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

Weathering

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

Bedrock-Regolith

(Ecuador)

The Rock Cycle

The Rock Cycle

WeatheringBreakdown of Rock near the Surface Due to

Surface Processes Chemical Alteration • Solution & Leaching • Biological Action • HydrationMechanical • 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. 2005. 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. 161-173.

Cheung, J.B., Chen, T.S. and Thirumalai, K., 1974,

Transient Thermal Stresses in a Sphere by Local Heating,

Transaction of the ASME, 930-934.

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 (σyy) 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

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 ProcessesLeaching 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.http://www.typepad.com/services/trackback/6a0133f32df47b970b0133f35829bb970b

Alpine Fault

Rapid Uplift~10 mm/yr

Slow Uplift~1 mm/yr

NW SE

Indo-AustralianPlate

Pacific Plate

SouthernAlps

Strong coupling between physical and chemical weatheringPhysical 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

WeatheringZoneDepth

ParentRock

Mineralogy

UpliftRate

Base CationRelease

WeatheringProcesses

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

0

2000

4000

6000

8000

100 1000 104

105

106

107

108

12000

10000

Effective Surface Age (yr)

Si Flux (mol/(haXyr)

Collisional Mountain Belts

Pacific Rim

Unglaciated Stable Craton

Inactive Mountain Belts

N = 103

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

100 1000 10,000 100,000 1,000,000

TotalOligoclaseBiotiteOrthoclase

Effective Surface Age (yr)

CollisionalMountain

Belt

TemperateCraton

TropicalCraton

Chemical

Wethering Rate

(mol/(m s))

3

2.0E-07

1.5E-07

1.0E-07

5.0E-08

1.0E-08

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