Soils & Mass Movements · Mass Movements •Mass Movement: The downslope movement of soil and weathered rock resulting from the force of gravity •All occur on slopes •Because

Soils & Mass Movements

Review: Weathering

• Weathering produces rock fragments (sediments)

• Sediments range in size from huge boulders to microscopic particles

• Physical Weathering: Minerals remain chemically unchanged• Breaking rock into smaller pieces • No change in composition

• Chemical Weathering: minerals in a rock are dissolved or otherwise chemically changed

Physical Weathering - Effect of Temperature

• Temperature plays a role in mechanical weathering

• Frost (Ice) Wedging: • When water freezes, it expands and increases in volume

• In many places on the earth, water collects in the cracks of rocks and rock layers

• If the temperature drops to the freezing point, water freezes, expands, exerts pressure on the rocks, and can cause the cracks to widen slightly.

• When the temperature increases, the ice melts in the cracks• Example: pot holes in roads

Frost Wedging

Physical Weathering - Effect of pressure

• Roots of trees and other plants can exert pressure on rocks when they wedge themselves into the cracks in rocks

• As the roots grow and expand, they exert increasing amounts of pressure which often causes the rocks to split

Physical Weathering

• Exfoliation: gravity causes rocks to break loose, coming off in layers• During the day the rock heats up, causing it to expand, but at night it

contracts when the temperature is cool

• When this is constantly repeated, the outer layer will often peel away from the rest of the rock

Chemical Weathering

• Process by which rocks and minerals undergo changes in their compositions

• Agents of chemical weathering: • Water• Oxygen • Carbon Dioxide (CO2) • Acid Precipitation

• The interaction of these agents with rock can cause some substances to dissolve, and some new minerals to form. The new minerals have properties different than those that were in the original rock.

Chemical Weathering: Effects of Water & Oxygen

• Water: • Water may dissolve many kinds of

minerals and rocks

• Oxygen:• Oxidation: the chemical reaction of oxygen

with another substance ~21% of Earth’s atmosphere is oxygen gas

• Example: Iron & oxygen create iron oxide (rust)

Chemical Weathering: Effects of CO2

• Carbon Dioxide: • Carbon Dioxide gas occurs

naturally in the atmosphere as a product of living organisms

• When carbon dioxide combines with water in the atmosphere, it forms a very weak acid called carbonic acid• This can dissolve minerals like

calcite that is found in marble and limestone

Chemical Weathering: Effects of Acid Precipitation

• Acid Precipitation: • Sulfur Dioxide and Nitrogen

oxides released into the atmosphere by human activities can also cause acid precipitation.

• Can slowly dissolve minerals in rocks, be harmful to many organisms, and destructive to human-made structures

Rate of Weathering

• The Natural weathering of Earth materials occurs slowly• It can take 2000 years to weather 1 cm of limestone

• Climate • The interaction between temperature and precipitation in a given climate

determines the rate of weathering

• Chemical weathering is rapid in climate with warm temperatures, abundant rainfall and lush vegetation

• Physical weathering is more rapid in cool climates. Physical weathering rates are highest in areas where water in cracks undergoes repeated freezing and thawing.

Rate of Weathering: Surface Area

• Mechanical weathering breaks rocks into smaller pieces. As the pieces get smaller, the surface area increases. When this happens, there is more total surface area available for chemical weathering.

• Weathering has more of an effect on smaller particles (with more surface area)

Erosion

• Erosion: the removal and transport of sediment

• Four main agents of erosion: • Moving Water

• Wind

• Glaciers (Ice)

• Gravity

Energy of transporting agents

• Water: • Fast-moving water can transport large particles better than slow-moving

water

• As water slows down, largest particles settle out first

• Rivers, streams, ocean waves

• Wind: • Can only move small grains

• Glaciers:• Move all materials with equal ease

• Dumps materials in unsorted piles as the glacier melts

• Deposits similar to landslides

How do you start describing the soil?

What do you see?

What is different from top to bottom?

How deep do roots go?

Soils

• Soil: made of loose, weathered rock and organic material in which plants with roots can grow

• Product of thousands of years of chemical and mechanical weathering and biological activity

Soil Development • The material from which a soil is

formed is called its parent rock

• Residual Soil: A soil whose parent material is the bedrock beneath the soil • Example: the soil in the Bluegrass region

of Kentucky

• Transported Soils: soils from transported materials • Example: the soils in the Midwestern US

have parent material that was deposited by the glaciers

Soil Development

• A soil develops as its parent rock is weathered away

• Worms and other organisms break down organic matter and add nutrients to the soil • Organic matter can include leaves,

twigs, roots, dead animals and insects

• As nutrients are added, the texture changes, and the soil’s capacity to hold water increases

Soil Profiles

• During the development of soil, layers form

• A soil profile is a vertical sequence of soil layers (a cross-section)

• New soils are not fully developed and don’t have distinct layers

• If there are distinct layers, the soil is mature

Soil Horizons

• Soil Horizon: a distinct layer within a soil profile • 4 major soil horizons in mature soil:

• O-horizon: top layer of organic material • Humus: organic material that forms from decayed plant and animal materials

• A-horizon (Topsoil): weathered rock combined with a rich concentration of dark brown organic material

• B-horizon (Subsoil): red or brown layer that has been enriched over time by clay and minerals deposited by water flowing from the layers above• Usually a blocky structure • Can create a hard layer call the hardpan

• So dense that it allows little or no water to pass through it

• C-horizon: little or no organic matter, often made of broken-down bedrock or rock fragments

• The unweathered parent rock lies directly beneath the C-horizon

Soil Horizons

Top Soil

Subsoil

Parent Material

A Horizon

B Horizon

C Horizon

Factors of Soil Formation

• Various factors can affect soil formation and product different types of soil, called soil orders.

