Intro to Geomorphology: Lecture 12
12/7/2015
Final Examination
• Final exam is to be held on Wedneday, December 9,
2014, 15:30-18:30 in AQ 3153 (Check for changes!).
• Exam will cover lectures, readings, laboratory
material.
• Sections: Multiple choice and one word answers, short
answer (some choice), 1 long answer question.
• Multiple choice questions and short answers are all on
Lectures 8-13. The long answer includes some
material from the mass wasting lectures.
1
Goals of Today’s Lecture: Course Review
1. To review the concept of the balance
between driving and resisting forces in the
landscape.
2. How has the emergence of life on Earth
affected its topography?
3. Course Evaluation
2
1
Intro to Geomorphology: Lecture 12
12/7/2015
The Basics of Geomorphology
Critical concepts:
1. A delicate balance (equilibrium) exists between geomorphic
processes and the landforms that they develop.
2. The perceived balance is created by the forces that drive landform
change and the resistance to change.
3. Changes in the driving forces or resisting forces can push a system
beyond defined limits (threshold), resulting in changes in landform.
Driving forces:
Resisting framework:
1. Climate –solar radiation drives the
climate system including surface
heating, precipitation, & wind.
1. Lithology – determines both erodibility
and the stable products of the
weathering process
2. Structure – faults, crustal warps, folds
etc. often have a first order control on
surface morphology
3. Internally generated resistance – the
mass of particles, bedforms in rivers,
vegetation in air and water flows, root
cohesion, interparticle cohesion, etc.
2. Gravity – gravitational force (Fg = mg) is
the force driving water movement and
drawing landscape materials to lower
elevation.
3. Internal Heat – drives the tectonic
system.
3
What controls rate of weathering?
Resistance to weathering: Rock strength and composition.
Driving force:
1) Exfoliation requires erosion which requires  water
2) Ice crystalization requires  water
3) Salt crystalization requires  water
4) Biota growth requires  water
5) All chemical weathering reactions require water
Weathering systems are controlled by the availability of
water and thus are climatically controlled.
4
2
Intro to Geomorphology: Lecture 12
12/7/2015
What controls downslope mass
movements?
Slope movement occurs when gravitational forces exceed
the frictional resistance of the material resting on the
slope.
Whether material moves depends upon:
1. Magnitude of the driving force: caused by mass &
density of the material involved and the slope angle
2. Material properties: controls frictional resistance and
response to imposed stress
5
Driving forces on a hillslope: Shear
Stress
   b g h sin 
τ = shear stress (force applied
over an area)
ρb = soil density
g = gravitational acceleration
h = soil depth

6
3
Intro to Geomorphology: Lecture 12
12/7/2015
Resisting forces on a hillslope:
Coulomb Equation
S  c  (  u ) tan 
S = Soil strength
c = soil cohesion
σ = normal force per unit
area
u = pore water pressure

tan Φ = the angle of internal
friction (resistance to
internal deformation of
hillslope material)
7
What controls flow and sediment movement in rivers?
• River channels are stable features in the landscape because of a balance
between the downstream driving force (gravity) which causes flow and
resistance to flow
• This resistance to down-slope flow is communicated to the fluid via the
generation of turbulent flow eddies at the boundary
Resistance
Elements
• Turbulent eddies extract energy from the mean flow and convert it to turbulent
energy maintaining the balance
8
4
Intro to Geomorphology: Lecture 12
12/7/2015
Driving forces in a river: Shear Stress

Fg
A
  g h tan 
τ = shear stress (force applied
over an area)
ρ = water density
g = gravitational acceleration
h = water depth
Fd
l
w

d
Fg
9
Resisting force =
frictional (drag) force
Step
Pool
Dunes
& Bars
Ripples
10
Gravel clusters and cells
5
Intro to Geomorphology: Lecture 12
12/7/2015
What controls ice movement and erosion?
