Transcript Volcanoes and Igneous Activity Earth

Chapter
5
Weathering, Soil, and
Mass Movements
Weathering
• Earth is continuously undergoing natural breaking down
process – Weathering
LepoRello
(Wikipedia)
• Weathering and Erosion superhoroes
• Weathering
– the breaking down of rocks and other materials
on the Earth’s surface
• A slow and continuous process
• Affects all substances exposed to the atmosphere
• The effects are not always easily observed
due to how long weathering takes.
• Two types of weathering
1.Mechanical
2.Chemical
• Weathering, Erosion, and Deposition
5.1 Weathering
Mechanical Weathering
 Mechanical weathering occurs when
physical forces break rock into smaller and
smaller pieces without changing the rock’s
mineral composition.
 In nature three physical process are
especially important causes of weathering:
frost wedging, unloading, and biological
activity.
NOTES:
Mechanical Weathering
• Forces of weathering break rocks into smaller pieces but
do not change the chemical makeup of the rock.
• Strictly a physical process
• Common agents of Mechanical weathering are ice,
plants, animals, gravity, running water and wind
• Like hitting a rock with a hammer and smashing it to
pieces.
• At the beginning of weathering, rock fragments are
usually angular and sharp. As weathering continues
they become smooth and rounded.
Increase in Surface Area by
Mechanical Weathering
NOTES
Causes of Mechanical Weathering
1. Temperature
 Changes in temperature can cause rocks to break apart.
 The outside of the rock is heated and expands while the inside
of the rock remains cool and does not expand. When the
temperature drops, the outside of the rock cools and contracts.
 This cycle repeats every day and causes the particles to crack
and flake off.
 Exfoliation – pieces break off in curved sheets or slabs
parallel to rock’s surface. Other agents also cause exfoliation.
5.1 Weathering
Mechanical Weathering
1. Frost wedging
• The mechanical breakup of rock caused by the
expansion of freezing water in cracks and
crevices
• Sections of rock that are wedged loose may
tumble into large piles called talus, which
typically form at the base of steep, rocky cliffs.
Frost Wedging
NOTES
2. Frost Action or Ice Wedging
 Water expands in volume by about 10% when it freezes
Repeated freezing and melting of water
 Water seeps into a small opening or crack in a rock, the temperature
drops below freezing, the water freezes and expands.
 The crack is made larger by the pressure of the water as it expands.
 Over time the cycle causes the rock to break into pieces.
 Talus – large piles of broken rock.
NOTES
 Cracks and potholes in roads and cement
driveways are often caused by frost action.
 Commonly occurs at high elevations and in
climates where the temperature regularly
varies above and below freezing –
Northern US.
5.1 Weathering
Mechanical Weathering
2. Unloading
• Reduced pressure on igneous rock causes it to
expand and allows slabs of outer rock to break
off in layers in a process called exfoliation.
3. Biological activity
• The activity of organisms, including plants,
burrowing animals, and humans, can also
cause mechanical weathering.
Unloading and Exfoliation
of Igneous Rocks
Weathering and
Biological Activity
4. Organic/Biological Activity
 Root-pry - Roots of plants loosen rock material by
growing in a crack in a rock and make the crack
larger are the plant’s roots grow and spread out.
 Digging of borrowing animals expose new rock
surfaces to weathering.
5. Gravity
 Landslides - Pull loosened rocks down mountain cliffs
 Landslide – large movement of loose rocks and soil – As the
rocks fall, they collide with one another and break into
smaller pieces.
 A type of abrasion
Mine
NG Video
Abrasion
 The wearing away of rocks by solid particles carried by wind,
water or other forces.
 Sand Blasting - Sharp edges of sand particles scrape off small
pieces of exposed rock.
 Running water causes abrasion by carrying along loose rocks
and other particles that scrape against each other and against
the riverbed. Causes the riverbed rocks to be smooth and
rounded.
Chemical Weathering
 The chemical makeup or mineral composition of the rocks is
changed
 Chemical reactions take place between the minerals in the rock and
water, carbon dioxide, oxygen and acids.
 Chemical reactions alter the internal structure of the original
mineral and lead to the formation of new minerals.
 Both chemical composition and the physical appearance of the rock
undergo changes.
 Minerals can be added or removed.
 Minerals can be broken down – decomposition
5.1 Weathering
Chemical Weathering
 Chemical weathering is the
transformation of rock into one or more
new compounds.
 Chemical Weathering of Granite
• Weathering of potassium feldspar produces clay
minerals, soluble salt (potassium bicarbonate), and
silica in solution.
• Quartz remains substantially unaltered.
5.1 Weathering
Chemical Weathering
 Weathering of Silicate Minerals
• Produces insoluble iron oxides and clay minerals
 Spheroidal Weathering
• Causes the corners and edges of rock to be
more rounded
Causes of Chemical Weathering
• Water – Hydrolysis
– Change in composition when they react with water is called
hydrolysis
– Causes most of chemical weathering with carbon dioxide
– Water can dissolve most minerals that hold rocks together
– Soluble – able to be dissolved in water
– Water forms acids with certain gases in the atmosphere
– Water can combine with a mineral to form a completely different
mineral
• Example – Water + Feldspar = Clay (called kaolin)
• Hydrogen ions from the water displace the elements potassium or
calcium in feldspar.
– Leaching – water carries dissolved minerals to lower layers of
rock.
• Bauxite (aluminum ore) form when leaching causes a mineral to
concentrate in a thin layer beneath the Earth’s surface.
Oxidation
– Metallic elements combine with oxygen
– Often occurs in rocks with iron-bearing
minerals
• 4Fe + 3O2  2Fe2O3 (rust)
– Occurs when oxygen chemically combines
with another substance
– Causes the red color in a lot of the soil of the
southwestern US.
Weathering of Silicates
Spheroidal Weathering
Carbonation
– Carbonic acid forms when carbon dioxide dissolves in
water – occurs when carbon dioxide in the air
combines with falling rain water.
• H2O + CO2  H2CO3
– Carbonation – carbonic acid reacts chemically with
other substances.
– Carbonic acid is able to dissolve certain rocks on or
beneath the surface of the Earth
– Carbonic acid is too weak to be harmful to humans
– Slowly decomposes feldspars and limestone
• Carbonic Acid + Calcite (limestone)  Calcium Carbonate
• This dissolving action sometimes produces underground
caverns
Reflecting lake in the Luray caverns in the northern Shenendoah valley.
David Jones
Spheroidal Weathering
Sulfuric Acid – Acid precipitation
– Sulfur oxides are in the air of polluted
areas
– A byproduct of burning coal as a source
of energy
– Sulfur dioxide dissolves in rainwater to
form sulfuric acid
– Much stronger than carbonic acid.
– Sulfuric acid corrodes rocks, metals and
other materials very quickly
• http://www.natgeoeducationvideo.com/film
/1233/acid-rain
• Clingman’s
Dome Video
Plant Acids
– Plants produce weak acids that dissolve
certain minerals in rocks
– Lichens and mosses produce acids
http://www.lostateminor.com/2013/07/18/karcher-cleans-mt-rushmore-for-free/
5.1 Weathering
Rate of Weathering
 Two other factors affecting the rate of
weathering are rock characteristics and
climate.
1. Rock characteristics
• Mineral composition and solubility
• Physical features such as joints
5.1 Weathering
Rate of Weathering
2. Climate
• Temperature and moisture are the most crucial
factors.
• Chemical weathering is most effective in areas
with high temperatures and abundant moisture.
5.1 Weathering
Rate of Weathering
 Differential Weathering
• Caused by variations in composition
• Creates unusual and spectacular rock
formations and landforms
5.2 Soil
Characteristics of Soil
 Soil is part of the regolith that supports
the growth of plants.
• Regolith is the layer of rock and mineral
fragments that covers most of Earth’s land
surface.
5.2 Soil
Characteristics of Soil
 Soil Composition
• Soil has four major components: mineral matter,
or broken-down rock; humus, which is the
decayed remains of organisms; water; and air.
