Goals of Today s Lecture

Transcription

1 Goals of Today s Lecture 1. To answer the question: Where do landscape materials come from? 2. To examine weathering and bedrock erosion processes at Earth s surface. 3. To start answering the question: How do landscape materials get from mountain tops to valley floors? 4. To discuss different types of mass movements Where do landscape materials come from? Granite Sand Enchanted Rock State Natural Area of Texas, USA 1

2 Where do landscape materials come from? There are two processes that are important: 1) Weathering or Soil Production: In situ disintegration or breakdown of rock material 2) Bedrock Wear: Erosion of rock material by water, wind, or ice These are not mutually exclusive processes. Only where rock is covered by soil does weathering operate as the sole process. In many environments, both weathering and bedrock erosion are occurring at the same time. Enchanted Rock State Natural Area of Texas, USA dz dt = U - E - qs All landscapes must obey this fundamental statement about sediment transport! Change in landscape surface elevation (rate) Bedrock erosion rate (P+W) Sediment flux divergence (written in 3D) Photo courtesy of Bill Dietrich Uplift rate of the landscape surface The whole landscape in one equation! 2

3 dz dt = U - E - qs All landscapes must obey this fundamental statement about sediment transport! Our discussion today will focus on Sediment production by weathering (P) component of the bedrock erosion rate. Photo courtesy of Bill Dietrich The whole landscape in one equation! Weathering in the Rock Cycle 3

4 Weathering in the Source to Sink Framework Types of weathering Physical (Mechanical): 1. Pressure release 2. Freeze-thaw 3. Salt-crystal growth 1. Pressure release 2. Freeze-thaw 3. Salt-crystal growth 4. Thermal expansion 5. Biological 4. Thermal expansion 5. Biotic Disintegration of rock into smaller pieces in situ 1. Solution Chemical: 2. Hydration 3. Hydrolysis 1. Solution 2. Hydration 3. Hydrolysis 4. Oxidation 5. Biotic 4. Oxidation 5. Biological Transformation/decomposition of one minerals to another in situ Review different types in Textbook is review from GEOG 111/EASC 101 will appear on exam! 4

5 Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces This increases the total surface area exposed to weathering processes. Role of Physical Weathering 1) Reduces rock material to smaller fragments that are easier to transport 2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering 5

6 What controls rate of physical weathering? Resistance to weathering: Rock strength, composition, fracture pattern. Joints in a rock are a pathway for water they can enhance mechanical weathering. The form and density of fractures is controlled by the rock type. What controls rate of physical weathering? Driving force of weathering: 1) Exfoliation requires erosion which requires water 2) Ice crystalization requires water 3) Salt crystalization requires water 4) Biota growth requires water Physical weathering systems are controlled by the availability of water and thus are climatically controlled. More on this later! 6

7 Chemical Weathering: breakdown as a result of chemical reactions. CaCO 3 +CO 2 +H 2 O ---> Ca HCO 3 - Calcium Carbonate Limestone or marble rock Carbon dioxide Water Carbonic Acid (H 2 CO 3 ) Calcium Bicarbonate Rock that can be carried in solution! Transformation/decomposition of one mineral into another! Typical Chemical Weathering Products Olivine + H 2 CO 3 (acid) Clay 7

8 Typical Chemical Weathering Products Feldspar + H 2 CO 3 (acid) clay Calcite to. Calcite + anything Nothing solid 8

9 Typical Chemical Weathering Products + anything Quartz Quartz What controls rate of chemical weathering? Resistance to weathering is controlled by rock type. 9

10 What controls rate of chemical weathering? Water is main driving force: Dissolution Many ionic and organic compounds dissolve in water (Silica, K, Na, Mg, Ca, Cl) Hydration and Hydrolysis both require water Acid Reactions Require water Water + carbon dioxide carbonic acid Water + sulfur sulfuric acid Water + silica silica acid Why is sand so prevalent at Earth s surface? Mean Lifetime of a 1mm crystal at surface (in years) Quartz 34,000,000 Kaolinite 6,000,000 Muscovite 2,600,000 Microcline (Alk. Feldspar) 921,000 Albite (Sodium Plagioclase) 575,000 Sandine (Alk. Feldspar) 291,000 Enstatite (Pyroxene) 10,100 Diopside (Pyroxene) 6,800 Forsterite (Olivine) 2,300 Nepheline (Amphibole) 211 Anorthite (Calcium Plagioclase) 112 It is composed of quartz, a relatively stable mineral! 10

