Active Tectonics And Mountains

Question 1

What is tectonic and isostatic uplift

Answer 1

-tectonic: movement of faults and plates, movement of land from earthquakes
-Isostatic: uplift due to the removal of weight and pressure like glaciers

Question 2

What is the earthquake cycle

Answer 2

-frictional properties of the earths brittle upper crust gives rise to stick-slip behavior as the sides of teh fault are loaded by relative plate motion
-the inter seismic phase is the time between earthquakes as the stress builds up
-the coseismic phase includes the earthquake event

Question 3

Why is it hard to predict earthquakes

Answer 3

-the local rock strength is neither constant nor uniform along a fault
-the date at which stress accumulated is not constant
-each earthquake affects the faults nearby

Question 4

What are other short term earthquake predictions

Answer 4

-periodic model: same size earthquake occurs after the same amount of stress has accumulated. Earthquakes at a constant interval
-time predictable model: strain required for earthquake is constant but the size of the earthquake can change
-slip predictable model: strain to cause earthquake varies but the earthquake always releases enough energy to return to the same state, the longer since the last earthquake the larger the earthquake

Question 5

What is ocean continental convergence

Answer 5

-the continental crust wraps as stress builds up along fault, earthquake release teh stress and the crust pings back
-interseismic phase: fault is locked, upper plate bends leads to subsidence close to fault and uplift farther inland
-coseismic phase: fault slips continental plate relaxed and pattern of uplift and subsidence at any one point near the fault.

Question 6

Phases earthquake cycle

Answer 6

-pre seismic: mostly elastic (recoverable) strain accumulation, no fault movement
-coseismic: rapid strain release in an earthquake (seconds/hrs)
-post seismic: relaxation and more rapid strain accumulation, but decaying with time (years/hrs)

Question 7

Continental-continental convergence

Answer 7

Thicker crust means that faults can be much deeper than oceanic crust and therefore behave differently (more ductile!).
Patterns of uplift vary in space and time. Size of earthquake affects how much mountain building is achieved.

Question 8

How do earthquakes build mountains?

Answer 8

-The 1983 Borah Peak earthquake caused a break/scarp which was 2-3 m high. The pattern of slip shows that the mountains have uplifted in this way over time by earthquake events.
- Mountains are built by repeated (relatively) small earthquakes – takes a long time!
- A large fault scarp was instantly created in the 2016 Kaikoura earthquake, New Zealand.
-Mountain belts are composed of multiple faults and folds. Once we know how a single fault grows and accumulates displacement, we can understand the dynamics of the entire range.

Question 9

Surface uplift of mountain ranges:

Answer 9

Tectonic rock uplift (caused by movement on faults) is not the same as surface uplift (which builds mountain ranges), because we must also allow for erosion (often called denudation).
If denudation is faster than rock uplift, we have negative surface uplift (i.e., the mountain elevation decreases), even if there is tectonic activity!

Question 10

Isostatic rock uplift:

Answer 10

• Continental crust ‘floats’ at the surface of the denser mantle. Adding mass to the crust (e.g., mountains, ice sheet) causes the crust to sink. Removing mass causes the crust to rebound or rise.
  -Denudation (erosion) of mountains can therefore lead to isostatic rock uplift/rebound (~80% of total denudation). Net surface lowering after denudation is smaller than you might expect.
• If erosion is non-uniform (e.g., valleys erode faster than peaks), then isostatic rock uplift can cause the peaks to rise higher than their original elevation – even without tectonic activity!

Question 11

Erosion and tectonic rock uplift

Answer 11

• Erosion can change the tectonic processes – by removing material from the mountain range and creating space for more.
• Tectonics causes the rocks (‘squares’) to deform and move to the surface. Through erosion, the rocks are removed from the mountain range.
  -The direction of precipitation delivery matters for how a mountain range evolves. Topography also influences precipitation!

Question 12

How does topography influence weather

Answer 12

the topography influences atmospheric circulation and thus the pattern of precipitation.
E.g., South Asian Monsoon
- The Tibetan Plateau helps to determine the strength of the monsoon.
Summer: - Hot air rises over Tibetan Plateau, forms low pressure; draws in moist air from Arabian Sea/Bay of Bengal. Rain in N India
Winter: - Cold, dry air over Tibetan Plateau forms high, blocks moist air masses to south. Dry in N India
Uplift of the Tibetan Plateau has had a big effect on regional and global climate over time.

