sedimentary rocks Flashcards
weathering definition
The in situ chemical alteration and mechanical and biological breakdown of rocks by expose to the atmosphere, water and organic matter
Types of weathering
chemical- the process of chemicals in rainwater making changes to minerals in a rock
Mechanical- process of rocks crumbling due to rain,water or other atmospheric conditions
biological- the weakening and subsequent disintegration of rocks by plants, animals and microbes
Chemical weathering types
Carbonation in rocks- CO2 reacts with water to make carbonic acid
CO2-> in atmosphere
Water-> in atmosphere and pore spaces in soil
Ground water is more acidic than rain water
limestone is susceptible
Hydrolysis in rocks- Water reacts with silicate minerals(especially feldspar
s)
The residual material is often clay
Speeds up with carbonic acid
Mechanical weathering types
Exfoliation(onion skin weathering)-
Outer layers kd rock broken down first
significant diff in temp between day and night in desert causes rock to expand and contract,outer layers more so.
Causing them to break off
Frost shattering- Water enters and cracks,joints and bedding planes
frost shattering only occurs in climates regularly above and below 0
when water freezes it expands by 9 %
expansion puts pressure on the rock leading it to fracture + create scree
Pressure release- rocks can be under pressure if buried by layers of rock or under glacier
if pressure is reduced when overplayed of rock are weathered or glacier melts
rock that was under pressure expands in direction pressure is released.
biological weathering types
Root action- roots can grow along bedding planes, joints and fractures
roots can push the rock apart and cause fracture–> root action
Trees can sway in wind and prise open fracture in rock, these are then susceptible to carbonation, hydrolysis and freeze thaw
Burrowing- animal activity brings rock to surface from depths, they can then easily be weather by other means. The burrow also allows atmospheric gases in increase chance of carbonation
How is sediment transported
Weathing is the break up of rocks in situ
erosion is the transport of rocks or their fragments
by rivers, wind, sea, gravity and ice
Types of erosion through transport
Abrasion- The grinding or wearing down of material by dragging or hurling
Attrition- rocks and stones are carried by waves hit eachother breaking them down-sediment oarticals wearing away due to collisions with eachother
Traction- the rolling or sliding of large grains along a river bed or shore
saltation- the bouncing of sand grains as they are picked up carried along and dropped repeatedly.
Solution and suspension
solution- when dissolved materials is carried by a river m- the transport of ions dissolved in water
Suspension- a method of transporting very fine sediments in a river the transport of material in air or water without touching Earth’s surface
grain shape and roundness
sedimentary rock grain can have a roundness ranging from very angular through to well rounded
Very angular,angular, sub-angular, sub-rounded, rounded and well rounded
They are also classes based on how spherical they are so they either have high or low sphericity
Grain size scale in sedimentary rocks
Wentworth-udder scale, it is used to measure grain size in phi
as the size increases in mm the phi number decreases
it is a lothericmic scale which is used when very large or small no are involved
8- 0.0039mm clay
4- 0.0625mm silt
3- 0.125mm very fine sand
2- 0.25mm fine sand
1- 0.5mm medium sand
0- 1mm coarse sand
-1- 2mm very covered sand
-2 - -8- >2mm gravel,pebbles or bigger
degree of sorting
Sediments can be either very well sorted, well sorted, moderately sorted, poorly sorted or very poorly sorted the more well sorted a sediment the similar the size of all the grains is
grain size analysis practical
weigh original sample
arrange the stack of sieves with the largest sieve size at the top and the smallest at the bottom
at the bottom of the stack there is a solid pan
take dry sediment sample and place it in the top pan agitate the sieve stack for a minute
seperate the sieves and record the mass of sediment in each sieve
this data can be used to produce histograms and cumulative frequency curves to determine degree of sorting
coefficient of sorting equation
coefficient of sorting(p) = (84phi-16phi)/2
<0.5 is well sorted
0.5-1.00 moderately sorted
>1 poorly sorted
characteristics of sediment transported by wind
Wind blown dune sand- High energy
grain size- smaller than sand (s-m)
composition- quartz(lots) mineralogically mature as softer mineral have worn away
roundness-very well rounded and high sphericity
sorting very well sorted
characteristics of sediments transported by ice
deposited as a glacial till- low energy
grain size- large range
composition- minerologically immature lots of minerals left
roundness- angular
sorting- poorly sorted
