Mass Movements Wasting Erosion Agents Agents Mass wasting

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Mass Movements/ Wasting

Mass Movements/ Wasting

Erosion Agents • Agents: – Mass wasting – Running water (streams) – Glaciers –

Erosion Agents • Agents: – Mass wasting – Running water (streams) – Glaciers – Wind – Ocean waves – Ground water – Video

Mass Movement Includes • Landslides • Rock falls • Avalanches • Mud flows •

Mass Movement Includes • Landslides • Rock falls • Avalanches • Mud flows • Debris flows • Creep

>100 km/year <1 cm/year 0% ~40%

>100 km/year <1 cm/year 0% ~40%

Mass Movements • Material moves downslope due to the pull of gravity • Can

Mass Movements • Material moves downslope due to the pull of gravity • Can happen almost anywhere • Commonly associated with other events (heavy rainfall or earthquakes, for example) and are therefore under-reported • Movements can either be catastrophic (slope failure) or slow and steady (creep) • The rate of the mass movement can be increased by various erosive agents (especially water)

Factors in Slope Stability Gravity Water Earth Materials Triggering Events

Factors in Slope Stability Gravity Water Earth Materials Triggering Events

Risk factors to increase likelihood of mass movement Gravity - hill slopes more vulnerable

Risk factors to increase likelihood of mass movement Gravity - hill slopes more vulnerable (on top of a hill, on the slope, or at the bottom of a hill), modified slopes (road cut, cut flat area to build on, coastal erosion, etc. ) Water - risk is higher when ground is saturated and/or during heavy rains, El Niño events Earth Materials - loose soils (particularly clay-rich) or fractured rock, and old landslides pose greater risk Triggering Events - heavy rain during storm, rain after big storms or fires, earthquakes (when ground is saturated? )…are all triggers

Gravity & steepening of a slope

Gravity & steepening of a slope

Angle of Repose Varies for Different Materials

Angle of Repose Varies for Different Materials

Internal Causes for Slope Failure • • Water (weight & interaction with clay minerals)

Internal Causes for Slope Failure • • Water (weight & interaction with clay minerals) Decreasing rock cohesion Incompetent/weak material Adverse geologic structures

Controlling Factors • Water adds weight – BUT a little water makes soil stronger

Controlling Factors • Water adds weight – BUT a little water makes soil stronger (damp sand castle) • “the water holds the grains together” – Too much water makes soil weak • “floats the grains apart” • Vegetation – Absorbs excess water from soil (increasing strength) – Binds soil together (increasing strength)

The Weight of Water • Sedimentary rocks commonly have porosities of 10 - 30%

The Weight of Water • Sedimentary rocks commonly have porosities of 10 - 30% • If pore spaces fill with water, the weight of the material is increased substantially, creating instability

Mudflow – Flow of watery debris – Occurs where lack of vegetation: • Dry

Mudflow – Flow of watery debris – Occurs where lack of vegetation: • Dry climates • Volcanoes • After forest fires – Very fast up to 80 km/hr – Triggered by heavy rainfall – Los Angeles

La Conchita, CA March 1995

La Conchita, CA March 1995

It happened again in 2004… in exactly the same place…

It happened again in 2004… in exactly the same place…

La Conchita, CA

La Conchita, CA

Debris flows or mud flows • Mass movements that behave like fluids • Unlike

Debris flows or mud flows • Mass movements that behave like fluids • Unlike slides, flows are not controlled by a failure surface, but instead are dominated by internal movements

Landslides in the Bay Area

Landslides in the Bay Area

1982 San Mateo County

1982 San Mateo County

Devil’s slide area on Highway 1 north of Half Moon Bay

Devil’s slide area on Highway 1 north of Half Moon Bay

Rock Falls

Rock Falls

Classification of Mass Wasting • Rate of movement – Soil creep to slumps to

Classification of Mass Wasting • Rate of movement – Soil creep to slumps to mudflows • Extremely slow (~1 mm/year) to very rapid (>100 km/hour) – Soil creep is slowest but ultimately most important - it happens everywhere – Others happen in wet and/or steep places

Controlling Factors • Most important: Water • Climatic controls – Ice – Vegetation –

Controlling Factors • Most important: Water • Climatic controls – Ice – Vegetation – Mr. Parr Song

Controlling Factors • Overgrazing – Reduces vegetation • Weakens soil • Allows water to

Controlling Factors • Overgrazing – Reduces vegetation • Weakens soil • Allows water to enter soil • Overloading – Dumping of mine debris – Heavy buildings on steep, denuded hillsides

Rotational landslide

Rotational landslide

Creep • Downslope movement of soil and uppermost bedrock • Creep happens at too

Creep • Downslope movement of soil and uppermost bedrock • Creep happens at too slow of a rate to observe directly • Instead, creep can be identified by it’s effect on objects

Most Common Mass Wasting • Soil creep Freeze – gentle slopes – vegetation slows

Most Common Mass Wasting • Soil creep Freeze – gentle slopes – vegetation slows movement – very slow flow (< 1 cm/year) • facilitated by water in soil • by freeze-thaw in colder climates – Freezing of water expands soil. Thawing drops the grains downhill. Thaw

Weathering • Breakdown of rocks at the Earth’s surface • Factors controlling weathering –

Weathering • Breakdown of rocks at the Earth’s surface • Factors controlling weathering – Presence of water – Gasses – Changing temperatures – Biological organisms – Pressure changes

Definitions • • Parent material is removed by erosion Transport by wind, water, glaciers

Definitions • • Parent material is removed by erosion Transport by wind, water, glaciers In-place weathering produces soil Differential weathering for different rock types

Mechanical Disintegration • • • Physically break rocks into smaller pieces Frost wedging and

Mechanical Disintegration • • • Physically break rocks into smaller pieces Frost wedging and heaving Pressure release Thermal expansion Biological organisms Salt Crystal growth

Frost Action • Water expands when it freezes • Necessary conditions – Temps that

Frost Action • Water expands when it freezes • Necessary conditions – Temps that go below freezing – Free water – Cracks for the water to seep into • Rocks spall off and form a talus slope

Figure 5. 4, p. 113

Figure 5. 4, p. 113

Pressure Release • Rocks that form under high pressure – Deep in the earth

Pressure Release • Rocks that form under high pressure – Deep in the earth – Subsequent uplift to the surface

Thermal Contraction/Expansion • Rocks expand when heated and shrink when cooled • Interior remains

Thermal Contraction/Expansion • Rocks expand when heated and shrink when cooled • Interior remains at a steady temperature • Resulting in fracturing • Important during fires • On the Moon

Biological Organisms • • • Tree roots and trunks Burrowing insects, worms, animals Earthworms

Biological Organisms • • • Tree roots and trunks Burrowing insects, worms, animals Earthworms Help to expose deeper layers to water Mix soils and circulate them to the surface

Chemical Weathering • Rainwater with carbon dioxide (CO 2) – Forms a weak acid

Chemical Weathering • Rainwater with carbon dioxide (CO 2) – Forms a weak acid • Acid attacks the rock – Results depend on what type of rock – Temperature, acidity, water, etc.

Granite and Acids • Granite in the presence of carbonic acid will weather and

Granite and Acids • Granite in the presence of carbonic acid will weather and break apart • The Iron (Fe) will rust (oxidize) – It usually remains in the soil • The Aluminum (Al), Potassium (K), and Silica (Si. O 2) turn into clays and remain in the soil

Some of the Products Wash Away • The Calcium (Ca), sodium (Na), and Magnesium

Some of the Products Wash Away • The Calcium (Ca), sodium (Na), and Magnesium (Mg) wash away – The Ca goes into the sea and is used by sea creatures to make their shells – The Na makes the sea salty – The Mg reacts with rocks at the spreading centers to make new rocks

Soil • What is left behind is soil • More heat, more water -->

Soil • What is left behind is soil • More heat, more water --> more stuff is washed away and the soil has fewer minerals in it. • Very dry areas, the Ca, Mg, and even Na remain and the soil is very salty

Badlands • Erosion from the Rockies, deposited in the great plains. • Soil is

Badlands • Erosion from the Rockies, deposited in the great plains. • Soil is very clay rich – The clays create a hard surface that channels the water, washes away plants – The clays are unstable, slopes slump down, washing away plants – Water doesn’t soak in well.

 • Bill Nye - Erosion • Magic School Bus - Erosion

• Bill Nye - Erosion • Magic School Bus - Erosion