Download Weathering and Erosion

Weathering and Erosion
Weathering
• Weathering is the chemical alteration
and physical breakdown of rocks and
minerals at or near the Earth's surface.
Erosion
• Erosion involves the transportation
- movement of weathered particles.
The Rock Cycle
• The three major types of rocks, igneous,
sedimentary, and metamorphic rocks
are interrelated by a series of natural
processes.
Igneous rocks
• Igneous rocks form from the cooling
and crystallization of hot molten lava
and magma. Igneous rocks undergo
weathering and erosion to form
sediments
Sedimentary Rocks
• Sediments are deposited and
by compaction and cementation
form sedimentary rocks.
Metamorphic Rocks
• Sedimentary rock become buried
by additional sedimentary
deposition, and when they are deep
within the Earth, they are subjected
to heat and pressure which
causes them to become
metamorphic rocks.
Magma
• With further burial and heating, the
metamorphic rocks begin to melt.
As melting proceeds with
increasing temperatures and
depths of burial, eventually the rock
becomes molten and becomes
magma.
Back to Igneous Rock
• Magma cools and crystallizes to
form igneous rock, or which is
erupted onto the Earth's surface
as lava, and cools and
crystallizes to form volcanic
igneous rock.
Types of Weathering
• Physical or mechanical
weathering
• Chemical weathering
Physical Weathering
• Reactions at the Earth's surface
tend to make large particles
smaller. A decrease in particle size
increases the total surface area of
the particles.
Consider a cube of rock that is 1"
on a side. The total surface area is
6 square inches. If this cube is
broken down into 8 cubes each
0.5 " on a side the total surface
area increases to 24 square inches.
Continued reduction in size leads to
an increasing total surface area.
This is important because chemical
reactions generally begin at the
surface of an object. The greater
the surface area the greater the
reactivity of the particles.
The Wind -- Mechanical Weathering in
Action!
So what causes physical
weathering?
• A number of processes are capable
of reducing particle size. A
particularly effective process
involves the repeated freezing and
thawing of water in cracks in
rocks.
• When water freezes there is a
9% increase in volume. The
expansion exerts a force on the
size of the crack which may
cause the crack to widen.
Ice wedging - water expands when it
freezes, which increase the size of
the cracks and causes crumbling of
the rock.
Exfoliation or unloading o rock breaks off into leaves or sheets
along joints which are to parallel the
ground surface;
o caused by expansion of rock due to
uplift and erosion; removal of pressure
of deep burial;
• Eventually large sheets of rock
may "slide" off the rock creating a
rounded upper surface. These
exfoliation domes are quite
common.
Enchanted Rock State Park - Texas
Thermal expansion o repeated daily heating and cooling of
rock;
o heat causes expansion; cooling causes
contraction.
o different minerals expand and contract
at different rates causing stresses along
mineral boundaries.
Sediment
• The loose material released by
physical weathering is termed
sediment.
• The size of sediment is described using the
following terms:
gravel - > 2 mm in diameter
sand - 1/16 mm to 2 mm in diameter
silt - 1/256 mm to 1/16 mm in diameter
clay - < 1/256 mm in diameter
Chemical weathering
• The reactive components in the Earth's
atmosphere include:
o Oxygen
o Carbon Dioxide - CO2
o Water - H2O
• Nitrogen, the most abundant consistent in
the atmosphere has little effect on the
chemical weathering of rocks and minerals.
Dissolution (or solution) o Several common minerals dissolve in
water
o halite
o calcite
o Limestone and marble contain calcite
and are soluble in acidic water
o Marble tombstones and
carvings are particularly
susceptible to chemical
weathering by dissolution.
Oxidation
• Oxygen combines with ironbearing silicate minerals
causing "rusting"
• Iron oxides are produced
• Iron oxides are red, orange, or
brown in color
Hydrolysis
• Silicate minerals weather by
hydrolysis to form CLAY.
• Feldspar alters to clay
(kaolinite) plus dissolved
materials (ions)
Clay
Biological weathering
Organisms can assist in
breaking down rock into
sediment.
What organisms?
• Roots of trees and other plants
• Lichens, fungi, and other
micro-organisms
• Animals (including humans)
Roots of Trees
Lichens on a rock
Soils
• Loose rock material at the surface of
the Earth is referred to as regolith.
Soil consists
of:
Regolith
becomes soil when a surface
layer that can support plant life has
developed due to a combination of
climatic and weathering factors.
Soil consists of:
•
•
•
•
•
minerals and weathered rock fragments
organic matter
gases
water
living organisms
There are five factors influencing
soil formation:
•
•
•
•
•
Climate
Parent rock
Organisms
Relief (slope, topography)
Time
What is a soil profile?
• A soil profile consists of several soil
horizons.
• They are numbered roughly
alphabetically, beginning at the ground
surface, going downward:
O horizon
o This is the layer of humus on the ground
surface.
A horizon
o Top soil.
• Rich in organic matter. Typically has dark
color.
• B horizon
o Subsoil.
o May contain soluble minerals such as
calcite
• C horizon
o Weathered bedrock or saprolite (rotten
rock).
• Bedrock lies below the soil profile.
Soil erosion
• Soil may be eroded due to the
action of water (runoff) or wind.
• Soil is transported by streams, and
most will eventually be deposited on a
floodplain or at the mouth of the river
in a delta. (Today, much eroded soil
will be trapped because of dams across
rivers, filling reservoirs. Loss of
sediment transport to the coast also
depletes beaches of sand, and can lead
to accelerated beach erosion
• The actions of wind and water
change the contour of the land.
Before
Katrina
After
Katrina
Wind Erosion
• The Dust Bowl of the 1930's was
due to wind erosion of soil
following extended drought and
over-tilling of the soil.
How can we slow soil erosion?
• Contour plowing
• Terraces
• Planting groundcovers; roots hold the
soil.
• Windbreaks
Contour plowing
Terraces
Planting Groundcovers
Windbreak
Properties of Water
• Water is a remarkable compound.
Near the surface of the Earth water
can exist in three phases - ice
(solid), water (liquid), and vapor
(steam).
Specific Heat
• Specific heat is a physical property
of matter.
• The specific heat of water is 1
calorie/gram °C = 4.186
joule/gram °C which is higher
than any other common substance.
• This means that the temperature of
water changes slowly.
• What does this high specific heat of water
effect the weather on Earth?
Specific Heat
• Water has a high Specific Heat. Imagine a
shoreline. The land and the water surfaces
receive equal thermal energy from the sun
but the land heats up quicker than the water;
in general, rocks have lower heat capacities
than water. Thus, water may remain quite
cool even in the summer.
• What would happen if the Earth had no
water?