Driving force: Fg
At any level in a glacier:
  ice gz sin 
If z is the full depth of ice:
 b   ice gh sin 
 = shear stress
b = basal shear stress
ρice = density of ice
g = gravitational acceleration
z = depth of the ice below surface
h = depth of the ice
θ
11
Resistance to flow is controlled by
Rheology: Physical Characteristics of Materials
1. The same stress always produces the same
strain (deformation)
Most earth materials exhibit mixed behavior. Yield
stress is sometimes referred to as the plastic limit and
the breaking stress is sometimes referred to as liquid
limit (Atterberg limits).
2. Sustaining a stress produces a constant strain
(deformation)
Yield
Stress
Plastic behavior is when a material can no longer
recover from a stress
Stress
3. Removing the stress always results in recovery
Breaking
Stress
Viscous behavior is when a material is fluid and
flows in response to stress. Newtonian fluids
flow at rates proportional to applied stresses
(i.e. water) while non-Newtonian fluids do not
(i.e. ketchup)
Strain
Failure
Elastic behavior follows these general rules:
12
6
Intro to Geomorphology: Lecture 12
12/7/2015
Resistance to ice
flow is also
controlled by:
Drag force at the
boundary
• Glaciers may freeze to their
boundary (cold-based) or
may be lubricated by water
flow (warm-based).
• Topography exerts a first
order control on ice flow.
• More complex boundaries
will retard ice flow more
than ‘smoother’ boundaries
because ice deformation
needs to be greater over
complex boundaries
13
Topography and Life
14
7
Intro to Geomorphology: Lecture 12
12/7/2015
Consider how some of the
processes we discussed this
semester are influenced by life.
Wind blown ripples in sand
15
Perturbation by organic detritus
16
8
Intro to Geomorphology: Lecture 12
12/7/2015
Plants force mounding and distortion of surface 17
Dune form prevails over local plant disturbance
18
9
Intro to Geomorphology: Lecture 12
12/7/2015
How does life impact topography?
19
http://www.ngdc.noaa.gov/mgg/image/globalimages.html
Has the emergence of life on this planet
affected the large scale topographic
organization of the earth?
20
10
Intro to Geomorphology: Lecture 12
12/7/2015
Which one is from an abiotic planet?
21
Conservation of mass equation
Dietrich and Perron begin to answer this question by
looking at the mass continuity equation for landscapes:
surface
elevation
change
through
time
=
uplift
rate
-
∆
∂z
=U-E∂t
· qs
incision
rates into
bedrock
-
spatial
gradient in
sediment
transport
rate
22
11
Intro to Geomorphology: Lecture 12
12/7/2015
23
So, life affects many geomorphic
processes.
But what would a world without life
look like?
24
12
Intro to Geomorphology: Lecture 12
12/7/2015
Hypothesis 1: Abiotic Earth would have little or no soil.
Lack of vegetation would
allow soil to be stripped from
the landscape and poorly
weathered rocky slopes
would remain.
25
On a steep, soil mantled hillslope
Soil
Broken
roots
Rotten
bedrock
Soil formation is predominantly a biogenic
processes
26
13
Intro to Geomorphology: Lecture 12
12/7/2015
We have these kinds
of landscapes on
Earth now.
Atacama Desert, Chile
1mm/year
precipitation
Justine
Owen and Ron
Amundson, 2004
27
28
1mm/year precipitation
14
Intro to Geomorphology: Lecture 12
12/7/2015
Hypothesis 2: Abiotic Earth would have more rocky channels.
Mt Hood, OR near Ramona Falls
If you took the NA Coastal mountain ranges and eliminated all
29
life for 1 million years, what would it look like?
A few hundred years later!
30
15
Intro to Geomorphology: Lecture 12
12/7/2015
Relation between slope and grain-size
• As a river cuts down through bedrock it receives
sediment from adjacent hillslopes.
The larger the rocks a river carries, the steeper
the slope
Slide Modified from original by Bill Dietrich
• Feedback processes over time cause the channel slope
to adjust to convey the sediment received.
