Active Tectonics And Mountains Flashcards
(33 cards)
What is tectonic and isostatic uplift
-tectonic: movement of faults and plates, movement of land from earthquakes
-Isostatic: uplift due to the removal of weight and pressure like glaciers
What is the earthquake cycle
-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
Why is it hard to predict earthquakes
-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
What are other short term earthquake predictions
-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
What is ocean continental convergence
-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.
Phases earthquake cycle
-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)
Continental-continental convergence
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.
How do earthquakes build mountains?
-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.
Surface uplift of mountain ranges:
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!
Isostatic rock uplift:
- 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!
Erosion and tectonic rock uplift
- 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!
How does topography influence weather
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.
4 different types of movement of sediment
- 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.
- Solute load in rivers: ions and dissolved components (>80% consists of four types: HCO3, SO4, Ca, SiO2). Globally ~20% of the suspended load.
- 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?)
- Aeolian transport: sand, silt, clay transported by wind
What is sediment load and yield
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.
What does seidment load and yield tell us
• 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)
Suspended sediment discharge Model:
- 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
What is the timescale of building a mountain
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.
What happens to Detritus from a mountain range:
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.
What’s the importance of Offshore sediment record:
- 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.
What do the sediment cores tell us?
- Sediment age: how old is the sediment that has been deposited in the sink (ocean basin, lake etc.)
- Sediment accumulation rates: how quickly is sediment deposited in the sink?
- Sediment size: Silt, sand, gravel etc. This can tell us about transport distance.
- 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.
What sets the morphology of a mountain landscape?
-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’ .
Ways to investigate mountain topography:
-historical maps and data
-topographic maps
-satellite imagery and data
-digital elevation models
-surveying
-Field data collection
Modelling mountain morphology
- 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.
Mountain topography: metrics
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.
