 Sulfuric acid is a strong acid that quickly
wears away rocks and metals
Plant acids
 Plans produce weak acids that dissolve
certain minerals
 Example: mosses, which grow in damp
areas, produce weak acids that seep into
rocks and dissolve certain minerals
Rate of weathering
 Rate of weathering: how fast weathering
takes place
 Depends on several factors:
 Type of rock
 Stable rock: rock that can resist chemical
weathering
 Time
 Size of exposed surface area
Assignment – pick one!
 Draw a comic strip (at least 6 frames) that
shows the difference between mechanical
and chemical weathering and gives 2
examples of each.
 Pretend you are a rock. Write a plea to
nature asking it to stop weathering you so
much. Your plea must show the difference
between mechanical and chemical
weathering and give 2 examples of each.
 Due Friday, 2/26
 What do you think happens to the small
pieces of rock that are broken off by
weathering?
They form soil!
Soil is formed when rocks are
continuously broken down by
weathering.
Why do we need soil?
Why we need soil
 Very important for all living things
 Plants need minerals and water found in
soil to grow
 Animals either eat these plants or eat
other animals that eat the plants
 Humans eat plants or eat the animals
that eat the plants
Soil
 Residual soil: soil that remains on top of
the rock it came from
 Transported soil: soil that moves away
from its origin
 Bedrock: layer of rock beneath the soil
Soil from organic material
 Humus: part of the soil formed by
decaying organic material
 Comes from living things
 Decay means to break down plants and
animals into what they are made of
 Humus is important for plant growth (fills soil
with nutrients plants need)
What do you think soil is
made of?
Ingredients of soil
 2 main ingredients:
 Humus (organic material)
 Pieces of weathered rock (80% of soil)
 Most abundant minerals: clay and quartz
 Other ingredients:
 Air
 Water
Where is the air and water?
 Pore spaces: space between soil
particles
 Fill with air and water
 Plant roots get the oxygen they need
from the air found in pore spaces
 Plants use minerals that are dissolved in
the water in pore spaces
Soil Composition Varies
 Different types of rocks being broken
down = different types of soil
 Different types of weathering = different
types of soil
 Mechanical weathering: soil is similar to rock
being weathered
 Chemical weathering: soil is different from
rock it came from
Soil Texture
 Different size particles give soil different
texture
 Largest particles: gravel (2-64 mm in
diameter)
 Gravel breaks down into sand (less than
2 mm in diameter)
 Silt is made of broken rock crystals (less
than 1/16 mm in diameter)
 Clay has smallest particles (less than
1/256 mm in diameter)
Soil has layers
 Layers of soil are called horizons
 You can take a cross section of soil to
see the different horizons – called a soil
profile
 Soil with 3 layers: mature soil
 Soil with 2 layers: immature soil
Uppermost layer
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Called the A horizon
Soil here is topsoil
Contains mostly humus
Living organisms add minerals to it
regularly
 Many pore spaces
 Fertile soil – good for planting
Middle layer
 Called the B horizon
 Soil here is called subsoil
 Minerals are washed by water from
topsoil down into B horizon: called
leaching
 Made of minerals from leaching, clay,
and some humus
Bottom layer
 Called the C horizon
 Made of partly weathered rock
 Continues until you reach the
unweathered parent rock
 Composition is similar to parent rock
below the soil
Factors affecting soil
formation
 Type of weathering
 Climate: How can rain and temperature
change things?
 Type of rock
 What the region looks like