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Chapter 10: Geology
Processes, Hazards, and Soil
Three Major Zones of the
Earth’s Structure
• Core – innermost layer; has a solid inner part
surrounded by a liquid core of molten material.
• Mantle– middle layer, a thick solid zone that
surrounds the earth’s core. Most of the mantle is
solid, but under the outermost part is a zone of
hot, melted rock that flows like soft plastic –
asthenosphere
• Crust –
– Continental Crust – underlies the continents
– Oceanic Crust – which underlies the ocean basins
Internal Processes
• The inside of the earth is constantly changing by
geologic processes most take thousands to millions of
years.
• Internal Processes- geologic changes originating from
the earth’s interior; they build on the planet’s
surface
– Heat provides energy; gravity also plays a role.
– Residual heat from earth’s formation is still being given off
as the interior core cools and the outer core cools and
solidifies.
– Decay of radioactive elements in the crust adds to the heat
flow from within.
• The internal heat within the earth’s core causes the
mantle to deform and flow slowly.
– Convection Cells – where large volumes of heated rock move
(resembles convection in the atmosphere)
– Mantle Plumes – where mantle rock flows slowly upward in a
column and then moves out in a radial pattern in all
Tectonics Plates
• Convection currents and mantle plumes move
upward as the headed material is displaced by
cooler, denser material sinking under the influence
of gravity
• The energy and heated material cause the
movement of the tectonic plates.
• Tectonic Plates – rigid plates about 100 km thick;
composed of the continental and oceanic crust and
the outermost part of the mantle  All parts
called the lithosphere
• Plates move constantly
• Some plates move faster than others
Theory of Plate Tectonics
• Plate Tectonics – the theory explaining the
movement of the plates and the processes that
occur at their boundaries
• Developed from the theory of continental drift throughout earth’s history, continents have split and
joined as plates have drifted thousands of km back
and forth across the planet’s surface
• Creates mountains, the oceanic ridge system,
trenches, and other features
• Causes volcanoes and earthquakes
• Concentrate many minerals we extract and use
• Also explains certain patterns of biological evolution –
we can trace life-forms that migrated from one area
to another
Plate Boundaries
• Divergent Plate Boundaries – plates move in
opposite directions (divide)
• Convergent Plate Boundaries – plates are pushed
together (collide)
– Subduction carries the oceanic lithosphere downward
into the subduction zone. A trench forms at the
boundary between the two converging plates
– Stresses in the plate undergoing subduction causes
earthquakes
• Transform Plate Boundaries – occur where plates
slide past one another along a fracture (fault) in
the lithosphere; most transform faults are on the
ocean floor.
External Processes
• External Processes – geological changes based directly
or indirectly on energy from the sun and on gravity
• Erosion - the process by which material is 1) dissolved,
loosened, or worn away from part of the earth’s surface
and 2) deposited in other places.
• Streams are the most important agent of erosion – produce valley,
canyons, and deltas
• Weathering – caused by mechanical or chemical
processes usually produces loosened material that can
be eroded.
• Mechanical weathering – in which a large rock mass is broken into
smaller fragments; frost wedging is when water collects in pores
and cracks of rock, expands upon freezing, and splits off pieces of
the rock.
• Chemical weathering – in which one or more chemical reactions
decompose a mass or rock ; usually a reaction of rock material with
oxygen, carbon dioxide, and moisture in the atmosphere and the
ground.
Minerals, Rocks, and the Rock Cycle
• Mineral – an element or inorganic compound that
occurs naturally and is solid. Most minerals occur
as inorganic compounds composed of various
combinations of elements.
• Rock – any material that makes up a large, natural,
continuous part of the earth’s crust; some contain
only one mineral, but most consist or two or more
minerals.
• Rocks are constantly exposed to various physical
and chemical conditions that change them over
time.
• Rock Cycle – the interaction of processes that
change rocks from one type to another
Three Major Rock Types
•Igneous
•Sedimentary
•Metamorphic
Igneous Rock
• Igneous Rock –formed below or on the earth’s
surface when molten rock material (magma) wells
up from the earth’s upper mantle or deep crust,
cools, and hardens into rock.
