Question: If cratering rate on Earth is same as our Moon, why are their so few craters preserved?

Answer: Erosion

Estimating natural erosion rates

• Accumulation of sediment versus time in alluvial fans.

• Ratio of cosmogenic isotopic creation rate/unit area to concentration/unit volume of sediments.

• Closure age of exposed minerals at surface plus increase of temperature with depth.

Erosion Rate for Cascades ~ 0.05 – .25 Km/Ma

mm/y = m/Ka = Km/Ma

Erosion Rate for Central France: 55 m/MA

1 t/ha/y => 30 m/Ma

1 mm/a = 1km/Ma

Summary of Erosion rates

• Himalaya: 2-3 m/Ky = 2-3 km/Ma

• Eastern edge of Bolivian Altiplano: 3 km/Ma

• Cascades: 300 m/Ma

• Massif Central (France): 50 m/Ma

• Atacama Desert (Chile/Peru): 0.5 m/Ma

• Central Australia: 0.5 m/Ma

Question

If erosion can flatten mountain topography in only a few million years, why are there

mountains at all?

Question: If erosion can flatten mountain topography in only a few million years, why are there mountains at all?

Answer: Uplift is continuous

Implication: Earth is a dynamic planet

L = cA½

Channel length and drainage area

Streamflow and drainage area…

QPA c

River basins in Kentucky, USA, from Solyom and Tucker, 2004

Q = 0.0171*A0.9932

R2 = 0.9977

Q = Fluvial Discharge P = Effective Precipitation Rate = Rainfall - lossesc = positive constant 1

Q Piaii

or

Channel Width:

W Qb

W Channel Width

Q = Fluvial Discharge PA c

b = positive constant ~ 0.5

Data from the Clearwater River, Washington State, from Tomkin et al., 2003.

Q = 0.1335 * A0.9

W=4.2*A0.42

W = b(AP)½Should actually be discharge = Area x precipitation

Spatially Variable Precipitation, Ellis, Densmore & Anderson, 1999

Pre

cip

Distance