Question 13

4 different types of movement of sediment

Answer 13

1. Suspended load in rivers: fine particles (silt, clay) that are transported in the river flow and slowly settle out in the oceanic water column. Easy to measure.
2. Solute load in rivers: ions and dissolved components (>80% consists of four types: HCO3, SO4, Ca, SiO2). Globally ~20% of the suspended load.
3. Bed load in rivers: coarse particles (sand-boulders) that are transported along the riverbed. Very hard to measure; ~10-50% of suspended load? (10% or less for large rivers?)
4. Aeolian transport: sand, silt, clay transported by wind

Question 14

What is sediment load and yield

Answer 14

Sediment load (or discharge) is the mass of sediment leaving a catchment per unit time (measured from river samples, lake basins, or accumulation in deltas)
-Sediment yield is the load, divided by the catchment area A. It provides a way of comparing basins of different sizes.

Question 15

What does seidment load and yield tell us

Answer 15

• Rates of erosion in the catchment
• Rates of sediment transfer (e.g. reservoir infilling)
• Baseline for understanding effects of climate change
• Baseline for understanding anthropogenic effects (e.g. agriculture)

Question 16

Suspended sediment discharge Model:

Answer 16

• Pelletier (2012) proposed a global model of suspended-sediment discharge (yield).
• Yield correlates most strongly with topographic slope (steep slopes, high yield)
• Rainfall and vegetation are also important, but don’t have the same range of variation as slope.
  E.g., Amazon Basin
• Sediment sources are in the basin headwaters (Andes)
• Sediment yields decrease progressively as you move downstream – little source, and increasing storage

Question 17

What is the timescale of building a mountain

Answer 17

Mountain ranges are built over geological timescales (millions of years) – but these measurements of sediment transport/erosion are made over human timescales
- Suspended sediment concentrations after the M7.9 Wenchuan earthquake increased by 3 – 7 times! This makes it difficult to know if our samples are representative of long-term rates.
-Modern measurements provide a snapshot of the current processes acting upon the Earth’s surface.

Question 18

What happens to Detritus from a mountain range:

Answer 18

Mountain building processes (inc. erosion) generate huge volumes of sediment that have to go somewhere.
- Sediment is transported by rivers from the mountains to the ocean. Sediment is thickest around continental shelves and near mountain ranges.
- Thick accumulations of sediment can be found in line with past and present glacier/ice sheet limits. These sediments can tell us about glacial processes and dynamics.

Question 19

What’s the importance of Offshore sediment record:

Answer 19

• Ocean sediment cores provide a valuable record of how the environment has changed over time – the chemical changes of the ocean (recorded by the sediment) help to reflect the evolution of the earth system.

Question 20

What do the sediment cores tell us?

Answer 20

1. Sediment age: how old is the sediment that has been deposited in the sink (ocean basin, lake etc.)
2. Sediment accumulation rates: how quickly is sediment deposited in the sink?
3. Sediment size: Silt, sand, gravel etc. This can tell us about transport distance.
4. Sediment provenance and geochemical composition: This tells us about the source area.
   - Mountain building is driven by climate and tectonics. Signatures of climate and tectonics can be found in the sediment record.
   - Tectonics: We might expect that, as we uplift mountain ranges, we see higher sediment accumulation rates.
   - Climate: We might expect higher sediment accumulation rates as glaciers grow throughout the Quaternary.

Question 21

What sets the morphology of a mountain landscape?

Answer 21

-Hillslopes: ‘a solid surface that lies at an angle to the horizontal’; ‘side or slope of a hill’; ‘surface which connects a ridgeline to its nearest stream’; surface which delivers water and sediment to the fluvial network’
-channels: ‘’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’ .

Question 22

Ways to investigate mountain topography:

Answer 22

-historical maps and data
-topographic maps
-satellite imagery and data
-digital elevation models
-surveying
-Field data collection

Question 23

Modelling mountain morphology

Answer 23

• One way to study sediment routing systems is to use model landscapes – ‘mountain in a box’.
• At set time intervals, telemetric lasers will measure the changing elevation of the sand in the box.
• Model experiments can show us the effects of varying different parameters – for example, increase rock uplift rates lead to steeper, higher and rougher topography.

