Knowt
Note
Studied by 7 people
EPSC Final Notes
Igneous Rocks and Volcanism
Intrusive vs extrusive rocks
Igneous rocks have 2 major categories based on where they cool
Extrusive = lava cools on the surface
aka volcanic
low ambient temperature, cool fast
chill too fast to make big crystals
Intrusive = rocks cool at depth
aka plutonic
high ambient temperature, cool slowly
crystals grow slowly and large
commonly occur together
magma chambers feed overlying volcanoes
magma chambers may cool to become plutons
many ingenious geometries are possible
they are linked in space in time → both form at subduction and at rift zones during continental collision and at hot spots
Felsic vs mafic magmas
Erupting magma through Earth’s surface is called lava
Magmas vary widely in composition but are mostly made of Si and O which are referred to as silicate magmas
Felsic magmas
higher Si, Na, K
melt at lower temperatures (700ºC)
Mafic magmas
lower S
higher Ca, Fe, Mg
melt at higher temperatures (up to 1200ºC)
lava formed the first surface rocks
our geological record goes back to ~4.3 Ga
these original rocks were mostly lost to our observation, weathered or covered by later rocks, cycled through tectonic processes
Dikes vs Sills
always younger than the rocks they intrude
tend top have uniform thickness that can be traced laterally
dike: cuts across rock layerings, causes rocks to spread sideways
sill: injected parallel to rock layering, creates vertical uplift
tabular intrusions
dikes and sills modify the invaded wall rock (aka country rock)
cause it to expand and inflate
they also thermally alter the wall rock
cut across rock layering and sometimes occur in swarms
common in extensional settings
sills are injected parallel to layering
below - basalt (dark rock) intruded light-coloured sandstones in Antarctica
The intrusion lifted the entire overlying landscape
Magma chamber processes:
fractional crystallization
partial melting
assimilation
fractional crystallization
the composition of the residual magma changes as it cools and crystallizes
high melting point minerals (Fe, Mg, Ca → mafic silicates) crystallize early and settle to base of magma chamber due to gravity
the remaining magma is enriched in Na, K, Al → felsic silicates & becomes more felsic
while cooling, the magma evolves in composition bc different minerals crystallize at different temperatures and have different melting points
the solid minerals are denser than the liquid, so they settle to the base of the magma chamber where a cumulate rock forms
at any point, an eruption of the liquid and suspended crystals may occur → the same magma chamber can make mafic or felsic lava
partial melting
rocks rarely melt uniformly
most magmas form from partial melting of existing rock → either in upper mantle or crust ← Si rich minerals melt first and Si poor minerals melt last
partial melting therefore yields a silica rich, felsic magma, and removing the partial melt from its source (eg by eruption) creates a mafic residual magma with high melting temperature
assimilation
magma incorporates the wall rock it passes through
blocks of wall rock (xenoliths) break off and fall into the magma
assimilation (melting) of these rocks alters the magma composition
Mafic xenoliths in granite. The one below has partially dissolved.