31
Biotic Earth
Mountain profile
Channel profile
Slide Modified from original by Bill Dietrich
Abiotic Earth
Larger rocks lead to
steeper channels and
higher mountains 32
16
Intro to Geomorphology: Lecture 12
12/7/2015
The abundant coarse sediments on
hillslopes would be delivered to channels,
which would steepen the long profile of
rivers.
But we have these
kinds of landscapes
on Earth now.
33
Hypothesis 3: Abiotic Earth would have few meandering channels.
Meandering River
(high bank strength)
Braided River
34
(low bank strength)
17
Intro to Geomorphology: Lecture 12
12/7/2015
Vegetation or
weathering of
bedrock to produce
clay and fine silt
(which is deposited
to form strong banks)
is necessary to
produce large
meandering rivers
with floodplains.
1 km
Fly River, Papua New Guinea
35
Vegetation can transform a braided
channel to meandering
Michal Tal and Chris Paola, 2004
36
18
Intro to Geomorphology: Lecture 12
12/7/2015
But we have these
kinds of landscapes
on Earth now.
Quill Creek, YT
37
Is there a topographic signature to
life on earth?
If life was the only thing missing (i.e. the ocean, atmosphere,
lithosphere coupling was exactly the same):
1) Rough, bedrock-dominated landscapes would dominate.
2) Channels would be rocky, steeper and mountains would tend to
be higher.
3) Meandering channels with floodplains would be rare.
38
19
Intro to Geomorphology: Lecture 12
12/7/2015
Frequency of occurrence
Would abiotic earth look different
from our biotic earth?
abiotic
biotic
Topographic features
On an abiotic Earth, we would not see landscapes outside our
39
current experience
But, if there was no life, the coupling between the
atmosphere, oceans and land surface would change.
“Desert World” model results by Kleidon et al. (2000)
Removing vegetation would fundamentally alter precipitation patterns!40
20
Intro to Geomorphology: Lecture 12
12/7/2015
Hypothesis 4. Abiotic Earth would have different
rainfall patterns which would change the height,
width and symmetry of mountains
Tectonic forces would
persist, but erosion
would cease!
41
5) Abiotic Earth would
have no ice?
Slide Modified from original by Bill Dietrich
The emergence of life
reduced greenhouse
gases, which may have
allowed the first cooling of
the earth and glaciation
42
21
Intro to Geomorphology: Lecture 12
12/7/2015
What if life had never emerged on Earth?
1.00
Early Earth experienced
reduced solar radiation, but
liquid water was present by
3.5 billion years ago.
0.90
0.80
0.70
4.0
From Boering, 2004
3.0
2.0
1.0
Time [Ga bp]
0.0
Slide Modified from original by Bill Dietrich
Luminosity \ L
The Faint Young Sun Paradox
Elevated greenhouse gases
(methane and CO2) kept the
Earth from freezing
(“snowball earth”).
Growth of continents (causing accelerated chemical weathering and
reduction of atmospheric CO2) and the biologically-driven consumption
of C02 and rise in 02 (reducing atmospheric methane) probably led to
the first glaciation about 2.3 billion years ago.
(e.g. Kump et al, 2000; Wiechert, 2002; Kasting and Siefert, 2003)
43
Heights of mountains are limited by development of glaciers
“ice buzz saw”
No ice
glacier
44
22
Intro to Geomorphology: Lecture 12
12/7/2015
6) Abiotic Earth remains methane-rich long enough to lose
its hydrogen and become dry
Without water, plate tectonics would not occur. The Earth would be
more like Venus or Mars flat.
Venus
Early Earth
45
Is there a topographic signature to
life on earth?
If the coupling between Earth’s atmosphere, oceans and
lithosphere changed, the Earth’s surface could be very
different without life.
4) In an abiotic world, mountain ranges and, perhaps, large scale
tectonic evolution would differ significantly.
5) Although not unique to Earth, are glaciers the result of life on
Earth? Is glacial topography the strongest signature of life on
Earth?
6) If water is completely lost, no plate tectonics would occur and the
world would look very different.
46
23