Post on 22-Feb-2016
description
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/Mamm/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