all 3 processes can occur together → fractional crystallization, partial melting, assimilation
partial melting of wall rock produces new magma that mixes with magma from below
blocks of rock fall into magma and dissolve → assimilation’
deep magma rises
Bowen’s reaction series
N.L. Bowen - conducted melt cooling experiments in the 1920s and discovered that minerals solidify in a specific order
continuous: plagioclase changes gradually from Ca rich to Na rich with decreasing temperature
discontinuous: minerals crystallize over a particular temperature range and then stop and then another mineral starts
olivine, pyroxene, amphibole, biotite
Decompression and Flux Melting
Tectonic setting
rock melting:
magma is not everywhere below the earth’s crust → only forms under particular conditions
melting is caused by pressure release (decompression melting), addition of volatiles (flux melting), and heat transfer
decompression melting
decrease in pressure (P) = decompression
the base of the crust is not hot enough to melt mantle rock but due to the high pressure the rock doesn’t melt
melting will occur if pressure is decreased
pressure drops when hot rock migrates to shallower depths
decompression melting occurs infantile plumes or beneath oceanic/continental rifts
rifts and hotspots
upward convection of the mantle at spreading centres leads to melting in response to decreasing pressure (temp changes little)
flux melting
caused by the addition of volatiles which greatly reduces the melting temperature
volatiles lower the melting point of rocks
common volatiles incl H2O and CO2
subduction carries water into the mantle thus melting the rock
the addition of volatiles decreases the melting point of a dry rock
heat transfer melting
magma from partial melting of mantle rocks rises up through the mantle
may pool at the base of the crust or rise through the crust
inc temp in nearby crustal rock to melting point
Properties of pyroclastic flows
pyroclastic flows:
Greatest Volcanic Hazard
Extreme temperature & velocity
Made of ash and gases
Volcanic hazards
pyroclastic flows = very hot and fast moving clouds of gas and ash
reach up to 700 km/h and 1000ºC
low density → can flow over topographic highs or small bodies of water
flows that glow red at night are called nuees ardentes = burning clouds
MOST DEADLY of all volcanic hazards
Properties of mafic and felsic lavas
basaltic lavas - low silica contents have low viscosity and tend to form thin, effusive lava flows
rhyolitic lavas - high silica content have high viscosity and form thick, slow moving and explosive volcanic eruptions
effect of viscosity
mafic lava
low silica - so low polymerization (process in which monomers combine to become polymers) in the melt
low viscosity = gases can escape
reduces pressure
usually has a quiet, effusive flow
like an open bottle of cocacola that has gone flat
felsic
high silica = high viscosity
too viscous for gas to easily escape so they tend to have higher volatile contents due to tectonic setting
higher pressure builds up
leads to explosive eruptions
like a closed bottle of cocacola shaken to release the gases
most volcanic glass (obsidian, pumice) is felsic but scoria is more mafic and characteristic of cinder cones
What eruptions are the most explosive
How does the addition of volatiles to a melt change its viscosity, eruption explosively, mineralogy
Sedimentary Rocks and Processes
Sedimentary rock formation steps: weathering → erosion → transport → deposition → lithification / diagenesis (burial, compaction, cementation)
Rock Types
rocks are classified into 3 groups - igneous, sedimentary, metamorphic
they are connected through the rock cycle
sedimentary rocks
made of sediments, these rocks are found only on the earth’s surface and cover about 75% of the total land surface
only sed rocks contain fossils, from which we can deduce earth’s history
all oil and natural gas reserves are found in sed rocks
most of the worlds caves are found in the sed rock limestone
form layers
layers are called beds, they record a history of ancient environments
sed rocks cover the underlying basement rock
types of sed rock
clastic (detrital) = rocks made of parts of other rocks, plants, or shells (eg mudstone, conglomerate, sandstone)
bioclastic = clastic rocks made from living things, organisms (eg coal, coquina, chalk)
chemical = rocks precipitated from seawater (eg limestone, chert, banded iron formations, saltstone)
rock features such as grain size, sphericity, and sorting tell us about the energy of the transport medium and the distance the fragments have travelled from their source
immature sediments have angular fragments, poor sorting and high feldspar contents
mature sediments have rounded, well sorted class, high quartz and clay contents
clastic sed rocks are classed based on grain size: clay/silt sone → sandstone → breccia/conglomerate
Weathering is enhanced by increased surface area (eg along joints)
steps in the formation of a clastic rock
weathering - generation of detritus by breaking up rocks
erosion - removal of sediment grains from weathering site
transportation - dispersal by gravity, wind, water, and ice
deposition - settling out of the transport medium eg due to a decrease in current energy
lithification - transformation of sediments into solid rock
weathering
physical - mechanical fragmentation of rock, cold and or dry climates
chemical - dissolution of minerals in water, hot/humid climates
weathering products are transported to sites of deposition by rivers, winds, and glaciers where they form sediments
surface area and weathering
fewer cracks, less SA → more cracks, more SA
joints and weathering
as rocks are exposed by uplift and weathering, the pressure from overlying rock is removed and expansion occurs
expansion leads to vertical and horizontal cracks called joints
these planes of weakness add more SA for weathering
joints are opened up by plants and ice
What rocks or minerals are most or least resistant to erosion
