Weathering In general, bedrock must be converted to soil/regolith before it can move downhill

Bedrock- Regolith (Ecuador)

The Rock Cycle

Weathering Breakdown of Rock near the Surface Due to Surface Processes Chemical Alteration Solution & Leaching Biological Action Hydration Mechanical Impact, unloading, thermal transients Wedging: frost, plant roots, salt crystal growth, expansion of hydrated minerals

Biomechanical effects: Roots

Mechanical displacements by roots

Large surface-parallel compression is commonly seen in crystalline bedrock: exfoliation. Stresses reflect uplift, unloading, and cooling history. Exfoliation (sheeting)

Frost-induced breakdown Large, sound boulder fractured within 20 years of deposition Why? Many boulders survive is seemingly similar environments for tens of thousands of years?

Icy Bay, Alaska Jim Roche found that weathering increased strongly with time (within 60 years) and proximity to ocean Rocks did not get saltier near ocean Why was weathering faster near ocean, when the whole area seems always wet?

Solar Weathering McFadden, L.D., Eppes, M.C., Gillespie, A.R. and Hallet, B Physical weathering in arid landscapes due to diurnal variation in the direction of solar heating. Geological Society of America Bulletin. V.117; 1-2, p

Cheung, J.B., Chen, T.S. and Thirumalai, K., 1974, Transient Thermal Stresses in a Sphere by Local Heating, Transaction of the ASME,

Temperature and resulting normal stress history at the center (core) of rock spheres of different sizes (dia. 0.05, 0.5 and 5.0 m) (positive stresses are tensile) subjected to diurnal, sinusoidal temperature variations spanning 25ºK over the entire boulder surface. Solar stresses: size effects

Thermal stresses in a boulder Distribution of the vertical component of normal stress (σ yy ) in a vertical meridional section of a 0.5 m boulder when tensile stresses (positive values) reach peak values either at the surface (cooling, right) or in the center (heating, left), respectively. Finite Element calculation by P. Mackenzie, UW

Spheroidal Weathering

Weathering Boulders: spalls & axial cracks

Weathering Rates

Differential Weathering and Erosion

Surface Area and Weathering

Surface-Volume Effects

Spheroidal Weathering and Exfoliation

What Affects Weathering Rates and Soil Type Climate Vegetation Drainage Time Parent Material Depth below ground surface

Soil Formation Young Soils Strongest Influence Is Parent Material Mature Soils Strongest Influences: Climate, Vegetation, Drainage

Soil Formation Processes Leaching from Surface K, Mg, Na Ca Si Al, Fe Accumulation beneath Surface Al, Fe in Humid Climates Ca in Arid Climates

Soil Horizons and Profiles Soil Horizons Layers in Soil Not Deposited, but Zones of Chemical Action Soil Profile Suite of Layers at a Given Locality

Principal Soil Horizons O - Organic (Humus) Often Absent A – Leaching –K, Mg, Na, Clay Removed E - Bleached Zone - Present Only in Certain Soils B – Accumulation –Absent in Young Soils –Distinct in Old Soils –Al, Fe, Clay (Moist) –Si, Ca (Arid) C - Parent Material

Weathering Forms: Inselbergs

Weathering Forms: Round Boulders & Tors

Coastal Weathering Forms

Cavernous Weathering: Baja Energetic wave activity: cobbles bounce up to 6-7 m

Diffusional Weathering Forms: Baja

Figure 1. Satellite photograph of weathering zones developed on marine terraces of differing ages (youngest profile is at the bottom of the photo near the Pacific Ocean, oldest is highest on the slope) near the city of Santa Cruz, California. The study of Maher et al. (2009) focused on Terrace 5 (red), the oldest profile at 226,000 years. Rates of weathering in the lab and in the field. How do they compare?

Figure 2: Reactive transport simulations (solid lines) of mineral profiles after 226,000 years of chemical weathering at Terrace 5, Santa Cruz. The simulations are able to match the observed profiles even while using laboratory-determined chemical weathering rates.

Strong coupling between physical and chemical weathering Physical Steady State Implies Chemical Steady State From P. Chamberlain

Parameters Affecting the Rate of Chemical Weathering Erosion Rate Depth of Weathering Zone Composition of Parent Rock & Constituent Minerals Mineral Dissolution Rates, Grain Characteristics Minerals are refreshed on a time scale given by the residence time, which is the ratio of soil thickness and erosion (& uplift) rate. Hence chemical weathering is fastest where erosion (or uplift) is fastest From P. Chamberlain

Effective surface age (or residence time) is ratio of soil thickness over erosion rate (e.g. for 1 m of soil eroding 1mm/yr, age is 1000 years). Chicken-n-egg issue: Weathering & soil formation are fast because erosion is fast, or vice versa?

Chemical Weathering Rate of Granitic Minerals From P. Chamberlain

- In general, bedrock must be converted to soil/regolith before it can move downhill - Rates of weathering are generally highest at surface - They depend on climate (temperature, moisture, vegetation) and rates of erosion (and uplift, assuming steady state). Weathering - Recap