GEOL 3010 - Exam 3

Mass wasting

The general process of earth materials moving downslope due only to gravity

Vertical elevations differences in a local landscape are referred to as

Relief

First order of relief

The coarsest level, consists of continental landmasses and ocean basins

Second order of relief

The intermediate level, includes mountain masses, plains, lowlands (continental features), mid-ocean ridges, oceanic trenches

Third order of relief

The most detailed level, includes individual mountains, cliffs, valleys, hills, and other smaller scale landforms

The most detailed order of relief applies to ______ order relief 

Third

Which of the following is an example of a first order of relief?

An ocean basin

Approximately what percentage of Earth’s surface is exposed above the mean sea level?

29%

Mean sea level

0m

Which of the following cannot impact the stability of slopes?

Atmospheric pressure

Why are cumulative effects of smaller mass wasting events greater than large ones?

Smaller events are more common

Why is smaller scale mass wasting so common?

1. Mass wasting can occur even on moderate slopes, which means it is a potential problem over a significant portion of the landscape

2. Mass wasting is often inadvertently triggered by routine human activities, such as road construction, deforestation, and open-pit mining that tend to destabilize slopes thereby triggering mass wasting

How does oversteepening a slope cause mass wasting?

As the steepness of a hillside increases, the component of gravity operating in the slope direction becomes greater, thereby increasing the potential for mass wasting

Factors that influence slope stability

Slope steepness, type of rock or sediment making up the slope, presence of water or ice, amount of vegetative ground cover

Common triggering mechanisms for slope failure

Earthquakes, heavy rains, and removal of vegetation by wildfires

What is the driving force behind mass wasting?

Gravity

What is the angle of repose for loose materials?

35 degrees

Angle of repose

The angle past which which loose sediment typically does not form slopes

Bedding planes

Horizontal surfaces that typically represent some change in the sediment during deposition, weakens sedimentary rock

Which of the following earth materials is most likely to produce a vertical cliff that is fairly stable?

Crystalline igneous rock such as granite

Why do humid climates have less mass wasting?

They have relatively consistent amounts of precipitation throughout the year and support dense vegetation that stabilizes slopes.

How can humid climates increase mass wasting?

During unusually large and prolonged rainstorms or rapid snowmelts, dense vegetation will increase infiltration since it reduces the ability of surface water to move downslope. It also adds significant weight to a slope and reduces friction through higher pore pressures.

How do arid climates lead to mass wasting?

When rainfall occurs, intense rains combined with sparse vegetation makes it easier for loose material.

Mass wasting due to vegetation removal

Occurs in humid climates where wildfires or logging activity suddenly remove vegetation from a hillside. Once the vegetation is gone, mass wasting can be triggered by a rainfall event.

Mass wasting in cold climates

Where freeze/thaw cycles are common, material can move downslope simply due to the expansion and contraction of water during freezing and melting events

Which of the following human activities commonly causes the over-steepening of slopes, which then leads to mass wasting?

Construction of roads

How does the nature of slope material influence mass wasting?

Materials with strong internal friction, such as a rock that is homogenous and composed of minerals that interlock and are resistant to weathering, form steep slopes while loose or unconsolidated sediments have lower frictional forces

Which of the following best explains why excessive amounts of water can destabilize a slope?

Increases weight and reduces friction

Rock

refers to large blocks of solid rock

Debris

refers to mixture of rock, earth, mud and plants

Earth

refers to loose sediment and weathered rock fragments

Mud

refers to mixture of water and finer-sized sediment

Regolith

The broken-up rock due to weathering

Bedrock

The parent rock from which weathered regolith and soils develop

Talus pile

Wedge-shaped deposit of rocks found at the base of exposed rock bodies

Earth falls and debris falls

Common in areas where migrating rivers undercut stream banks made of unconsolidated materials

Where are slides more likely to take place?

When the zone of weakness, or weakness plane, is inclined in the sme direction as the slope of the land surface

Translational landslide

A mass that slides downward and outward on top of an inclined planar surface (rock slides)

Rotational landslide

A slide in which the surface of a rupture is curved concavely upward and the slide movement is roughly rotational (earth and debris slides)

Parts of a slump

• Scarp - curved and distinct scar which marks where the disturbed material becomes detached from the undisturbed material

• Toe - a bulge developed at the base of a slump

Most common mass wasting involving flow

Mudflow

A ___ is similar to a mudflow, but the primary difference is that it contains particles ranging in size from clay (mud) and sand to large boulders and trees.

Debris flow

Earth flow

involves a large section of an unstable hillside that flows downslope as a more coherent and viscous mass

Where are earth flows common?

On slopes that are underlain by sediment richer in clay minerals and held in place by thick vegetation. Here the extensive root system helps bind the loose particles together, and therefore minimizing the potential for mudflows.

Why is the presence of clay minerals important in reducing friction?

Their plasticity greatly increases as they take on water

Solifluction

When the uppermost zone of permafrost ground thaws during the summer, it becomes saturated and begin flowing downslope over the frozen base

What best describes the way in which mass-wasting events are classified?

Type of material and type of motion

Which of the following best describes a mass wasting event referred to as a fall?

Material tumbling through air or down a steep slope

Gradual subsidence

Associated with the compaction of pore spaces within a sedimentary sequence, compaction occurs in fine-grained sediment when overburden pressure forces individual clay mineral particles to become aligned in a parallel manner and more compact

Which of the following best describes a mass wasting event referred to as a slide?