11 Linkage between climate and weathering Mechanical weathering Enhanced where there are frequent freeze-thaw cycles Bierman and Montgomery Textbook Chemical weathering Most effective in areas of warm, moist climates decaying vegetation creates acids that enhance weathering Least effective in polar regions (water is locked up as ice) and arid regions (little water) Yukon Vancouver Amazon Altiplano, Andes 11

12 How does weathering fit into our generalized continuity equation of the landscape? Bill Dietrich P>Transport Weathering is P Bedrock landscape P<Transport Capacity dz dt = U - P - q s Soil-mantled landscape Bill Dietrich dz dt = U - P - W - q s How can we predict P? Bierman and Montgomery Textbook 12

13 Soil Production Function P 0 Pe H P G. K. Gilbert, 1870 H An exponential decay in the soil production rate with soil depth for a given climate and rock type. Heimsath, et al., 1997, Nature. A practical reason to know the soil production rate Lynn Betts, USDA-NRCS USDA-NRCS Bierman and Montgomery Textbook 13

14 How do landscape materials get from mountain tops to valley floors? The processes that move materials into stream, creeks, and rivers are collectively called mass movements or mass wasting. This includes all sorts of landslides, debris flows, and rock falls. Goals of Mass Movement Lectures 1.Introduction to mass movements Impacts of mass movements Types of mass movements 3. Slope stability analysis (next week) 4. Geomorphic transport laws for mass wasting processes (next week) 14

15 Frank Slide, Turtle mountain, Alberta. Canada s Worst Natural Disaster 15

16 z t = U - E - qs All landscapes must obey this fundamental statement about sediment transport! Our discussion will focus on mass wasting processes that cause erosion and deposition at the Earth s surface. Photo courtesy of Bill Dietrich The whole landscape in one equation! Mass Movement Mass movements are important processes in all types of landscapes, in all climatic settings, and even in the ocean. 16

17 Mass Movement Simply put, mass movement will occur when the resisting forces holding rock in place are overcome by the gravitational forces. This generally happens when the resisting forces are reduced due to water pressure. We will formalize this idea mathematically when we consider how to predict when a slope will be unstable through slope stability analysis. Rates of mass movement Conceptually, mass movement can be though of as working at two levels: 1. The obvious we can see the evidence very clearly (ie: houses falling down a cliff in North Vancouver). 2. The hidden movements that of themselves are so small that they cannot be seen very easily, but over time can be significant. 17

18 The Obvious The Hidden Photo by: Joan Miquel Borce, France 18

19 Frequency and magnitude of geomorphic processes The most frequent events (hidden) do not do the greatest amount of work (not surprising) The largest events (most obvious) do the most work, but they are infrequent. Moderately sized transport events (often hidden) do the most geomorphic work in the landscape as a consequence of the frequency of moderate sized events From: Wolman, M. G. & Miller, J. P. (1960). Magnitude and frequency of forces in geomorphic processes. Journal of Geology, 68, Classification of Mass Movements FLOWS Debris Flows Earth Flows FALLS Rock Falls Rock topples HEAVES Results in creep SLIDES Slump Spread 19

20 Flows Spatially continuous movement in which surfaces of shear are short lived, closely spaced and usually not preserved. The distribution of velocities resembles that in a viscous fluid. Examples of flows: Debris flow tracks Scars formed by debris flow in greater Los Angeles during the winter of USGS 20