carbonate rocks (limestones) are less resistant to weathering than silicate rocks
degree of weathering depends on SA
Immature vs mature sediments
with increasing distance from their source, sediments become rounder, better sorted and finer, with a mineralogy more resistant to weathering
what minerals are present in the most mature sediments
Depositional environments: what sediments and rock types correspond to each and why
locations where sediment accumulates
differ in
physical, chemical, bio characteristics
sediment delivery, transport, and depositional conditions
energy regime
environments include
terrestrial
coastal
marine
terrestrial
glacial sed = formed due to glacier movement
glaciers carry sed carved out of glacial valleys, sed are dumped during melting, creates glacial till (poorly sorted Gravel, sand, silt, and clay)
immature sediments due to short transport distances
rivers = immature or mature seds
maturity depends on distance travelled → longer distances offer more time to break down into smaller debris → size of particles deposited depend on stream’s energy, low energy streams will drop out larger particles and carry finer ones
high energy rivers = large clasts during floods, poorly sorted coarse conglomerate is characteristic, cobbles and boulders are immobile
low energy rivers: channelized sed transport, sand and gravel fill conc up channels, find sand, silt, and clay are deposited on nearby flood plains
immature river sed: steep high energy mountain streams can transport coarse, unsorted debris
mature river sed: broad, slow moving rivers have lower energy and transport finer particles
channel sediments
as rivers meander across a floodpain, the channel will migrate
shales and siltstones - younger floodplain deposits
sandstone - channel fill
shales and siltstones - older floodplain deposits
a single conc down channel + younger and older floodplain deposits above and below that are related to channels outside of the image
alluvial fans
sediments that pile up at a mountain front due to seasonal streams
rapid drop in stream velocity creates a cone shaped wedge
sed become conglomerate and arkose
immature - coarse, poorly sorted, high in feldspar
deltas
sed are dep near or at sea level
deltaic seds accumulates where a river enters the sea
sed carried by the river is dumped when velocity drops
deltas grow over time, building out into the basin
beaches
beach sands are deposited from seawater
sed are constantly being processed by wave action, filtering out fine, silty particles
common result is well sorted and well rounded sand
beach ripple marks are often preserved in sed rocks
deposition from currents
water or wind flowing over sed creates bedrooms - rhythmic sed features formed at the sed fluid interface
ripple marks - cm scale ridges & troughs
dev perpendicular to flow
ripple marks are freq preserved in sandy sediments
found in modern beaches
found on bedding surfaces of ancient sed rocks
dunes - bigger ripples
windblown, well sorted sand
faster sed transport velocity causes ripples to flatten out
occur in deserts or beach regions
often preserve large internal cross beds
the direction the beds are dipping indicates paleocurrent
cross beds - created by ripple and dune migration
sediment moves up to the gentle side of a ripple or dune
sediment piles up and then slips down the steep face
slip face continually moves down current
added sed forms sloping cross beds
turbidity currents and graded beds
sediment moves on underwater slopes as a pulse of turbulent water
as the pulse wanes, water loses velocity and grains settle
coarsest material settles first, med next and then fines
process forms graded bedding in turbidite deposits
reefs
shallow water carbonate rich envs
most sed are carbonates - shells of organisms, CaCO3
warm clear marine water → red free of clastic sed
protected lagoons accumulate mud
wave tossed reefs are made of coral and reef debris
main source of limestones
shallow marine environments
shallow marine clastic deposits - finer sands, silts, muds
fine seds deposited offshore where energy is low
silts and muds turn into siltstones and mud or shales
usually supports an active biotic community
—> organic material —> black shales and fossils
deep marine envs
fine sediments settle out far from lan
skeletons of planktonic organisms accumulate as a calcareous mud on the seafloor and make chalk or chert
fine silt and clay lithifies into shale
In summary:
sed rock types and features are closely linked to their depositional envs
- terrestrial
glacial, streams/river, alluvial fans → river envs can be high energy or low energy & this relates to the size of the sediments transported
- coastal
beaches, river deltas, reefs
sands, shale / clay stone, limestone
- marine
shallow and deep water
shale, chalk, chert
bedding can be graded or cross bedded, and can have ripple marks if deposited from currents
Sedimentary basins: types and formations
basins form where tectonic activity creates space
topographically low areas that accumulate sediment
thick layers of sediments up to 10-20 km
foreland basins
edge of collisional mountain belt
flexure of the crust from loading creates a downwarp
fills w debris eroded from mountains
fluvial, deltaic, and lake sediments fill foreland basins
rift basins
occur at divergent (pull apart) plate boundaries
curst thins by stretching and rotational normal faulting
thinned crust subsides
sediment fills the down dropped troughs
subsidence = sinking of the land caused by tectonic activity and sediment loading
sed basins
intercontinental basins - plate interiors far from margins
may be linked to failed crustal rifts
continue to subside for millions of years after formation
passive margins - continental edge far from plate boundary
underlain by crust thinned by prev rifting
thinned crust subsides as it cools
subsiding basin fills w sediment from rivers entering sea
Water and Ice: Glaciers
Alpine vs continental glaciers
what is the difference
how do they grow/shrink and flow
what part of the glacier moves the fastest?