Material moving down-slope along a surface

Cutbank

A highly unstable overhang created when a stream channel migrates and undercuts its bank

How can water strengthen a slope?

A thin film of water forms within the pore spaces, and the electrical attraction between the water molecules and mineral grains makes the sediment stronger

How can water destabilize the slope?

The additional weight of the water and the reduction of the frictional forces between the individual grains

How does climate influence the stability of the slope?

It determines how and when precipitation

Which of the following best describes a mass wasting event referred to as a flow?

Material moving as a viscous fluid

Which of the following forms of mass wasting involves both sliding and flowing?

Slump

Which of the following forms of mass wasting involves the slow, down slope movement of individual soil and rock particles?

Creep

Unlike mass wasting, gradual subsidence and sudden collapse involve the movement of earth materials without any slope. Sudden collapse is typically caused by which of the following?

Collapse of caves in limestone rock

The use of engineering controls is one of the two basic methods for minimizing mass wasting hazards. Which of the following is the other basic approach used to minimize mass wasting hazards?

Avoid building in hazard zones

Which of the following engineering controls would be best suited to strengthen a slope that has been over-steepened by cutting into the hillside during highway construction?

Build a retaining wall

Imagine a section of highway with a large outcrop of fractured rock that poses a serious rockfall hazard. Which of the following engineering controls would be the best choice for minimizing the chance of large rocks falling onto the roadway?

Install rock bolts

Suppose that engineers are unable to keep earth materials from coming down onto a highway. In this case, which of the following would be the most economical and effective technique for reducing the hazard?

Build a protective structure

Highway tunnels are very expensive, but are often times more cost effective over the long-term compared to the cost of repeated repairs and maintenance in areas where mass wasting is difficult to control. (True or false)

True

It is nearly impossible for scientists and engineers to predict where slope failure may occur. (True or false)

False

Pumping oil from the subsurface can result in subsidence (True or false)

True

Which common feature can weaken sedimentary rocks?

Bedding

What common material found in the environment can add weight and reduce friction, thereby triggering mass wasting?

Water

What feature associated with volcanoes can trigger a mass wasting event?

Seismic waves

Which geologic process contributes to the occurrence of rockfalls?

Freeze-thaw cycles

Which common feature often contributes to the occurrence of rockslides in sedimentary rocks?

Bedding

Where does solifluction occur?

In areas of permafrost

Which process contributes to the development of soil creep?

Freeze-thaw cycles

Which type of geologic hazard can be triggered by submarine mass-wasting?

Tsunami

Land subsidence can be associated with which type of geological feature?

A cave

How does removal of water help to stabilize a slope?

Increases friction

What are retaining walls used for?

To strengthen over steepened slopes

Rock bolts are used to prevent mass-wasting in what type of material?

Loose rock

Which of the following is not a strategy for controlling excess water in an unstable slope?

Install rock bolts

How does vegetation help to stabilize slopes?

It takes up water and reduces the fraction of rain or snow that infiltrates into the subsurface

Protective structures used to control mass-wasting include all of the following except

Terraces

What is the basic role of streams in the Earth system?

drain water from the landscape and transport sediment

Water cycle volumes and percentages

• Evaporation from the ocean - 419 volume, 86 percent

• Precipitation over the ocean- 382, 78 percent

• Evaporation and transpiration from land - 69 volume, 14 percent

• Precipitation over land - 106 volume, 22 percent

• Surface runoff and groundwater flow - 37 volume, 8 percent

Overland flow

Some of the precipitation that falls on the land surface moves downslope in thin sheets in a process called overland flow. During an overland flow event the water will eventually move into a low area of the terrain and begin flowing as a discrete body or stream channel.

Plant transpiration

process in which water is taken up by root systems and then released through plant leaves

Evapotranspiration

a collective term (evaporation + transpiration) for all the processes by which water in the liquid or solid phase at or near the land surface becomes water vapor.

Capillary rise

the rise in a liquid above the level of zero pressure due to a net upward force produced by the attraction of the water molecules to a solid surface

Groundwater baseflow

Groundwater that discharges into the surface environment

Stream hydrograph

Plot of discharge versus time

Things humans have historically used rivers to do

1. Transport goods

2. Obtain food

3. Farm in floodplains

4. Harness the energy of falling water to build mills

What puts large numbers of people and buildings at risk of flooding?

The frequency of occurrence combined with human activity concentration along waterways

Which of the following terms is used to describe the volume of water flowing past a specific point in a stream over a given time period?

Discharge

Why groundwater baseflow is important to maintaining a healthy stream ecosystem in humid climates?

provides streams with a steady flow of water during dry periods

What is a drainage basin?

the land area that collects water for a given network of streams.

What is the headwaters of a drainage basin?

collection of small channels in the upper part of a basin that merge into larger streams.

What are characteristics of streams near their mouths?

low velocity, high discharge, and meandering channels

Why stream valleys become wider over time?

meandering streams undercut their banks and cause mass wasting.

Why average discharge of the Amazon River is about 12 times of that of the Mississippi River?

the Amazon’s drainage basin lies mostly within a tropical rainforest

How do hydrologists measure the severity of floods?

the stage above which a stream overflows its banks

What best explains how hydrologists determine the frequency of flooding?

calculate recurrence intervals from historical discharge records

Which of the following natural factors affect the degree or severity of flooding?

rainfall volume and infiltration capacity of the ground