1) Climate: • Most significant factor • Dry, arid climate vs. tropical determine

the minerals present in the B-horizon• Heavy rainfall intensely weathers soils

where lots of minerals have been flushed out

Factors of Soil Formation

2) Topography: • Slope and orientation of land

• On steep slopes, weathered rock is carried downhill• Hillsides tend to have shallow soils while valleys and flat areas have thicker soils with

more organic material

• Slopes that are oriented towards the sun receive extra sunlight allows more vegetation to grow

• Slopes without vegetation tend to lose more soil to erosion

3) Parent Material:• The texture and composition of soil depend partly on the parent rock

• Residual vs. Transported soil

Factors of Soil Formation

4) Biological Organisms: • Organism including fungi, bacteria as well as plants and animal interact with

soil • Microorganisms decompose dead plants and animals • Different types of biological organisms in a soil can result in different soil

orders

5) Time: • New soils are often found along rivers where sediment is being deposited • After tens of thousands of years of weathering, more of the original minerals

in a soil are changed or washed away

Soil Texture

• Particles of soil are classified according to size as clay, silt, sand, with clay being the smallest and sand being the largest.

• Soil texts affects is capacity to retain moisture and therefore its ability to support plant growth • Example: Soil that is very sandy holds

water easily, but dries out quickly

• Texture can vary with depth

Soil Fertility

• Soil Fertility: The measure of how well a soil can support the growth of plant

• Determined by: • Topography

• Availability of minerals and nutrients

• Number of microorganisms present

• Amount of precipitation

• Level of acidity

Soil Color The minerals, organic matter, and moisture in each soil horizon determine its color

• Dark layers are usually rich in organic matter • O-horizon & A-horizon

• Red and yellow soils might be the results of oxidation of iron minerals

• Yellow soils are usually poorly drained and are often associated with environmental problems

• Grayish or bluish soils are common in poorly drained regions where soils are constantly wet and lack oxygen

• Munsell System of Color Notation: used to describe soil color based on hue, value

(light/dark) and intensity

Mass Movements

• Mass Movement: The downslope movement of soil and weathered rock resulting from the force of gravity • All occur on slopes

• Because few places on Earth’s surface are completely flat, almost all of Earth’s surface undergoes mass movement

• Can range from motions that are barely noticeable to sudden slides, falls, and flows.

• Materials that are moved can range in size from fine-grained mud to large boulders.

Factors that Influence Mass Movements

1) Material’s Weight 2) Material’s resistance to sliding or flowing

- Amount of friction - How cohesive the material is - Anchored to bedrock

3) Trigger - Earthquake shaking material loose

4) Water- Saturation increases the weight of soils and sediment- Water also acts as a lubricant between the grains, reducing the friction

Mass Movements occur when the forces pulling material downslope are stronger than the material’s resistance to sliding, flowing, or falling

Types of Mass Movements1) Creep: slow, steady, downhill flow of

loose, weathered Earth materials, especially soils

• Movement might be as little as a few centimeters per year • Usually noticeable only over long

periods of time

• Observe the positions of structures and objects over time to tell if creep has occurred• Creep can cause once-vertical utility

poles and fences to tilt, and trees and walls to break

Types of Mass Movements

• Can flow a few centimeters per year to hundred of kilometers per hour

A. Earthflows: moderately slow movements of soils

B. Mudflows: swiftly moving mixtures of mud and water• Triggered by earthquakes or similar vibrations • Tend to occur in drier regions that experience

infrequent but heavy rainfall• Lahar: mudflows that accompany volcanic eruptions

• Heat from erupted materials melts snow and ice on top of a volcano. Water flows down the slopes of the volcano, carrying mud along with it

2) Flows: materials have been saturated with water and flows as if they were a thick layer

Types of Mass Movements

3) Slides: a rapid, downslope movement of Earth materials

A. Landslide: occurs when a relatively thin block of soil, rock, and debris separate from the underlying bedrock• Material rapidly slides downslope as one

block with little internal mixing • A landslide mass eventually stops and

becomes a pile of debris at the bottom of a slope, sometimes damming rivers and causing flooding

• Causes almost 2 billion dollars in damage cost in the US per year

• Causes 25-50 deaths per year in the US

• Avalanches: a landslide that occurs in mountainous areas with thick accumulation of snow• About 10,000 avalanches occur each year in

the mountains of the western US

• Radiation from the Sun can melt surface snow, which then refreezes at night into an icy crust. Snow that falls on top of this crust can build up, become heavy, slip off, and slide downhill as an avalanche.

• Large avalanches occur at slope angles between 30 and 45. (If greater than 45, not enough snow can accumulate to create large avalanches)

B. Rockslide: types of landslide when a sheet of rock moves downhill on a sliding surface

Types of Mass Movements

4) Slumps: when the mass of material in a landslide moves along a curved surface

• Material at the top of the slump moves downhill, and slightly inward, while the material at the bottom moves outward

• Tends to occur because a slope becomes too steep for the bottom to support the soil at the top of the slope

• Frequently occurs on along highways where the slopes of soils are extremely steep

Human Activity

• While mass movements are natural processes, human activities often contribute to factors that cause mass movements

• Construction of buildings, roads, and other structures can make slopes unstable

• Poor maintenance of septic systems, which often leak, can trigger slides

Reducing the Risks • Humans can minimize the destruction caused by mass movements by not building structures on or near the base of steep and unstable slopes

• A series of trenches can be dug to divert running water around a slope and control its drainage

• Landslides and rockslides can be controlled by covering steep slopes with materials such as steel nets and constructing fences along highways

• Installation of retaining walls to support the bases of weakened slopes

• Best way to reduce the number of disasters related to mass movements is to educate people about the problems of building on steep slopes