Transcript Chapter 5: Weathering, Soil, and Mass Movements

Chapter 5:
Weathering,
Soil, and Mass
Movements
5.1: Weathering
• The breaking down and changing of
rocks at or near Earth’s surface
• Part of the rock cycle
2 Types of Weathering
• Mechanical
– Physical forces break rock into smaller and
smaller pieces without changing the rock’s
mineral composition
– 3 physical processes that cause it:
1. Frost Wedging
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When water freezes and expands it enlarges cracks in rock
Rock breaks into pieces after many freeze-thaw cycles
Most common in mountainous regions in middle latitudes
Talus: large piles of rock that have been wedged loose
– Form at base of steep, rocky cliffs
2. Uploading
• Pressure exerted on igneous rock is reduced
– Exposed through uplift and erosion of overlying rock
• Causes outer layers of rock to expand more than the rock
below
– Slabs of outer rock separate like layers and break loose
(called exfoliation)
– Produces large, dome-shaped rock formations
3. Biological Activity
• Living organisms
• Plants  roots
• Burrowing animals move rocks
to surface
• Decaying organisms produce
acids that cause chemical
weathering
• Humans  deforestation,
blasting for minerals or creating
roads
Chemical Weathering
• Transformation of rock into one or more new
compounds
Water
• Most important agent
• Absorbs gases from the atmosphere and ground
which will chemically react with various minerals
• Ex: oxygen  oxides; carbon dioxide  carbonic
acid; sulfur oxide and nitrogen oxides  acid rain
Granite
• Feldspar converted to clay minerals
• Quartz remains unchanged
– Grains released from granite
– Rivers transport debris to sea
• Become component of beaches and sand dunes
Silicate minerals
• Make up most of the crust
• Na, Ca, K, and Mg dissolve and are carried away by
groundwater
• Fe reacts with oxygen to produce iron oxide
• Al, Si, and O combine with water to produce clay
minerals
Spheroidal Weathering
• Corners and edges of rock weather most rapidly
– Become more rounded taking a spherical shape
• Sometimes causes outer layers of a rock to
separate from main body
– Minerals in rock turn to clay which swells when water
is added
Rate of Weathering
• Mechanical weathering
–Accelerates chemical weathering by
increasing surface area of exposed rock
Rate of Weathering
• Rock
characteristics
–Influence ability of
water to penetrate
rock
–Mineral
composition
affects rate
Rate of Weathering
• Climate
– Temperature and
moisture
• Control frequency of
freeze-thaw cycles
• Affect amount of frost
wedging
• Affect rate of chemical
weathering
• Influence kind and
amount of vegetation
– Slow in arid and polar
regions
Differential Weathering
• Different parts of rock weather at different rates
• Differ in mineral composition
• Variations in number and spacing of cracks
5.2-Soil
• Characteristics:
– Regolith:
• Layer of rock and mineral fragments
• Covers nearly all of Earth’s land surface
• Soil is part that supports the growth of plants
–Thin layer of loose material
Soil Composition
• 4 major components:
proportions vary in different
soils
– Mineral matter or broken down
rock
– Organic matter (humus)  from
decayed remains of organisms
• Important source of plant nutrients
• Increases soil’s ability to retain water
– Water  provides moisture for
chemical reactions
– Air  source of carbon dioxide for
photosynthesis
Soil Texture
• Depends on size and
nature of soil particles
• Proportions of different
particle sizes
– Based on % of clay, silt,
and sand in soil
• Influences soil’s ability to
support plant life
Soil Texture
• Clay
– Smallest grains
• Silt
– Medium-sized
grains
• Sand
– Largest grains
• Loam
– Mix of clay, silt, and
sand
– Best soil for
agriculture
Soil Structure
• Particles usually form clumps
• Determines how easily a soil can be cultivated and
how susceptible it is to erosion
• Affects the ease with which water can penetrate
the soil
– Influences movement of nutrients to roots
Soil Formation
• Parent material:
– Source of mineral matter
– May be bedrock (residual soil) or
unconsolidated deposits
(transported soil)
• From gravity, water, wind, or ice
– Affects rate of weathering and rate
of soil formation
– Transported soil develops faster
than residual soil
– Chemical makeup affects soil’s
fertility
• Influences types of plants soil can
support
Soil
Formation
• Time
– Longer a soil forms, the thicker it becomes
• Climate
– Greatest effect on formation
• Influence rate, depth, and type of weathering
– Amount of precipitation influences soil fertility
• Affect rate nutrients are removed from soil
– Effect on types of organisms that live on or in soil
Soil
Formation
• Organisms
– Type and number impact physical and chemical properties
– Plants are main source of organic matter in soil
• Releases nutrients when they decay
– Produces acids that speed up weathering
• Contributes to soil fertility
– Microorganisms (fungi, bacteria, protists) decompose dead
organisms
• Can convert nitrogen gas into usable nitrogen compounds
– Burrowers mix mineral and organic matter
Soil Formation
• Slope
– Affects rate of erosion and
water content of soil
– Steep slopes increased
erosion
• Holds little water
• Thin soils
– Flat areas  little erosion
• Poor drainage
• Thick and dark soils
– Indicates a lot of organic
matter
– South-facing  receive more
sunlight
• Warmer and drier
– North-facing  less sunlight
Soil Profile
• Soil varies in
composition, texture,
structure, and color at
different depths
– Divides soil into
zones/distinct layers
(soil horizons)
• A cross section of soil is
known as a soil profile
• Vertical section through
all soil horizons
• O Horizon- organic material
– Leaf litter; accumulated decomposed
materials
– Freshly fallen and partially decomposed
leaves, twigs, animal waste, fungi, organic
materials
• A Horizon
– Topsoil
– Upper part of organic matter (humus)
– Full of insects, fungi, and microorganisms
• break down some complex organic compounds into simpler
inorganic compounds soluble in water.