alpine
mountain glaciers on peaks and in valleys
flow from high to low elevation
continental
ice sheets covering large areas of land
flow outward from the thickest central part
glaciers flow by…
basal sliding - meltwater forms at the base of the glacier dec friction and allowing ice to slide
plastic deformation - glaciers deform under their own weight and are pushed by gravity
flow rates vary widely - 10-300m / yr
flow rate is controlled by:
steepness of slope angle
amt of basal water
location w/i glacier
greater velocity in centre of ice
friction slows the ice at the margins
glaciers are like bank accounts for water
zone of accumulation - area of net snow addition
colder temps prevent meltong’
snow remains in summer months
one of ablation
area of net ice loss
accumulation>ablation - glacial toe advances downslope
accumulation<ablation - glacial toe retreats upslope
Glacial landforms: how are they created?
ice ages and interglacials repeat cyclically due to changes in earth’s orbit and rotation - milankovitch cycles
alpine glaciers carve bedrock into cirques, aretes, and horns as well asu shaped and hanging valleys
erosion polishes bedrock and wears down mountains thus producing rock flour and striations in rock surfaces
glacial till is unsorted sediment left behind by retreating glaciers to form lateral, median and end moraines
glaciers move due to basal meltwater and plastic deformation
Glacial till and moraines
moraines are comprised of glacial till - extremely poorly sorted sediment
erratics are boulders dropped by glacial ice - they’ve often been carried long distances are different from the local bedrock
Water and Ice: Streams
Streams evolve along their length and as they age
graded (mature) streams are steepest at the headwater and the gradient gradually decreases toward the mouth
ungraded (immature) streams have steep sections at various points and can have rapids and waterfalls
Meandering streams
how do they develop and evolve over time
meanders evolved during floods or times of higher discharge
faster current on outer curve causes erosion - cuts into the bank
slower water on inner curve deposits point bars - stream moves towards centre of channel
migrate downstream with time
Immature streams have higher energy due to the steep gradients and are able to move larger clasts
Water and Ice: Groundwater
How is groundwater recharge and discharge related to the location of the water table?
the water table is a subsurface boundary that defines the depth below which the ground is saturated with water
not flat- generally deeper under hills and shallower in valleys
may coincide with the ground surface → swamps, ponds, rivers
in the saturated zone - beneath the water table, all pores are full of water
in the unsaturated zone, above the water table, pores contain air
the water table is changeable
water table position changes with the degree of recharge
during seasonally rainy periods, the water table rises
during prolonged droughts, the water table falls
ponds and rivers dry up if the water table falls below their bases
How does groundwater flow relate to the hydraulic gradient?
groundwater flow is controlled by the hydraulic gradient and rock permeability. groundwater storage is controlled by porosity
groundwater flows slowly under the influence of gravity
flow in the unsaturated zone is straight down
in the saturated zone, flow follows curved paths governed by gravity and pressure
from high to low hydraulic head, even if upward
this is the gravitational potential energy that drives fluid flow, is a measure of the pressure exerted by weight of the water column
h = z + P/pg
h is a function of the elevation of the measurement point z, fluid pressure P, water density p and Gravitational acceleration g
hydraulic head is proportional to the height of the water column
the difference between the water level’s surface elevation and a reference point
hydraulic head at point a > point b - groundwater flows from high to low hydraulic head, or down the hydraulic gradient
What are aquifers, aquitards, and artesian wells?