• Granite – formed underground, Lava rock
• Forms the bulk of earth’s crust
• Source of many nonfuel mineral resources
Sedimentary Rock
• Sedimentary Rock – formed from sediment when
preexisting rocks are weathered and eroded into
small pieces, transported from their sources, and
deposited in a body of water
• Sandstone and Shale from deposited layers of sediment
• Dolomite and Limestone formed from the compacted shells,
skeletons, and other remains of dead organisms
• Lignite and Bituminous coal formed from plant remains.
Metamorphic Rock
• Metamorphic rock – produced when a preexisting
rock is subjected to high temperatures (which may
cause it to melt partially), high pressures,
chemically active fluids, or a combination of agents
• Anthracite – form of coal
• Slate
• Marble
Earthquakes
• Fault – fracture in the earth’s crust
• Earthquakes are caused by the faulting or the abrupt
movement on a fault.
• Energy is released as shock waves, which move
outward form the earthquake’s focus – the point of
initial movement
• Epicenter – is the point on the surface directly above
the focus
• Magnitude – used to measure the severity of an
earthquake
• Measures the amount of energy released in an
earthquake as indicated by the size of vibrations
when they reach the seismograph.
• Each unit represents an amplitude that is 10 times
greater than the next smaller unit.
More About Earthquakes
• Aftershocks – gradually decrease in
frequency over a period of up to several
months
• Foreshocks – can happen from seconds to
weeks before the main shock
• Earthquakes cause:
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Shaking
Permanent displacement of the ground
Rock slides
Urban fires
Flooding
Tsunamis
Reducing Earthquake Hazards
• Examine historical records and make
geological measurements to locate
active fault zones
• Make maps showing high-risk areas
• Establish building codes that regulate
the placement and design of buildings
• Predicting when and where
earthquakes will occur
Volcanoes
• Volcano – occurs where magma, molten rock,
reaches the earth’s surface through a central
vent or a long crack
• Can release debris ranging from large chucks of lava rock to
ash
• Liquid lava
• Gases into the environment – Sulfur dioxide can remain in
the atmosphere for up to three years.
• Volcanic activity is concentrated in the same areas
as earthquakes
• Creates highly fertile soil produced by the
weathering of lava
Reducing Volcano Hazards
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Land –use planning
Better predictions of volcanic eruptions
Effective evacuation plans
Studying phenomena that precede an
eruption:
• Tilting or swelling of the cone
• Changes in magnetic and thermal properties of the
volcano
• Changes in gas composition
• Increased seismic activity
Soil Basics
• Soil – a complex mixture of eroded rock,
mineral nutrients, decaying organic matter,
water, air, and billions of living organisms,
most of them microscopic decomposers
• Renewable resource – produced very
slowly by:
• Weathering rock
• Deposit of sediments by erosion
• Decomposition of organic matter in dead organisms
• Soil Horizons – a series of zones each with
a distinct texture and composition
• Most mature soils have at least three of the possible
horizons
Soil Basics - Continued
• Soil Profile – a cross-sectional view of the
horizons in a soil
• Thick topsoil layer with lots of humus – a fertile
soil that produces high crop yields
• The roots of most plants and most of a soil’s
organic matter are concentrated in these two
upper layers
• As long as vegetation anchors theses layers, soil
stores water and releases it in a nourishing trickle
or a devastating flood
• Infiltration – the downward movement of water
through soil
• Water seeps down and dissolves various soil components in
the upper layers and carries them to lower layers in a
process called leaching
Soil Horizons
• Surface Litter Layer – O Horizon
– Freshly fallen and partially decomposed leaves, twigs,
animal waste, fungi, organic materials
• Topsoil Layer - A Horizon
– Porous mixture – usually darker and looser than deeper
layers
– Partially decomposed organic matter – called hummus
– Some inorganic mineral particles
– Recycled by bacteria and other 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
– Dark brown or black topsoil – nitrogen-rich and high in
organic matter
– Gray, bright yellow, or red topsoils - low in organic
matter and need nitrogen enrichment to support most
crops
Soil Horizons (continued)
• Subsoil - B Horizon – contains most
of the soil’s inorganic matter, mostly
broken-down rock consisting of
varying mixtures of sand, silt, clay,
and gravel
• Parent Material – C Horizon – 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
Differences Between Soils
• Soils can vary in their content of:
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Clay (very fine particles)
Silt (fine particles)
Sand (medium-size particles)
Gravel (coarse to very-coarse particles)
• The amounts of the different sizes and types of
mineral particles determine the soil texture
• Loams – soils with roughly equal mixtures of clay,
sand, silt, and humus
• The best soils for growing most crops because they hold
lots of water, but not too tightly for plant roots to
absorb
Soil Measurements
• Soil Porosity – a measure of the volume of pores
of spaces per volume of soil and of the average
distances between those spaces
• Porous soil – has many pores and can hold more water
and air
• Soil Permeability – the average size of the spaces
or pores in a soil determines permeability; the
rate at which water and air move from upper to
lower soil layers
• Soil Structure – the ways in which soil particles
are organized and clumped together.
