Intro to Geomorphology (Geos 450/550)Lecture 1: Controls on large-scale topography, course intro• histogram of Earth's topography• isostatic model for difference in elevation between

continents and oceans• thermo-isostatic model for the topography of mid-

ocean ridges• overview of syllabus (4 out of 5 field trips required)• field trip #1

An elevation map of the surface of Earth

An elevation map of the surface of Earth

Histogram/frequency distribution/PDF of elevations

Frequency distribution of elevation from ETOPO5

elevation (m)

# pi

xels

in E

TO

PO

5

-8000 -6000 -4000 -2000 0 2000 4000 6000 80000

5000

10000

15000

20000

25000

30000

Histogram/frequency distribution/PDF of elevations

The mountains of the oceans: mid-ocean ridges

Melting beneath mid-ocean ridges

The melt rises out of the mantle because it is buoyant (less dense than the mantle)

1300°C

LiquidusWet S

olidusD

ry Solidus

Geothermfor Old Ocean Plate

Temperature-depth plot for

mantle beneath oceanic plates

The mountains of the oceans: mid-ocean ridges

Heat flow in Earth’s crust is governed by the

diffusion equation:

k is W/(m °K) (~3), ρ is kg/m3 (~ 3x103), c is J/(kg °K) (~103)

z

q

t

Tc

z

Tkq

ρ

Fourier’s Law

conservation of energy

Shape of mid-ocean ridges (Turcotte and Schubert, 4-202):

2/1

0

0v κ

ρρ

αρ2

u

xTTw

wm

mm

This model combines diffusive heat flow, temperature-dependent density, and isostasy.

Boiling it down to the essence:

Faster spreading ridges are broader, e.g.

EPR spreading = 6 cm/yrMAR spreading = 1.5 cm.yr

Model predicts that EPR should be 4x broader.

2/1

0

u

xw

Shape of mid-ocean ridges: comparison to data

0 500 1000 1500 2000 2500 3000 3500 4000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

EPRMAR

elev

atio

n (m

)distance from ridge axis (km)

Shape of mid-ocean ridges: effects of spreading rate

0 20 40 60 80 100 120 140 1600.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

0 0.5 1 1.5 2 2.5 3 3.50

10203040506070

time (min)

win

d sp

eed

at 1

cm

(m

/s)

wind speed at 1 cm (m/s)

# of

Tectonics(uplift/subsidence)

Structure andcomposition ofrock materials

Climate(water & temperature)

geomorphology

Modern geomorphology combines:

Field measurements(soil profile descriptions)

Remote sensing(airborne and ground-basedlidar, GPR, seismic refr. etc.)

Geochemistry(cosmogenic dating,immobile element conc.)

Numerical Modeling

Field trip #1: Tortolita Mountain piedmont

Pelletier et al. (2005)

Progressive stages of calcite accumulation in soils

Coatings on underside of gravel

Fine-texturedparent material

Coarse-texturedparent material