Chapter 10: GeologyProcesses, 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 directions.

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:• 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

• 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:

• 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 fast-flowing 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 • 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

• 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