• Soil moisture carries these nutrients into the roots of plants
and transported through stems and into leaves
– Lower part is mix of mineral and organic matter
– Dark brown to black in color; highest nutrient levels
– Porous mixture – usually darker and looser than deeper layers
– Some inorganic mineral particles
– Gray, bright yellow, or red topsoils - low in organic matter and
need nitrogen enrichment to support most crops
• E Horizon- zone of leaching
–Lighter in color; accumulated
minerals
• B Horizon
– Subsoil
• Similar to topsoil; fewer nutrients; lighter in color
– Poor in humus but rich in minerals
– Fine clay particles washed out of A horizon by water
that filters through pore spaces
– Lower limit of most roots and burrowers
– contains most of the soil’s inorganic matter, mostly
broken-down rock consisting of varying mixtures of
sand, silt, clay, and gravel
• C Horizon
– Contains partially weathered parent material
• Resembles parent material
– Mixture of partially broken down rock and soil
– Lies on a base of unweathered parent rock called
bedrock
• Two top layers of most well-developed soils teem
with bacteria, fungi, earthworks, and small insects
• R/D Horizon- bedrock
–Solid bedrock; not broken down
yet (letter R or D may be used)
Soil Types
• Pedalfer
– Form in temperate
areas with more than
63cm rain/yr
• Eastern US
– Large amounts of iron
oxide and aluminumrich clays
– Brown to red-brown
color
Soil Types: Pedocal
• Western US
• Drier areas
– Grasses and brush
vegetation
• Less clay
• Abundant in calcite or
calcium carbonate
• Light gray-brown color
Soil Types: Laterite
• Hot, wet tropical areas
– Increases chemical weathering
• Large quantity of water filters
through soils
– Most calcite and silica are removed
from soil
• Deeper than soils in temperate
areas
• Full of iron oxide and
aluminum oxide
• Orange or red color
• Very hard and waterproof
when dried
• No organic matter
Soil Erosion
• Role of Water
– Move soil from one place to another
• Carries away dislodged particles (sheet erosion)
– Forms tiny streams called rills
• Further erosion creates trenches called gullies
– Called sediment once particles reach a stream
Soil Erosion
• Rates of Erosion
– Human activities that remove vegetation have
increased erosion
• Farming, logging, and construction
– Soil easily carried away by wind and water without
plants
– Wind erodes more slowly than water
• Increased by prolonged drought
– Rate depends on soil characteristics, climate, slope, and
type of vegetation
Soil Erosion
• Sediment Deposition
– Rivers that accumulate sediment must be dredged to
remain open for shipping
– Reservoirs become less useful for storing water,
controlling floods, and generating electricity
– Some contaminated with pesticides
• Endangers organisms that live in or use water
– Excessive nutrients stimulate growth of algae and
plants
Controlling Erosion
• Soil conservation
– Preserve environments
– Protect the land
– Ex: windbreaks  planting rows of trees
– Ex: terracing hillsides
– Ex: plowing along contours of hills
– Ex: rotating crops
Mass Movements
• Slopes are always changing
• Transfer of rock and soil downslope due to gravity
– Can be fast or slow
• Combined actions of weathering and mass
movement produce most landforms
– Weathering weakens and breaks rock apart
– Mass movement moves debris downslope
Triggers of Mass Movements
• Gravity is force behind it
• Water
–Saturating surface materials with water
• Heavy rains
• Rapid melting of snow
–Pores in sediment become filled with
water
Triggers of Mass Movements
• Oversteepened slopes
–25-40° is stable slope
–If steepness of slope exceeds stable range
increase chances of mass movement
Triggers of Mass Movements
• Removal of vegetation
–Plants make slopes more stable
• Root system binds soil and regolith together
–Increases chances of mass movement
Triggers of Mass Movements
• Earthquakes
–Dramatic trigger
–Can dislodge enormous amounts of rock
and unconsolidated material
Types of Mass Movements
• Based on the kind of material
that moves, how it moves, and
speed of movement
• Rockfalls
– Rocks or rock fragments fall
freely through the air
– Common on slopes too steep for
loose material to remain on the
surface
– Many result from mechanical
weathering caused by freezethaw cycles or plant roots
– Can trigger other mass
movements
Types of Mass Movements
• Slides
–Block of material moves suddenly along a
flat, inclined surface
• Segments of bedrock  rockslides
–200 km/hr (fastest mass movements)
–Occur in high mountain areas
–Can be triggered by rain or melting snow
Types of Mass Movements
• Slumps
– Downward movement of a block of material along a
curved surface
– Does not travel very fast or far
– Leaves a crescent-shaped cliff above it
– Common on oversteepened slopes with a lot of clay
Types of Mass Movements
• Flows
–Material containing large
amount of water which
moves downslope as a
thick fluid
Types of Flows
• Mudflows
– Flows that move quickly
– Common in semiarid mountainous regions
• Sparse protective vegetation
– Heavy downpour or rapid snowmelt can flood canyons
with mix of soil, rock, and water
• Follows contours of canyon taking boulders and
large trees with it
Types of Flows
• Earthflows
– Move relatively slowly
(mm/day to several
m/day)
– Movement may continue
for years
– Occur on hillsides in wet
regions
– Forms tongue-shaped
mass
• Few m to 1 km long
• 1m-10m deep
Types of Mass Movements
• Creep
– Slowest type (few mm or
cm/yr)
– Freezing expands water in soil
• Lifts soil particles at right
angles to the slope
– Thawing causes contraction
• Allows particles to fall back
to lower level
– Causes structures to tilt
downhill
– Can displace fences, crack
walls, and underground pipes