aquifers - sed or rock w high porosity and permeability that easily transmits water
unconfined aquifer - intersects the surface in contact w the atmosphere - easily contained
confined aquifer - beneath aquitard, isolated from the surface, less susceptible to pollution
perched aquifers- lens shaped aquitards may exist in the unsaturated zone, preventing downward infiltration to the regional water table → lead to perched water tables above and typically exist only after a period of recharge
aquitard - impermeable or low permeability sed or rock that hinders water flow
artesian wells - flow without pumping, tap confined, tilted aquifers that contain groundwater under pressure from upslope recharge
unconfined aquifers - well water rises to water table, confined aquifers - well water rises to a higher level, the potentiometric surface, due to higher pressure, if ground surface is below this level, a flowing or artesian well occurs
Water and Ice: Limestone Caves
How are limestone caves formed
caused by the dissolution of dense, jointed limestone by groundwater
over millions of years, acidic groundwater or underground rivers slowly dissolve away the limestone through weathering
mainly happens in hot, humid climates like reef environments
What are sinkholes and how are they formed
if a growing limestone cave extends close enough to the surface, the top collapses and produces depressions = sinkholes
Features of limestone caves and karst terrane
karst topography is terrain characterized by highly eroded limestone bedrock or cliffs with caves, sinkholes, and underground rivers
caused by the dissolution of dense, jointed limestone by groundwarer
solution or karst caves are the worlds most common cave type
Calcite: CaCO3 + 2H+ = Ca2+ CO2
Water and Ice: Costal Erosion
What events can lead to increased coastal erosion?
destructive waves - high energy or storm waves, stronger backwash than swash —> seaside arches and caves
constructive waves - low energy and quiet, stronger swash than backwash —> seaside beaches
What are the processes involved in the formation of coastal caves, arches, and stacks?
cracks start to form in seaside cliffs due to wave action and abrasion, these cracks become caves and then arches, stacks, and stumps
cliff erosion process
hydraulic action - rock is broken by force of waves/compression of air into cracks in rock → esp joints in igneous rock
abrasion - sand carried by the waves wears down the rock like sandpaper
corrosion (dissolution) - the rock face is dissolved by water → esp limestone
attribution - eroded fragments are broken into smaller pieces by collisions when carried by the waves
climate change causes this to speed up
Longshore drift, development of spits and bars
spits form where coastline changes direction or where the coast has a bay
sand moved along the coast by longshore drift is deposited in the water at the headland or across the mouth of the bay
a headland spit will curve inward when winds change direction’
a spit that closes off a bay is a bar and the bay becomes a lagoon
longshore drift creates a current that carries sand along the shoreline & creates spits, sand bars and lagoons
Metamorphic rocks and processes
What are metamorphic foliations and how are they developed?
the solid state recrystallization of a parent rock that exists already or a photolithography, to a rock w a new mineralogy and/or texture (shape, size, arrangement of grains) in a process called metamorphism
protoliths can be igneous, sedimentary, or metamorphic
What metamorphic rocks develop without a foliation (eg due to contact metamorphism?)