Composition by Volume of
Good-Quality Soil
5.2 Soil
Characteristics of Soil
 Soil Texture
• Texture refers to the proportions of different
particle sizes.
- Sand (large size)
- Silt
- Clay (small size)
• Loam (a mixture of all three sizes) is best suited
for plant life.
Soil Texture
5.2 Soil
Characteristics of Soil
 Soil Structure
• Soil particles clump together to give a soil its
structure.
5.2 Soil
Soil Formation
 The most important factors in soil
formation are parent material, time,
climate, organisms, and slope.
1. Parent material
• Residual soil—parent material is the bedrock
• Transported soil—parent material has been
carried from elsewhere and deposited
Parent Material and Soils
5.2 Soil
Soil Formation
2. Time
• Important in all geologic processes
• The longer a soil has been forming, the thicker
it becomes.
3. Climate
• Greatest effect on soil formation
5.2 Soil
Soil Formation
4. Organisms
• Organisms influence the soil's physical and
chemical properties.
• Furnish organic matter to soil
5. Slope
• Angle
- Steep slopes often have poorly developed soils.
- Optimum slope is a flat-to-undulating upland
surface.
5.2 Soil
Soil Formation
5. Slope
• Orientation, or direction the slope is facing,
influences soil formation.
- Soil temperature
- Moisture
5.2 Soil
The Soil Profile
 Soil varies in composition, texture, structure,
and color at different depths. Soil horizons
are zones or layers of soil. A soil profile is a
vertical section through all the soil horizons.
• The A horizon is commonly know as topsoil.
• The B horizon is subsoil and contains clay
particles washed out from the A horizon.
• The C horizon is between B horizon and
unaltered parent material.
Soil Profile
A Soil Profile Showing
Different Horizons
5.2 Soil
Soil Types
 Three common types of soil are pedalfer,
pedocal, and laterite.
1. Pedalfer
• Best developed under forest vegetation
• Accumulation of iron oxides and aluminum-rich
clays in the B horizon
5.2 Soil
Soil Types
2. Pedocal
• Accumulates calcium carbonate
• Associated with drier grasslands
3. Laterite
• Hot, wet, tropical climates
• Intense chemical weathering
5.2 Soil
Soil Erosion
 Water erodes soil.
 Rates of Erosion
• Human activities that remove natural vegetation,
such as farming, logging, and construction, have
greatly accelerated erosion.
 Sediment Deposition
• Reservoirs fill with sediment.
• Sediments are contaminated by pesticides
and fertilizers.
5.2 Soil
Soil Erosion
 Controlling Erosion
• Planting rows of trees called windbreaks
• Terracing hillsides
• Plowing along the contours of hills
• Rotating crops
5.3 Mass Movements
Triggers of Mass Movements
 The transfer of rock and soil downslope
due to gravity is called mass movement.
 Among the factors that commonly trigger
mass movements are saturation of
surface materials with water,
oversteepening of slopes, removal of
vegetation, and earthquakes.
5.3 Mass Movements
Types of Mass Movements
 Geologists classify mass movements
based on the kind of material that moves,
how it moves, and the speed of
movement.
 Rockfalls
• A rockfall occurs when rocks or rocks fragments
fall freely through the air.
5.3 Mass Movements
Types of Mass Movements
 Slides
• In a slide, a block of material moves suddenly
along a flat, inclined surface.
• Slides that include segments of bedrock are
called rockslides.
 Slumps
• A slump is the downward movement of a block of
material along a curved surface.
Heavy Rains Can Trigger Slumps
5.3 Mass Movements
Types of Mass Movements
 Flows
• Flows are mass movements of material
containing a large amount of water.
• Mudflows move quickly and carry a mixture of
soil, rock, and water that has a consistency of
wet concrete.
• Earthflows move relatively slowly and carry
clay-rich sediment.
5.3 Mass Movements
Types of Mass Movements
 Creep
• Creep is the slow, downhill movement of soil
and regolith.
Creep