21 Some Cool Debris Flows llgraben, Switzerland, 28 July 2014 Badakshan District of Varduj, Afghanistan, June 2007 Debris flows typically have a point source Originate when poorly consolidated rock or soil masses are mobilized by the addition of water by: Periods of extended rainfall Localized areas of intense rainfall Ponding on surface upstream of flow Snowmelt or rain on snow Source area for debris flow near Bamfield 21

22 Debris flow track near Bamfield Debris flow track near Bamfield; looking upslope 22

23 Debris flow track near Bamfield; looking downslope Anatomy of a debris flow deposit 23

24 Anatomy of a debris flow channel Debris flow failure mechanisms Stock and Dietrich (WRR, 2003) Many debris flows originate at channel headwaters (hollows) Most debris flows originate on slope >15% But, they may also be formed by other types of initial failure upstream of the debris flow location. 24

25 Examples of flows: Earthflow Typically high viscosity flows formed from weathered volcanic rock Confluence of Muskwa and Chisca rivers, northern British Columbia. Martin Geertsema, 2002 A Cool Earth Flow 25

26 Anatomy of an Earthflow Large slow moving flows common in the western part of the interior plateau of BC Several km in length and typically composed of ~10 6 m 3 of material. Form in weathered volcanic rock that forms clay materials Often have a defined slide plane and shear surfaces Movement and rotation of blocks mean there is mixing Flows occur over several thousands of years Have velocities up to 1 m/a. Bovis, 1986 Churn Creek Earth Flow 26

27 Grinder Earth Flow Anatomy of an Earthflow Large slow moving flows common in the western part of the interior plateau of BC Several km in length and typically composed of ~10 6 m 3 of material. Form in weathered volcanic rock that forms clay materials Often have a defined slide plane and shear surfaces Movement and rotation of blocks mean there is mixing Flows occur over several thousands of years Have velocities up to 1 m/a. Pavillion Earth Flow Bovis,

28 Examples of flows: Earthflow Toe of Drynoch Earthflow along Thompson River June Ryder Falls Falls begin with the detachment of rock from a steep slope along a surface on which little or no shear displacement takes place. The material then falls or rolls through the air. Topple is a forward rotation, out of the slope, of a mass of soil or rock about a point or axis below the center of gravity of the displaced mass. Rockfall in the Talkeetna Mountains, Alaska 28

29 Talus Slopes Fraser Canyon Talus Slopes Fraser Canyon 29

30 Examples of falls: Talus cones nr. Lillooet, southwestern B.C., Canada. Angle of repose Fraser Canyon nr. Quesnel Heaves Periodic expansion and contraction of a soil or sediment mass that is usually linked to clay swelling and dewatering or freezing and thawing. Heave leads to downslope creep of hillslope materials as the strength of the materials is decreased. 30

31 Solifluction: downslope movement caused by vertical heave as soil freezes and downslope meovemtn when the soil thaws. Gelifluction: Slippage of the soil along a slide plane when it is thawed Both are simply a form of creep induced by freeze-thaw heave cycles. Examples of heave: Soil creep Downslope creep of soil at surface of vertical shale beds South of Dawson, Yukon, Canada Frank Nicholson 31

32 Ian Alexander Slides Downslope movement of soil or a rock mass occurring dominantly along a surface of rupture or relatively thin zones of intense shear. A) Pure slide (translational) B) Rotational slide 32

33 Deep-seated landslide: Hope Slide, BC Examples of slides: shallow-seated landslide, Briones Regional Park, CA 33

34 Examples of slides: shallow-seated rotational landslide, Marin Headlands, CA Examples of slides: deep-seated landslide, Keetmanshoop, Southern Namibia Carmen Krapf 34

35 Examples of slides: Deep-seated rotational landslide La Conchita slump. March 4, 1995 Santa Barbara, California. Christiane Mainzer Examples of slides: Deep-seated rotational landslide, La Conchita Ann Dittmer 35

36 Classification of Mass Movements Goals of Mass Movement Lectures 1.Introduction to mass movements Impacts of mass movements Types of mass movements 3. Slope stability analysis (next week) 4. Geomorphic transport laws for mass wasting processes (next week) 40