Geology: Processes, Hazards, and Soils
 Three concentric zones:
 Core—solid inner core of iron and liquid outer core
(iron) which gives the Earth its magnetic poles.
 Mantle—thick solid zone consisting of iron, silicon,
oxygen, and magnesium. The outermost layer of the
mantle is a thin plastic layer of partially molten rock—
the asthenosphere
 Crust—outermost and thinnest zone of the Earth
 Continental crust—29%
 Oceanic crust—71%
 Chemical makeup of the Crust:
 Oxygen
 Silicon
 Aluminum
 Iron
 Calcium
 Sodium
 Potassium
46.6%
27.7
8.1
5.0
3.6
2.8
2.6
 Plates—major areas of the Earth’s crust whose
boundaries are determined by lines of earthquakes
and volcanoes
 Lithosphere—crust and the rigid outermost part of
the mantle above the asthenosphere
 Plate Tectonics—the theory that explains the
movement of the plates and the processes that occur at
their boundaries (developed from the continental drift
concept).
 Plate movement produces mountains, volcanoes, the
oceanic ridge system, trenches and other earth
features.
 Help explain how certain patterns of biological
evolution occurred.
 1. Divergent Plate Boundary—plates move apart in
opposite directions.
 2. Convergent Plate Boundary—plates are pushed
together (subduction zone) forming a trench and
causing earthquakes and volcanoes.
 3. Transform Fault—plates move in opposite but
parallel directions (slide past each other).
 External Processes—those geologic changes based
directly or indirectly on energy from the sun and on
gravity (rather than heat from the Earth’s interior)
 Whereas internal processes generally build up the
earth’s surface, external processes tend to wear it
down.
 Erosion—material is :
 1. dissolved, loosened, or worn away from one part of
the earth’s surface and
 2. deposited in other places by water or wind.
 Weathering—caused by mechanical or chemical
processes that produce loosened material that can
then be eroded.
 Mechanical weathering—a large rock mass is broken
into smaller fragments similar to the results you would
get by using a hammer by the freezing and thawing of
water, plant roots, etc.
 Chemical weathering—one or more chemical
reactions decompose a mass of rock.
 The earth is composed of minerals and rocks. It is the
source of almost all the nonrenewable resources we
use: fossil fuels, metallic minerals, and nonmetallic
minerals.
 Mineral—an element or inorganic compound that
occurs naturally and is solid.
 Rock—any material that makes up a large, natural,
continuous part of the earth’s crust.
 Igneous rock—formed below or on the earth’s surface
from molten rock material.
 Sedimentary rock—formed from sediment when
preexisting rocks are (1)weathered and eroded
(2)deposited in body of water, and (3)over time
compressed into rock.
 Metamorphic rock—preexisting rock is subjected to
(1)high temperature, (2) high pressure, (3)chemicals,
or a combination of these to change the structure of
the rock.
 The interaction of processes that change rock from one
type to another.
 You will need to
Be able to label
This diagram…
Earthquakes and Volcanic Eruptions
 Stress in the earth’s crust can cause solid rock to
deform until it suddenly fractures and shifts along the
fracture, producing a fault. The faulting or later
abrupt movement causes an earthquake.
 Focus—the point of initial movement of the earth
below ground.
 Epicenter—the point on the surface directly above the
focus.
 Magnitude—the measurement of the severity of an
earthquake.
 Richter scale—a scale used to measure the magnitude
of earthquakes.
 Each unit on the Richter scale represents an amplitude
that is 10 times greater than the next smaller unit.
 An active volcano occurs where magma reaches the
earth’s surface through a central vent or a long crack.
 Volcanic activity can release:
 1. ejecta—debris ranging from large chunks of rock to
ash that may be extremely hot.
 2. liquid lava
 3. gases such as water vapor, carbon dioxide, and sulfur
dioxide.
Formation and Types
 A complex mixture of eroded rock, mineral nutrients,
decaying organic matter, water, and living organisms.
 Although soil is a renewable resource, it is produced
very slowly by:
 1. weathering of rock
 2. deposit of sediments by erosion
 3. decomposition of organic matter
 Mature soils are arranged in a series of zones called
soil horizons.
 A cross-sectional view of the horizons in a soil is called
a soil profile.
 http://www.youtube.com/watch?v=6Kr3Wj7SeSc
 Soil Texture—soils vary in their content of clay, silt,
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and sand. The relative amounts of the different sizes
and types of mineral particles determine soil texture.
Soil with roughly equal mixtures of clay, sand, silt, and
humus are called loams.
Loams are most often the desired soil texture for
agriculture.