foliation develops when new minerals are formed and are forced to grow perpendicular to the direction of greatest stress - esp elongate or platy minerals
phyllite display crenelated - wrinkled - layers and a satin sheen due to aligned micas and chlorite —> they are fine grained
similarly, the platy and elongate minerals in a schist are aligned into planes, but the minerals are med grained and can be seen with the naked eye, creating a scaly appearance
the gneissic banding, the felsic & mafic minerals have sep into layers, minerals are coarse grained
non foliated meta rocks
heat comes from magmatic intrusions emplaced in the crust
eg quartzite, hornfels and marble - their minerals are equant and don’t show alignment
develop in contact metamorphic zones around intrusions
Types of metamorphic rocks and their protoliths
burial - several kms deep in sed basins
low P, low T, zeolite facies
contact - baked zone around an intrusion
low p, high t (path 1)
produces hornfels and other non foliated rocks like marble and quartzite
regional - convergent + divergent boundaries
collision - moderate p-t (path 3)
subduction
in subducting slab, high p, low t (path 5)
in overlying continent, moderate p-t (path 2)
spreading centres - low p, high t (path 1)
Changes in rocks as they are metamorphose to higher grades:
grain size increases to decrease the energy at grain surface contacts
minerals become unstable and recrystallize to form new minerals that are stable at the new conditions (e.g. clay → mica → amphibole (with increasing water content))
Faults, Folds, Orogenesis
Stress type: extension, compression
the degree of deformation of brittle or ductile nature of the rocks response to stress depends on rocks composition and strength, the pressure, the temp, and the rate of application of the stress
the result of the stress = strain
extensional stress = stretching strain
compressional stress = shortening strain
Dip slip
dip slip is measured along dip
normal faults
special class = horses and grabens
reverse faults
special class = thrust faults
faults showing displacement (slip) in the vertical direction = dip slip faults
they may be either normal faults produced by extension or reverse faults resulting from compression
the sip in a dip slip fault is up or down the dipping plane
normal → stress: extension, strain: stretching
reverse → stress: compression, strain: shortening
Brittle vs ductile deformation
the 2 major types of deformation → brittle and ductile
faults = structures that occur bc of brittle response to differential stress causing them to fracture
folds are the ductile response to this stress
Know your dip-slip faults
what are the differences
are they caused by compression or extension
in what tectonic setting are they found
normal (extension) or reverse (compression) → faults showing vertical displacement (slip) in the vertical direction are dipslip faults
marker beds determine whether dip slip faults are normal or reverse faults → marker bed has a distinctive appearance and can be recognized on both sides of the fault
normal faults - the hanging wall moves down relative to the footwall → accommodate pulling apart aka crustal extension and the below fault shows displacement and drag folding
Horst and graben fault blocks
created by pairs of normal faults w opposite dip directions
reverse vs thrust faults
in reverse faults, the hanging wall moves up relative to the footwall
reverse faults = fault dip is steeper than 45 degrees
thrust faults = special class of reverse with a dip less than 45 degrees
reverse faults accommodate crustal shortening (compression)
thrust faults = low angle faults that can be kms to 100s of kms long and may dip at just a few degrees, making it hard to identify
during continental collison, mountain building occurs as a result of the propagation of sets of thrust faulrs
the thrusting often displaces soft sed rocks over a basement of hard igneous and metamorphic rocks
the thrust slices are detached by faulting away from the basement
What are the parts of a fold?
folds occur in many shapes, sizes, and geometries
commonly formed in orogenic settings (Orogeny is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges.) and may record many events of deformation
anticlines - fold limbs dip outwards
synclines - fold limbs dip inwards
anticlines have an ant hill shape
synclines have a sink shape
anatomy of a fold
hinge line - a line along which the curvature of the fold is greatest
limbs - the less curved sides of a fold
axial plane - connects hinges of successive layers
How are folds classified?
