Rocks and Weathering Flashcards
What is weathering?
Breakdown of rocks, occurs insitu Mostly occurs due to elements of the weather (rain water and temperature)
What is erosion?
Movement to wear rocks down. Occurs by flowing water, waves in the sea, wind, ice, animals burrowing and people’s feet
What is chemical weathering?
Rocks decomposing through chemical reactions into different substances
Example of chemical weathering?
Granite chemically weathers to kaolin
What is biochemical weathering?
Chemical breakdown of rocks caused by plants or animals
What is physical weathering?
When rocks disintegrate to produce smaller often angular fragments of the same rock
Example of physical weathering?
Limestone into screw
What is biophysical weathering?
Breakdown (physically) of rocks caused by plants or animals
Describe a collision zone?
No subduction Two continental plates converge Crust squeezed and squashed upwards and downwards Earthquakes (as land rises) No volcanos (no gap for lava to rise)
What is a turbidity current?
Currents that move material to bottom of trench (due to gravity)
What is a turbidite?
Material amassed at the bottom of a turbidity current
What is stoping?
When the crust burns it’s way through crust and erupts
How does an accretionary wedge form?
Oceanic plate descends, part of the continental plate is scrapped off, filling the trench
Define oceanic plate?
Average thickness of 10km Composed of sima (magnesium silicate) Geologically young ( less than 200m) Average density of 3000kg per m3 E.g. Pacific plate
Define continental plate?
Geologically old(more than 1,500m) Composed of sial (silicate and aluminium) Average density of 2,700 kg per m3 Average thickness of 40km E.g. Eurasian Plate
How does seafloor spreading occur?
Magma rises, plugs the gap and solidifies, creating new sea floor
Describe a conservative plate boundary?
Plates sliding past eachother No volcanos or fold mountains Earthquake prone E.g.
San Andreas in California - same direction different speeds
north american plate slides 1cm every year
pacific plate slides 6cm a year
Describe a constructive plate margin?
Plates moving apart Sea floor spreading occurs Volcanos occur as under the ridge there are lots of individual magma chambers Volcanic islands e.g. Iceland can occur
What are continental shield areas?
Less active regions of continental crust
Explain how ocean ridges form?
Formed at divergent plate margins Convection currents in the mantle move plays apart Rising magma cools to create new seafloor (seafloor spreading)
Describe how pressure release may contribute to the weathering of rocks?
Brought about by the removal of overburden This then allows the rock structure to expand upwards Producing weaknesses such as joints and cracks
What chemical weathering is prominent in granite?
Hydrolysis
What chemical weathering is prominent in limestone rocks?
Carbonation
Briefly, how are volcanos produced at subduction zones?
The melting of the subsisted plate will produce magma which may be extruded through faults and cracks in the continental plate to form volcanos (stoping)
Briefly, how are fold mountains are produced at subduction zones?
The sediments carried on the continental plate may be crushed and folded upwards to produce gold mountains
Describe hydration?
Certain rocks are able to absorb water into their structure causing them to swell and become suceptible to further breakdown
Describe carbonation?
Rainwater contains carbon dioxide in solution Forming carbonic acid A weak acid which reacts with calcium carbonate rocks such as limestone
When will physical weathering not take place?
When it is too warm Or when there is insufficient moisture Or too high temperatures
Describe the lithosphere?
Outermost layer composed of the crust and the rigid upper part of the mantle Seven major plates Eight minor plates Number of smaller fragments (micro plates) Rigid
Describe oceanic crust?
Thinner than continental plates, only reaching a thickness of 50-100km thick Composed of younger and denser rock of basaltic origin
How are convection currents generated within the mantle?
As the radioactive isotopes break down, the immense heat generated convection currents within the mantle
What is a hotspot?
Vertical plume of magma that can be found in the middle of plates or at plate boundaries e.g. Hawaii
What is theory refers to the slab pull mechanism?
Dragging theory
An example of a constructive plate boundary?
The North American and Eurasian plates are moving away from each other along the Mid Atlantic Ridge
Example of a conservative plate boundary?
San Andreas fault Pacific plate is moving rapidly northwest than the North American plate
Describe a conservative plate boundary?
Two plates moving side by side Often plates will move at different rates or in different directions Which puts increased stress on both plates As friction increases earthquakes can occur
Describe a constructive plate boundary?
When tectonic plates move apart creating an upwelling of magma from beneath Seismic activity occurs - ridges, islands, volcanos
What is a destructive plate boundary?