The soil triangle is used to determine soil texture.
http://www.youtube.com/watch?v=VEgHmgnrWz
k
% sand
example
% silt
75
% clay
10
15
42
_____
__________________
Soil Texture
sandy loam
37
______
52
21
__________________
______
35
50
__________________
 Soil texture helps determine soil porosity—a measure
of the volume of pores or spaces per volume of soil
and of the average distances between those spaces.
 A porous soil has many pores and can hold more water
and air than a less porous soil.
 The average size of the spaces or pores in a soil
determines soil permeability—the rate at which
water and air move through the soil.
 Soil pH—the acidity or alkalinity of a soil measured
on a pH scale, influences the uptake of soil nutrients
by plants.
 The movement of soil components, especially surface
litter and topsoil from one place to another.
 Moving water causes most erosion.
 Soil scientists distinguish several types of water
erosion.
 Splash Erosion—caused by the impact of rain drops.
 Sheet Erosion—occurs when surface water moves
down a slope or across a field in a wide flow and peels
off fairly uniform sheets of soil.
 Rill Erosion—occurs when flowing surface water cut
small channels in the soil.
 Gully Erosion—occurs when each successive rain cut
the channels of rill erosion deeper until they become
gullies or ditches.
 The two major harmful effects of soil erosion are:
 1. loss of fertility and its ability to hold water
 2. runoff of sediment that pollutes water, kills fish and
shellfish, and clogs irrigation ditches, boat channels,
reservoirs, and lakes.
 In tropical and temperate areas it takes 200-1000 years
(depending on climate and soil type) for one inch of
topsoil to form.
 The earth is losing 7-21% of its topsoil from actual or
potential cropland each decade.
 In developing countries, poverty and erosion interact
in a destructive positive feedback cycle.
 About 1/3 of the nation’s original prime topsoil has
been washed or blown into streams, lakes, and oceansmostly as the result of over cultivation, overgrazing ,
and deforestation.
 Soil on cultivated land is eroding 16 times faster than it
can form. The erosion rate is faster in heavily farmed
areas like the Great Plains.
 Process whereby productive potential on arid or
semiarid land falls by 10% or more
 Practices that leave topsoil vulnerable to
desertification:
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1. overgrazing
2. deforestation without reforestation
3. surface mining without reclamation
4. irrigation techniques that lead to increased erosion
5. salt buildup and waterlogged soil
6. farming on land that has unsuitable terrain or soil
7. soil compaction by farm machinery and cattle hooves
 Accumulation of salts in the soil.
 Irrigation of land can help produce crop yields of 2-3
times greater than rain watering alone. However,
irrigation water not absorbed into the soil evaporates,
leaving behind a thin crust of dissolved salts in the
topsoil.
 Salinization can stunt plant growth, lower crop yields,
and eventually kill plants and ruin land.
 Involves reducing soil erosion and restoring soil
fertility.
 Farming techniques to reduce erosion:
 Terracing—converting steep slopes into a series of
broad, nearly level terraces or steps that run
perpendicular to the slope
 Contour Farming—Plowing and planting crops in rows
across the contour of gently sloped land. Each row acts
as a small dam to help slow runoff.
 Strip Cropping—planting alternating strips of a row
crop (such as corn) and a grass crop (such as alfalfa)
 Agroforestry (alley cropping)—several crops are
planted together between trees and shrubs
 Windbreaks (shelterbelts)—trees or shrubs planted to
reduce wind erosion.
 Gully Reclamation—restoring severely eroded bare
land by planting fast-growing trees or shrubs to stabilize
the soil, and building small dams and channels to divert
water from the gully.
 Fertilizers partially restore plant nutrients lost by
erosion, crop harvesting, and leaching.
 Fertilizers can be either organic—from pant and
animal materials, or commercial inorganic—chemicals
produced from various minerals.
 Animal manure—the dung and urine of cattle,
horses, poultry, and other farm animals.
 Green manure—fresh or growing green vegetation
plowed into the soil.
 Compost—produced when microorganisms break
down organic matter such as leaves, food wastes,
paper, and wood.
 Spores of mushrooms, puffballs, and truffles—the
rapidly growing mycorrhizae fungi help plants take in
nutrients and be more disease resistant.
 Commercially made from various minerals.
 Typically contain nitrogen, phosphorus, and
potassium
 Percentage of each nutrient represented by the
numbers on the container, the first for nitrogen, 2nd for
phosphorus, and 3rd for potassium
Organic Fertilizer
Inorganic Fertilizer
 Improves ability to retain
 Easily transported,
water
 Improves soil structure
 Contains a wide range of
nutrients
 Stimulates beneficial soil
bacteria
stored, and applied
 Can target specific soil
nutrient deficiency
 Does not add humus to the soil
 Reduces soils ability to hold water
 Lowers oxygen content
 Typically supplies only 3 plant nutrients
 Releases nitrous oxide, a greenhouse gas from the soil.