F2 Key idea 1 (sedimentary) Flashcards
(24 cards)
weathering
occurs because rocks at the earths surface are out of equilibrium with the conditions (unstable)
break down of rocks in situ (haven’t moved)
physical weathering
when physical forces break apart the rock into smaller pieces
retaining original composition
freeze/ thaw
water penetrates gaps in the rock (joints, bedding planes, faults, cleavages)
temp falls below 0, turning water to ice
(diurnal temp fluctuation, below 0 at night, above 0 in day)
ice occupies 9% greater than water
immense internal stress in the rocks, forces then apart
cracks grow bigger overtime
insolation/ temperature change
high levels of insolation - high temps during the day leads to expansion
cooling at night leads to retraction
weakness the layers surrounding the rock that will eventually crack and weather away leaving behind rock fragments (scree)
chemical weathering
composition of the rock is altered by chemical action
hydrolysis
chemical reaction between a mineral and acidic water (rain)
ions in the water react with ions in the mineral, breaking it down
e.g feldspar in granite react and produce clay minerals (kaolinite)
carbonation
(mainly in limestone)
rainwater falling through the atmosphere picks up C02 to form a weak carbonic acid
carbonic acid reacts with rock (like limestone) causing it to dissolve gradually
can result in the collapse of caves as water infiltrates the soil
oxidation
ability of minerals to incorporate oxygen atoms into atomic structure (mostly effecting Fe rich minerals)
iron reacts with oxygen in the atmosphere to form iron oxide (rust) which is a reddish brown colour
solution
minerals dissolve directly in water, transporting away the products
e.g. halite and gypsum are very soluble, later forming evaporites
erosion
abrasion and attrition
removal of material
abrasion - rocks and sediment grind against the surface, wearing it away
(cliffs, river beds, or glacial valleys)
attrition - when rocks knock against each other/ collide and break into smaller, smoother pieces
transportation
traction, saltation, suspension, solution
movement of material
traction - when larger rocks and boulders are rolled along the river or sea bed
high energy environments
suspension - when small particles like silt and clay are carried (suspended) within the water current
makes water look muddy
solution - when minerals are dissolved in water and carries along invisibly
saltation - when small pebbles or sand grains bounce along the river or sea bed
current lifts them briefly then they fall back down
deposition
happens when the energy of moving water decreases, dropping heavier material/ sediment
flocculation
flat surfaces of clay particles have a small negative electrostatic charge, so two particles repel one another
turbulent water (e.g. estuaries) forces the particles together and they become attached, known as coagulation, creating larger molecules called flocs
salt particles speed up this process
precipitation
water contains dissolved minerals (solution)
if the water evaporates or changed temp/ chemistry, it would not longer be able to hold onto those minerals
the minerals then precipitate out of the solution, forming solid deposits (evaporates such as halite and gypsum)
post depositional diagenesis
and pressure solution
process that turns loose sediment into sedimentary rock
compaction - grains are squeezed together, dissolving the minerals, precipitating into the pore spaces
cementation - minerals (e.g. calcite or quartz) crystallise between grains, ‘gluing’ them
recrystallisation - some minerals change form or grow larger
pressure solution - pressure increases (greatest ar grain-to-grain contact, causing mineral to dissolve slightly, reducing pore space helping cementation
aeolian environments
aeolian - wind dominated (desert dunes)
well sorted, wind can’t carry as wide a range
rounded due to wind abrasion
aeolian sands tend to be quartz rich, low feldspar
iron oxide coating
fossils are rare, mostly trace fossils
marine environments
can be shallow or deep (in water)
moderately to well sorted in shallow
poorly sorted in deep marine deposits (turbidity flows)
fine grained material (shales and mustiness) common in low energy areas
quartz is common, beaches and shallow seas
clay minerals in deeper marine mudstones
calcite form limestones in warm, shallow seas
marine environments are very fossil rich
shale good for preservation
fluvial environments
(rivers, alluvial fans, floodplains)
moderate to poorly sorted, carrying a range of grain sizes
sub rounded to rounded, deposits often showing fining upwards
quartz rich due to durability in transport
may include feldspar and rick fragments
fossils less common, may include plant fossils, peace fossils
formation of coal
forms where swampy forests grow
1. plants die and fall into oxygen deficient waters, cannot decay due to lack of oxygen, so layers build up
2. the weight of the top layers, water and soil compresses lower layers of plant matter
3. the heat and pressure produce both chemical and physical changes
4. oxygen is forced out, leaving carbon rich deposits, overtime becoming coal
identification of different coal
brown coals
peat - original plant structure still clearly recognisable and feels spongy or moist as well as crumbly
lignite - darker brown or dull black with woody appearance
black coals
bituminous - black, often layered or banded and has a shiny (vitreous) lustre, is brittle and has sooty residue
anthracite - hardest, black and very shiny (metallic) with a conoidal fracture, can’t be scratched easily
evaporites
formed in environment around water
water evaporated in hot conditions to solar energy and leaves solution behind
e.g. Halite and Gypsum
sand bar theory - sand bar cuts off entrance between sea and lagoon where the lagoon water evaporates and precipitates thin beds of evaporites
relative order of precipitation
calcite - precipitates with only 50% of seawater evaporated
gypsum - 80%
Halite 90%
potassium and Mg salts - 95%
classification of limestone
chalk - soft, white to light grey and very fine -grained, as well as very powdery, rubbing off on fingers
has a strong reaction with HCI (95% calcium carb.)
deep marine (microscopic fossils)
shelly limestone - often grey, beige, or brown with visible shell fragments
coarse, rough and uneven
shallow marine, reef lagoon, fossils may stick out or are embedded in matrix
Oolitic limestone - light-coloured
feels smooth or slightly grainy made of Ooids (tiny, rounded calcite grains) cemented by calcite so has a strong fizz reaction to HCl
warm shallow marine, high energy where Ooids form by rolling in supersaturated water (more dissolved Ions)
cements
red - iron oxide, must be dry when lithified
yellow/ brown - hydrated iron oxide, when lithification takes place in well oxygenated water
white - calcite, marine sediments
white or colourless - silica, very hard and un reactive
black - iron sulphide and carbon, common causes of black colour in mudstones