• Soil Acidity or Alkalinity – pH influences the
uptake of soil nutrients by plants
Soil Erosion
• Soil Erosion – the movement of soil
components, especially surface litter and
topsoil, from one place to another.
– Causes the buildup of sediments and
sedimentary rock on land and in bodies of water
– Two main agents: Flowing water and wind
– Some is natural and some is caused by human
activities
– Roots of plants help anchor the soil
– Farming, logging, construction, overgrazing by
livestock, off road vehicles, burning vegetation,
and other activities can destroy plant cover and
leave soil vulnerable to erosion.
Types of Water Erosion
• Three types of water erosion:
– Sheet erosion – occurs when surface water moves
down a slope or across a filed in a wide flow and
peels off uniform sheets or layers of soil
– Rill erosion – occurs when surface water forms fastflowing rivulets that cut small channels in the soil
– Gully erosion – occurs when rivulets of fast-flowing
water join together with each succeeding rain cut
the channels wider and deeper until they become
ditches or gullies
• Two harmful effects of soil erosion:
– Loss of soil fertility and its ability to hold water
– Runoff of sediment that pollutes water, kills fish, and clogs
irrigation ditches, boat channels, reservoirs, and lakes
How Serious is Global Erosion?
• Top soil is eroding faster than it forms on about
38% of the world’s croplands
• 17% of the world’s land was degraded by soil
erosion
• NW China – a combination of overplowing and
overgrazing is causing massive wind erosion of
topsoil
• Creates dust plumes of eroded soil which block out the sun
and reduce visibility in China’s northeastern cities and
reduce visibility and increase air pollution
• Nearly 40% of the land used for agriculture is
seriously degraded by erosion, salt buildup, and
waterlogging
• Soil degradation has reduced food production on
about 16% of the world’s cropland
Economic and Ecological
Effects of Soil Erosion
• Loss of soil organic matter and vital plant
nutrients
• Reduced ability to store water for use by crops
• Increased use of costly fertilizer to maintain
fertility
• Increased water runoff on eroded mountain slopes
• Increased soil sediment in navigable waterways –
decreases fish production and harms other forms
of wildlife
• Increased input of sediment into reservoirs
Soil Erosion in the U.S.
– 1/3 of the nation’s original prime topsoil
has been washed or blown into streams,
lakes, and oceans as a result of
overcultivation, overgrazing, and
deforestation.
– Soil is eroding 16x faster than it can
form.
– Great Plains – has lost 1/3 of its topsoil
in 150 years
– Soil erosion decreased by 40% between
1985 and 1997 – soil erosion costs $3.4
million per hour
Desertification
• Desertification – the productive potential of arid
or semiarid land falls by 10% of more because of:
• Natural climate change than causes prolonged drought
• Human wasting or degrading of topsoil
• About 40% of the world’s land and 70% of all dry
lands is suffering from desertification
– Threatens the livelihoods of at least 135 million people
in 100 countries and causes economic losses of $42
billion per year
• Ways to slow desertification:
• Reduce overgrazing
• Reduce deforestation
• Reduce destructive forms of planting, irrigation, and
mining
• Plant trees and grasses that will anchor the soil,
hold water, and reduce global warming
Salinization and Waterlogging
• 17% of the world’s cropland that is irrigated produces
40% of the world’s food
• Irrigated land can produce crop yields two to three
times greater than those from rain watering.
