Mountains Lecture 5- Deciphering the fingerprints: how erosional processes shape mountain range form Flashcards
What is a hillslope?
‘A soil surface that lies at an angle to the horizontal’
‘Side or slope of a hill’
‘Surface which connects a ridgeline to it’s nearest stream’
‘Surface which delivers water and sediment to the fluvial network’
What is a channel?
‘Where flow is concentrated’
‘A landform where water and/or sediment converges and moves downslope or through a valley’
‘Where a narrow body of water is situated’
Name 6 ways moutain landscape morphology can be investigated.
Historical maps and data
Topographic maps
Satellite imagery and data
DEM
Surveying
Field data collection
What is one way to study sediment routing systems?
Model landscapes- ‘mountain in a box’
At set time intervals, telemetric lasers will measure the changing elevation of the sand in the box.
E.g. Badlands in Southern Spain has been modelled.
What can model experiments show us?
The effects of varying different parameters.
For example, increase rock uplift rates lead to steeper, higher and rougher topography.
What can we divide mountainous landscapes into?
Channels and hillslopes.
Describe hillslopes.
Have low upslope contributing areas and a range of gradients
Describe channels
Start at a given contributing area and their gradients decrease downstream
What happens to drainage areas as you move through a valley?
It gets bigger.
How can we analyse slope change with distance downstream?
Using longitudinal profiles of a river, which follows a drop of water (or sediment grain) from the drainage dicide to the river mouth.
What happens on the hilltop (mountain topography)?
At the headwaters (top of valley, near the ridgeline), the elevations drop rapidly.
The slope of the hillslope is nearly uniform.
What happens in the river channel (mountainous landscapes)?
Downstream, the elevation drops more gradually - low and always decreasing slope.
Do channels and hillslopes show different patterns in slope?
Yes
What happens to channel gradient with increasing drainage area?
Decreases
What happens to hillslope gradient with increasing drainage area?
Increases/remains high (moving from ridgeline downslope)
What does the sediment routing system describe?
The pathways that sediment travels from the mountain peaks to the river/ocean basin.
The processes of transport vary through a sediment grain’s journey.
What processes move sediment change through hillslopes?
- Sediment production and diffusive sediment transport
- Landslides
What processes move sediment change through channels?
- Debris flows
- Fluvial flows
What is hillslope evolution controlled by?
Thickness of the soil and the rate at which it is produced by weathering bedrock.
This typically happens at 10s to 100s microns per year.
How does sediment production and diffusive transport work?
Rock interacts with water, air and life in the near surface. Physical processes cause rock to fracture and fragment. Chemical processes cause chemical alteration to rock.
What happens when soil production is rapid?
The landscape is covered or ‘mantled’ by soil - smooth, round hillslopes.
What is the rate of sediment (soil) transport dependent on?
Slope and happens grain-by-grain (diffusive sediment transport).
In soil mantles landscapes, what is sediment transport often influenced by?
Biogenic processes.
What may be the main cause of sediment transport in some landscapes?
Gophers.
What is a landslide?
A landslide is the movements of masses of
rock, earth or debris down a slope under the effect of gravity (Cruden & Varnes, 1996)
Are there different types of landslides and what do they depend on?
There are different types of landslides depending on the material they move
(rock, debris, mud) and the type of failure
mechanism (e.g. slide, fall, flow).
How do landslides work?
As hillslopes get steeper, loose sediment is transported in discrete shallow landslides rather than grain-by-grain. This produced hillslopes that are much more planar.
What are shallow landslides very efficient at?
Removing material from the hillslope - the transport rate (sediment flux) increases very quickly with increasing hillslope gradient.
What is the threshold slope called?
The angle of repose.
What does shallow landsliding require?
A loose layer of soil at the surface.
If the soil and other loose material (regolith) is removed (by landsliding), we are left with a bedrock landscape.
What do bedrock hillslopes look like?
May have a thin or discontibuous regolith cover, but not a smooth, rounded landscape.
How do bedrock hillslopes evolve?
Via deep-seated/bedrock landslides, which can involve entire hillslopes. Produces planar (no rounded) hillslopes.
What do stronger rocks or less landsliding lead to in bedrock hillslopes?
high relief and steeper gradients.
E.g. Karakoram Highway in Himalayas.
Are soil-mantles, high relief landscapes rare or common in mountain settings?
Rare, hence why we see lots of steep, bedrock mountain peaks.
What is threshold slope?
When the slope gets very steep, particles start to slide downslope. This transport gets very
fast as the slope approaches a critical value.
What are debris flows?
Many steep channels (slopes > 10%) in mountainous regions are dominated by debris flows.
Mixtures of sediment and water that behave as a slurry or semisolid mass, rather than as a simple fluid.
What happens if sediment cannot be transported within a channel (debris flow)?
The bed builds up and the channel aggrades.
What would happen without debris flows?
Sediment would be trapped on hillslopes (at least until the next large landslide) instead of being delivered to the channel network.
What are debris flows a way of?
Hillslopes ‘handing off’ sediment from hillslopes to larger channels.
E.g. 2008 Wenchuan earthquake hazard cascade.
What are characteristics of debris flows?
Debris flows are a significant hazard for mountain communities due to their ability to rapidly transport large boulder.
What are fluvial channels?
Channels which transport sediment and incise bedrock through the action of flowing water.
Much more efficient at transporting water than hillslopes (once there is water in the channel).
Are sediment transport rates in channels orders of magnitudes greater than those on hillslopes?
Yes
What do geomorphic processes in the channel network depend on?
Gradient of the river
Distance from the divide/ridgeline
Presence/absence of sediment
Nature of the sediment
What general downstream trends do many mountain rivers show?
- decreasing slope
- decreasing exposure of bedrock
- increasing sediment cover
- decreasing grain size
What is topographic relief?
The difference in elevation between the highest and lowest point of a valley, landscape or mountain range.
As we transition from the hillslope to the river channels, how does the relationship between slope and drainage area change?
Move down the hillslope (away from the ridgeline), the slope increases with drainage area. We move into the river channel, where the slope then starts to decrease with increasing drainage area.
What is the ‘angle of response’?
The critical hillslope gradient for landsliding to occur.
What are debris flows made of?
Sediment and water that behave as a slurry or semi-solid mass.
How does grain size of sediment generally change with distance downstream in mountain environments?
Decreases.