Occurs from the convergence of oceanic and continental plates As they are forced under each other the denser oceanic plate subduction mms unser the thicker less dense continental plate The subduction plate becomes increasingly magmatic due to intense heat and pressure
Example of a destructive plate boundary?
Nazca plate moves towards the South American plate
Example of a subduction volcano?
Láscar in Chile
Example of a fold mountain range due to destructive plate boundary?
Andes Nazca plate and South American plate
Describe a collisional plate boundary?
Occurs when plates of equal thickness and densities collide Though this happens extremely slowly, the extreme pressure is great enough to develop huge mountain chains
Describe a fold mountain range that is formed at a collisional plate boundary?
The Himalayan mountain range Formed due to the Indian plate and the Eurasian plate
Subduction destructive plate boundary? Oceanic-oceanic
The melted subduction plate may result in submarine volcanos that may eventually surface to form islands These islands occur along a boundary, if several chains form it is an island arc
Describe a Benioff zone?
Done if seismicity corresponding to a subduction plate Develop along the boundary between the two plates as friction dictates
Describe an oceanic trench?
Long narrow depressions in the surface of the seafloor along a plate boundary formed due to the defending oceanic plate Range between 6,000 and 11,000 deep
What angles does subduction usually occur at?
30-70 degrees Gradient increases in depth
What are fold mountains?
Formed by the compression of marine sediments trapped between the converging lithospheric plates at a convergent boundary Distorted under extrememe pressure and thrust upwards
What is pressure release?
The process where overlying rocks are removed by erosion
Where does thermal expansion occur?
Found in areas with a large diurnal temperature range such as hot deserts
Where is weathering greatest?
There are extremely weather conditions Poorly resistant types Rock has been exposed for a long time
Define weathering?
The natural process of decomposition and disintegration of rock in situ
What is mechanical weathering?
The disintegration of rock into smaller particles through mechanical processes without a change in the chemical composition of the rock
Where is mechanical weathering most common?
High altitude areas Arctic areas Deserts with sparse vegetation
Main types of mechanical weathering?
Freeze thaw Salt crystallisation Thermal expansion Pressure release
Where is freeze thaw most effective?
In periglacial and alpine trogons where modifier is plentiful and there are frequent fluctuations above and below freezing point
What is scree or Talia?
The name given to a collection of shattered material that lies at the foot of a steep slope
Where is salt crystallisation common?
Coastal environments Where sea cliffs are exposed to sea spray but can be found whenever there is water with a high proportion of sale (due to high evaporation rates)
How does salt crystallisation weathering works?
As temperature rises salt crystals expand that have entered pore spaces on the rock and grow exerting pressure on the rock
Where does chemical weathering most likely to take place?
Warm Moist Vegetated areas
How does thermal expansion work?
Rocks heat up and expand by day but contract at night weakening the outer layers of rock and creating peeling or exfoliation
What is block disintegration?
In heavily jointed rock where splits form along weakness and break into large easily defined pieces
Describe exfoliation?
Layers of rock may peel off the exterior surfaces exposing new surfaces underneath
Define pressure release?
The process where overlying rocks are removed by erosion
How can you identify pressure release?
Large horizontal cracks and joints (not bedding planes)
Describe how pressure release works?
The weight of overlying layers squashes lower layers during formation and forces the rock to contract under pressure This erosion of these upper layers allows the unroofed lower layers to relax and crack
Describe chemical weathering?
The decomposition of rock resulting from a chemical change
What is hydrolysis?
Reaction with water Breaking of a bond in a molecule using water
Describe carbonation?
The process where carbon dioxide often produced carbonic acid This weak acid solution reacts with calcium carbonate Rock will dissolve away
How can you increase acidification?
The greater the concentration of acid
When does mechanical weathering not happen?
During extreme cold During prolonged warm/cold Little moisture available Too much moisture to freeze Thick vegetation cover protects against insulation
Out of sedimentary, metamorphic and igneous rocks, which is weaker?
Sedimentary tend to have a weaker structure than those formed with intense heat and pressure
What type of rock out of metamorphic, sedimentary and igneous rocks?
Most structurally resilient
Why is granite prone to hydrolysis?
Feldspar is present in it
What type of weathering is limestone most susceptible to?
Acid carbonation
How does quartz influence weathering?
Rocks composed of quartz may be more resistant to weathering
What types of weathering is quartz not affected by?
Dissolution Hydrolysis Oxidation
How can you determine the rate of decomposition?
Degree of resistance Chemical composition Mineral content Climate