• Irrigation water is a dilute solution of various salts, and
too much salt can be toxic
• Irrigation water not absorbed into the soil evaporates
leaves behind a thin crust of dissolved salts in the
topsoil – salinization – the accumulation of salts
• Can stunt crop growth
• Lower crop yields
• Eventually kills plants and ruin the lands
• Waterlogging – supplying large amounts of irrigation
water to leach salts deeper into the soil
• Water accumulates underground
• Gradually raises the water table
Soil Conservation
• Soil Conservation – involves reducing soil erosion
and restoring soil fertility
• Conventional-tillage Farming –farmers plow the
land and then break up and smooth the soil to
make a planting surface
• Conservation-tillage Farming – minimum-tillage or
no-till farming
• Goal is to disturb the soil as little as possible while
planting crops.
• Minimum – break up and loosen the subsurface soil
without turning over the topsoil, previous crop
residues, and any cover vegetation.
• No-Till Farming – special machines inject seeds,
fertilizers, and weed kills into slits make in the
unplowed soil.
Methods to Reduce Soil Erosion
• Terracing – can reduce soil erosion on steep slopes
by converting the land is a series of broad, nearly
level terraces that run across that land contour
• Retains water for crops at each level
• Reduces soil erosion by controlling runoff
• Contour Farming – involves plowing and planting
crops in rows across the contour of gently sloped
land
• Each row acts as a small dam to help hold soil and slow
water runoff
• Strip Cropping – involves planting alternating
strips of a row crop and another crop that
completely covers the soil - the cover strip traps
soil that erodes from the row crop, catches and
reduces water runoff, and helps prevent the
spread of pests and plant diseases
Methods to Reduce Soil Erosion
• Alley Croping – Agroforestry – in which several crops are
planted together in strips or alleys between trees and shrubs
• Windbreaks or Shelterbelts of trees
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Can reduce wind erosion
Help retain soil moisture
Supply some wood for fuel
Provide habitats for birds, pest-eating and pollinating insects,
and other animals.
• Gully Reclamation – involves restoring severely eroded bare land
by planting fast-growing shrubs, vines, and trees to stabilize
the soil, building small dams at the bottoms of gullies to collect
silt and gradually fill the channels, and building channels to
divert water from the gully
• Land Classification – can be used to identify easily erodible land
that should be neither planted in crops nor cleared of
vegetation
Maintaining and Restoring Soil Fertility
• Organic Fertilizers – from plant and animal
materials
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Animal manure
Green manure
Compost
Spores of mushrooms, puffballs, and truffles
• Commercial Inorganic Fertilizers –
produced from various minerals
• Crop Rotation – planting areas or strips
with nutrient depleting crops on year; the
next year with legumes
• Reduces erosion by keeping the soil covered with
vegetations
• Helps reduce crop losses to insects by presenting
them with a changing target.
Inorganic Fertilizers
• Commercial Inorganic Fertilizers – Contain:
• Nitrogen, phosphorus, potassium
• Other plant nutrients in trace amounts
• Easily transported, stored, and applied
• Disadvantages
– Doesn’t add humus to the soil
– Reduce the soil’s content of organic mater, so it reduces its
ability to hold water
– Lowers the oxygen content of the soil
– Supplies only 2 – 3 of the 20 or so nutrients needed by plants
– Requires large amounts of energy to produce, transport, and
apply
– Releases Nitrous oxide, a greenhouse gas that can enhance
global warming from the soil
– Can cause water pollution – cause cultural eutrophication –
causing algae blooms that use up oxygen dissolved in the water
– Rain water seeping through the soil can leach nitrates in
commercial fertilizers into groundwater