fold classification
based on shape and orientation
plunge angle of hinge - plunging or non plunging
dip of axial plane - upright (symmetric), inclined (asymmetric)
inter limb angle - gentle, open, close
plunging and non plunging folds
the hinge lines of anticlines and synclines may be horizontal or plunge (analogous to the dip of a plane)
horizontal folds (eroded) will be seen on Maos as the rep of beds
in plunging folds the fold intersects the horizontal as a U
How are mountains formed and distributed around the planet
mountains frequently occur in elongate linear belts
these belts correspond to past or present subduction zones and continent collision zones where tectonic forces cause the process of orogenesis (mountain building)
continental collision
follows ocean basin closure
complete subduction of oceanic lithosphere
brings 2 blocks of continental lithosphere together
in mountain building related to continent - continent collision, sets of thrust faults are present on either side of the mountain range
mountains form via continent collision, subduction and related volcanic arcs, and rifting
How does the continental crust under mountains differ from that in the plains
continental collision follows ocean basin closure
complete subduction of oceanic lithosphere
brings 2 blocks of continental lithosphere together
buoyant continental curst shuts down subduction
How do mountains formed from compression differ from those in extensional settings
leads to shortening and recumbent folds
convergent margin horizontal compression causes horizontal shortening & vertical thickening
these processes can double crustal thickness
thick crustal root develops beneath mountain ranges
What rock types are found in the mountains
Why do mountains have a maximum height
mountains have a finite lifespan
young mountains are high steep and still growing up
older mountains are lowered by erosion
topographic elevation is caused by a balance between the effects of gravity and buoyancy on the vertical position of the lithosphere - isostasy
mountains are so high bc of igneous rocks thickening the crust - removal of lithospheric mantle due to density inc can cause uplift —> mountains reflect a balance btw uplift and erosion - they are steep and jagged due to erosion which is controlled by rock characteristics - resistant layers cause cliffs and easily eroded rocks form slopes
Geologic Time
Geological principles for relative age dating
uniformitatianism
This principle holds that the geological processes operating today have been operating for most of Earth history
This means that our observations of current geologic processes can be applied to the same types of systems preserved in the rock record, e.g., sedimentation, rivers and glaciers, volcanism
superposition
The Principle of Superposition applies to sedimentary and volcanic rocks and is based on the idea that younger rocks lie on top of older rocks
original horizontality
The Principle of Original Horizontality holds that although sedimentary and volcanic rocks may not be horizonal at present because of deformation, they were deposited horizontally
original continuity
The Principle of Original Continuity states that although erosion may locally remove sedimentary strata, they were once continuous and therefore can be correlated from one locality to the next
inclusions
If a rock contains inclusions of another rock type, the inclusions must have formed earlier
cross cutting
If an intrusive rock like a dike crosscuts the layering in the country rock, the layering must have been there first
faunal succession
is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances (Wikipedia).
Index fossils
may be used to correlate stratigraphies
must be distinctive, abundant and widespread, limited to a particular time and contain hard parts that preserve well
How are fossils formed, what are they made of?
species dies, the flesh rots and leaves the bones behind, the bones are covered with sediment which cuts them off from the air and prevents further decay, groundwater fills in spaces in the organism and precipitates minerals like chert / calcite to preserve fine structure, tectonic uplift and erosion expose the fossils at the surface
Are fossils more common in terrestrial or marine rocks
marine rock since they are formed in rivers, etc and also there are more aquatic creatures that have been preserved through fossil
Types of fossils: body, cast and mould, trace
mold fossils - a fossilized impression made in the sed, a negative image of the organism
cast fossils - formed when a mold is filled in
trace fossils - fossilized burrows, footprints, etc
Do all fossils represent extinct species
Although fossils represent dead remains of the organism, it does not always mean that the organism in question has become extinct. Fossils of organisms that can even be found today means that the organisms have been on the earth since a very long time.
Bones are buried to stop decay and are replaced by minerals precipitated from groundwater
What are some events that might lead to mass extinction
mass extinctions occur periodically in response to major changes in global ecosystems
may be caused by:
start / end of glaciation
flood basalts erupting
global warming from co2
ocean acidification
ocean anoxia
meteorite impact
shock waves and impact winters
What effect might a massive volcanic eruption have on the planet’s climate
would heat up the planet’s climate - could lead to mass extinction if hot enough
What about a meteorite impact
meteorite hitting earth would kill us all
Do glacial cycles cause extinctions
yes because we would either freeze or burn to death depending on how much water we have and where we are in the glacial cycle
Fossil Fuels
What circumstances lead to the formation of coal
coal has been used for fuel since pre historic times
it is composed of mostly carbon & hydrogen w some O, N, S, H2O and impurities
coal layers = seams
origin of coal
preservation of terrestrial plant material in a low oxygen, peat accumulating env with stagnant water = peat swamps
coal seams are formed in situ (in place)
the organic matter is attacked by anaerobic bacteria, causing partial decomposition
over time, bacterial decay splits off impurities and concentrates the carbon
petrification
coalification
burial and compression cause peat to change in comp and become coal
follows bacterial decomposition in peat swamps
grades
burial, compaction and heating
squeezes out water and gases
methane
hydrogen sulphide
carbon dioxide
increases c content
inc the coal grade and energy content
How does coal formation relate to sedimentary diagenetic processes
Coal formation is intricately tied to sedimentary diagenetic processes, as coal itself is a sedimentary rock formed from the accumulation and alteration of organic matter over millions of years. Here's how coal formation relates to sedimentary diagenetic processes:
Organic Matter Accumulation: Coal forms from the accumulation of plant debris in ancient swamps and marshes. Over time, this organic material accumulates in layers, forming what is known as peat.
