Geology: Processes, Hazards, and SoilsAPES Chapter 10

Geologic Processes:

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 EarthContinental crust—29%Oceanic crust—71%

Earth’s Structure:

Chemical makeup of the Crust:

Oxygen 46.6% Silicon 27.7 Aluminum 8.1 Iron 5.0 Calcium 3.6 Sodium 2.8 Potassium 2.6

Internal and External Earth Processes:

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 Tectonics:

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.

Three Types of Plate Boundaries:

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.

Geologic Processes on the Earth’s Surface

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.

External Processes: Erosion and Weathering

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.

Minerals, Rocks, and the Rock Cycle

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.

Minerals and Rocks

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.

Three Major Rock Types:

The interaction of processes that change rock from one type to another.

Rock Cycle

Natural Hazards: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.

Earthquakes:

Focus—the point of initial movement of the earth below ground.

Epicenter—the point on the surface directly above the focus.

Earthquake Features:

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 lava3. gases such as water vapor, carbon dioxide,

and sulfur dioxide.

Volcanoes:

Soil Resources: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 rock2. deposit of sediments by erosion3. decomposition of organic matter

Soil

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.

Soil Texture—soils vary in their content of clay, silt, 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.

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.

Soil Erosion and Degradation

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.

Soil 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.

Types of Water Erosion:

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.

Global Soil Erosion:

About 1/3 of the nation’s original prime topsoil has been washed or blown into streams, lakes, and oceans-mostly 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.

Soil Erosion in the U.S.

Process whereby productive potential on arid or semiarid land falls by 10% or more

Practices that leave topsoil vulnerable to desertification:1. overgrazing2. deforestation without reforestation3. surface mining without reclamation4. irrigation techniques that lead to increased

erosion5. salt buildup and waterlogged soil6. farming on land that has unsuitable terrain

or soil7. soil compaction by farm machinery and

cattle hooves

Desertification:

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.

Salinization:

Soil Conservation

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

Soil Conservation:

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.

Fertilizers:

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.

Organic Fertilizers:

Commercially made from various minerals.Typically contain nitrogen, phosphorus, and

potassiumPercentage of each nutrient represented by

the numbers on the container, the first for nitrogen, 2nd for phosphorus, and 3rd for potassium

Inorganic Commercial Fertilizers:

Organic FertilizerImproves ability to

retain waterImproves soil

structureContains a wide

range of nutrientsStimulates beneficial

soil bacteria

Easily transported, stored, and applied

Can target specific soil nutrient deficiency

Advantages of Each:Inorganic Fertilizer

Does not add humus to the soilReduces soils ability to hold waterLowers oxygen contentTypically supplies only 3 plant nutrientsReleases nitrous oxide, a greenhouse gas

from the soil.

Inorganic Fertilizer Disadvantages: