Tearing Down Mountains I: Weathering, Mass Movement, & Landslides

1. Tearing Down Mountains I:Weathering, Mass Movement, & Landslides • Presented by Dr. Sridhar Anandakrishnan • The Pennsylvania State University

2. Erosion • Erosion • Weathering • Transportation • Deposition on the Earth’s surface • Landscapes created and destroyed • Involves atmosphere, water, gravity

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

4. Outline • Mass wasting • Tetons National Park • Soils • Badlands National Park • Rain • Redwoods National Park

5. Selected National Parks Badlands Tetons Redwoods

6. Mass Wasting • Masses of debris or bedrock moving downhill • Landslides and slower movements • Driven by gravity

8. Classification of Mass Wasting • Rate of movement • Soil creep to slumps to mudflows • Extremely slow (~1mm/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

10. Controlling Factors • Most important: Water • Climatic controls • Ice • Vegetation

11. 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)

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

20. 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

21. Most Common Mass Wasting Freeze Thaw • Soil creep • 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.

25. Outline • Mass wasting • Tetons National Park • Soils • Badlands National Park • Rain • Redwoods National Park

26. Selected National Parks Badlands Tetons Redwoods

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

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

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

30. 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

32. Figure 5.4, p. 113

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

36. 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

37. 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

39. Chemical Weathering • Rainwater with carbon dioxide (CO2) • Forms a weak acid • Acid attacks the rock • Results depend on what type of rock • Temperature, acidity, water, etc.

40. 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 (SiO2) turn into clays and remain in the soil

41. 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

42. 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

43. 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.

44. Wind and Weather

45. Wind and Weather • Hot @ equator, cold @ N/S Pole • Because the sun shines straight down @ equator • Curve of earth makes sunlight spread out @ Poles • Hot air rises @ equator, forms convection cell • Cools off in upper atmosphere, is shoved N/S • Comes down in the mid-latitudes • Returns to Equator

47. World’s Deserts Image courtesy USGS

48. Rain Shadow Desert • Death Valley dry and hot because it is in the rain shadow of the Sierras. • Warm, wet air rises from the CA coast and cools. • Cool air holds less water, so it rains. • Water vapor to rain releases heat • opposite of the way evaporation of sweat takes in heat...

49. Rain Shadow Desert (2) • So, the air that gets to the top of theSierras is dry and “warm” • It only cools by 0.6C/100m because of the rainfall • If it had been dry to begin with, it would have cooled by 1C/100m. • When that dry, “warm” air goes down the other side it warms by 1C/100m • Death Valley is HOT AND DRY