Burial and Compaction: As more layers of organic material accumulate, the weight of overlying sediments increases. This leads to compaction of the organic matter below, reducing pore spaces and increasing pressure.
Diagenesis: Diagenesis refers to the physical and chemical changes that occur in sedimentary rocks due to increased pressure and temperature over time. In the case of coal formation, diagenetic processes play a crucial role in transforming peat into coal. The heat and pressure from overlying sediments cause chemical changes in the organic matter, driving off volatile components like water, methane, and carbon dioxide. This process, known as coalification, gradually transforms the peat into lignite, then sub-bituminous coal, bituminous coal, and finally anthracite coal, with each stage characterized by increasing carbon content and energy density.
Hydrothermal Alteration: In some cases, hydrothermal fluids can infiltrate coal-bearing formations, leading to additional chemical alterations. These fluids can introduce new minerals or modify existing ones, affecting the quality and composition of the coal.
Structural Deformation: Tectonic forces can deform sedimentary rocks, including coal seams. Structural deformation may lead to folding, faulting, or fracturing of coal beds, influencing their thickness, continuity, and accessibility.
Overall, coal formation is a complex process influenced by various sedimentary diagenetic processes acting over geological timescales. Understanding these processes is essential for both the exploration and exploitation of coal resources (ChatGPT).
What are the 3 essential rock components of a petroleum system and what are their properties? What types of rock might work for each
1 - source rock
must be organic rich to generate the oil/gas
must contain sufficient organic matter to produce oil / gas (1-10 percent)
must exist at the appropriate depth and temp conditions to produce oil and gas
2 - reservoir rock
a - porosity, to hold the hydrocarbons
b - permeability, to allow fluid flow
sufficient porosity to hold economic quantities of oil and gas
sufficient permeability to enable extraction of commercial amounts of oil / gas
types of reservoirs = 1. siliciclastic, 2. carbonate, 3. fractured
3 - cap rock (seal)
reservoir must be sealed by an impermeable rock to prevent escape of the petroleum
must have v low permeability to prevent the escape of oil / gas
must be located above the reservoir rock in a physical config that will trap the oil / gas
types of cap rock = 1. shale, clay (fine sed), 2. salt, anhydrite (evaporites), 3. unfractured carbonate or granite
if any of these are absent the petroleum system will fail
What else is necessary for a petroleum system to be successful
other essential ingredients in petroleum system:
production & preservation
sufficient heating in source rock to form oil and gas
no loss, biodegradation or over maturation
migration of hydrocarbons out of the source rock into more permeable reservoir rock
trapping by impermeable cap rock above the source
timing of trap formation - must occur before hydrocarbon migration
How are oil and gas trapped in the subsurface
petroleum traps = any barrier that impeded the upward movement of oil / gas and allows either / both to accumulate
incl a low permeability cap rock to prevent hydrocarbon flow
no trap is completely impermeable over geological time → can leak or be breached
traps can e structural (anticline, fault) or lithological (salt dome, stratigraphic trap)
anticlinal trap
anticline = folded rock seq w an upward facing dome
permeable and impermeable rock layers
permeable layer contains water, oil, gas
oil and gas rose above water into dome of anticline
further migration of oil and gas is prevented by impermeable cap rock
fault trap
seq of permeable and impermeable rock layers offset by faulting
oil and gas want to move upward along perm reservoir rock layer BUT further migration of oil / gas is prevented by cap rock which cuts off the left side of the reservoir rock due to fault movement
salt dome trap
halite (salt), NaCl is an impermeable mineral and is soft enough to flow underground and make sat domes
intrusions of impermeable salt block off underground avenues of escape for oil / gas