Question 24

Mountain topography: metrics

Answer 24

At the most basic level we can divide the landscape into channels (which concentrate flow) and hillslopes (everything else). Each is dominated by a particular set of geomorphic processes.
- Hillslopes have low upslope contributing areas and a range of gradients (flat to steep).
- Channels start at a given contributing area, and their gradients decrease downstream.

Question 25

What happens to the drainage basin the further down the valley

Answer 25

Location 1: Hillslope – no drainage area
Location 2: Hillslope – small drainage area
Location 3: Channel – medium drainage area
Location 4: Channel – large drainage area
Key concept: As you move through the valley, your drainage area gets bigger.

Question 26

How does the slope change the further down the valley

Answer 26

On the hillslope at the headwaters (top of valley, near the ridgeline), the elevations drop rapidly (steep slopes). The slope of the hillslope is nearly uniform.
- In the river channel downstream, the elevation drops more gradually – low and always decreasing slope.
- With increasing drainage area, hillslope gradient increases/remains high (moving from ridgeline/plateau downslope).
- With increasing drainage area, channel gradient (river steepness) decreases.

Question 27

What is sediment routing

Answer 27

• The sediment routing system or network describes the pathways that sediment travels from the mountain peaks to the river outlet/ocean basin. The processes of transport vary through a sediment grain’s journey.

Question 28

Hill slope evolution

Answer 28

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.
- Hillslope evolution is controlled by the thickness of the soil and the rate at which it is produced by weathering of bedrock.
-When soil production is rapid, the landscape is covered or ‘mantled’ by soil – smooth, rounded hillslopes.
-The rate of sediment (soil) transport is dependent on slope and happens grain-by-grain (diffusive sediment transport).
- In soil mantled landscapes, sediment transport is often influenced by biogenic processes.
- In some landscapes, gophers (animal) might be the main cause of sediment transport!

Question 29

What happens with debris flow in a channel

Answer 29

Many steep channels (slopes >10%, or 4-6˚) 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.
- If sediment can’t be transported within the channel, then the bed builds up and the channel aggrades.
- Without debris flows, sediment would be trapped on hillslopes (at least until the next large landslide) instead of being delivered to the channel network.
-Debris flows are a way of the hillslopes ‘handing off’ sediment from hillslopes to larger channels

Question 30

What are fluvial channels:

Answer 30

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).
- Sediment transport rates in channels are orders of magnitude greater than those on hillslopes.
- The geomorphic processes in the channel network depend on the gradient of the river, the distance from the divide/ridgeline, the presence/absence of sediment and the nature of the sediment.
Many mountain rivers show general downstream trends of:
1) Decreasing slope
2) Decreasing exposure of bedrock
3) Increasing sediment cover
4) Decreasing grain size

Question 31

What is Uniformitarianism

Answer 31

• Uniformitarianism suggests that events we have experienced are representative of Earth History.
• We can take modern observations (and knowledge surrounding this) and infer something about past processes, conditions, events etc.
  -But is this the only way to look at Earth processes? There are some large events – where there is no recent or historical analog – which have had a profound impact on the evolution of the earth system.
• If these large events (for which there is no modern analogue) are important in Earth history, then we might develop an alternative view – call it neocatastrophism

Question 32

Predicting earthquake frequency-magnitude

Answer 32

• Frequency-magnitude plots using a y-axis log scale suggest that as we increase earthquake magnitude, the number of events decreases – following a straight line (equation of a line is y = n +mx)
• We can use the equation of our trend line to predict the number of earthquakes of each magnitude
• This relationship is known as the Gutenberg-Richter law (mid-20th century). Can be defined for global, region or local datasets and tells you the expected number of earthquakes of a given size.

Question 33

Basic ideas around earthquakes

Answer 33

1. To understand the impact of large events, we need to know their distribution and their effectiveness
2. Earthquake frequency decreases with increasing magnitude; an increase in magnitude by 1 unit means a decrease in frequency by 10x
3. This magnitude-frequency relationship (known as the Gutenberg-Richter law) gives us a way of forecasting the likelihood of earthquakes of any given size