Mechanical Weathering Processes

Transcription

1 Mechanical Weathering Processes

2 How Rock Fragmentation Influences Weathering 1. Smaller particles transport more easily. 2. Fractured materials have decreased strength, and concentrate stresses at crack tips which drives greater fracturing. 3. Water and air can penetrate, and attack, fractured rock more efficiently/effectively. 4. Fracture increases surface area for attack by chemical weathering processes.

3 Mechanical Weathering Processes

4 Hot Processes Differential Thermal Expansion Rock, like most substances, expands when heated. most minerals have a coefficient of thermal expansion of approx. 1-5 e-5 / C: A 1m-long slab of rock will lengthen by 1mm if heated by 10 C. Rock doesn t just breathe with heating and cooling. Why? 1. Non uniform heating pattern 2. Minerals vary in their coefficient of thermal expansion

5 Hot Processes Mechanism #1: Granular Disintigration This mechanism arises from mineral differences in rocks. Granites with >10% biotite break down rapidly into grus, small little granitic pebbles of feldspar, while granites with <10% biotite are quite resistant. Biotite has a very high coefficient of thermal expansion, so when the biotite crystals are heated, they flex more than the other mineral grains and stress gradients are set up. This propagates cracks. Biotite grains behave as miniature crowbars working to wedge themselves free. Plot shows differences in mafic vs. felsic portions of a boulder heated with sunlight. Note shading events (S) and wind cooling (bars).

6 Hot Processes Mechanism #2: Spallation: Fires on Alluvial Fans Brush Fires on Alluvial Fans burn hot but are of short duration. Boulders don t burn, but witness the burning of brush and trees. Heat conduction is diffusive square root of time dependence of length scale. What about spall thicknesses next to large trees?

7 Effects of Temperature Variation at a Boundary Surface temperature variation recorded in subsurface thermal profile

8 Daily Heating Patterns and Crack Orientation

9 Growth of Ice Lenses

10 Frost Cracking Window Frost cracking is most effective within the window of -3 C to -10 C. At higher temps, water is thermodynamically stable in thin films and microcracks it won t freeze. At lower temps, effective viscosity of water in the thin films increases, slowing transport to growing ice lenses.

11 Frost Cracking Experiments

12 Frost Cracking Modeling Numerical model used to explore depth over which temperatures within the optimal frost cracking window occur. There is a trade off between extreme temperature fluctuations at the surface and slow temperature oscillations at depth.

13 Bi Directional Freezing Experiments Permafrost Simulation

14 Frost Cracking and Scree Production

15 Tree Roots and Rock Fracture In forested landscapes, windy ones in particular, one might imagine that the depth to which wind stress impacts soil and rock ought to depend on the architecture of the root network.

16 Tectonic Rock Crusher Motion of a rock mass through a listric fault can result in cracking of rock as it is put through the compression of the interior of the bend rock in such settings often arrives at the surface pre fractured.

17 Sheet Jointing Why are domes so common in massive rock bodies? Recall: By "massive", we mean a rock that is little or not at all broken by joints, cracks, foliation, or bedding, tending to present a homogeneous appearance.

18 Domes Resulting from Sheet Jointing It is largely because of a surface parallel sheeting pattern which evolves to break off curved (almost spherical) fragments of the rock.

19 Mechanism of Sheet Joint Development The requirement for generating surface parallel sheeting joints is that a tensile stress must arise that is normal to a convex, traction-free surface, ant that this stress exceeds the tensile strength of the rock.