Post on 17-Apr-2020
transcript
Weathering Notes
Weathering
Weathering is the breaking down on rocks into smaller pieces
Two types of weathering
o Physical (mechanical)
When the mineral composition of the rock isn’t changed
Ex: Smashing a rock into two pieces
o Chemical
When the mineral composition of the rock is changed
Ex: When a rock rusts
Physical Weathering
Frost Wedging
o When water seeps into the cracks of rocks and freezes the water expands, splitting the rock
o Occurs fastest in climates that constantly fluctuate between freezing and thawing
Ex: mountains in the summer or early spring/late fall in mid‐latitudes (NYS)
o The debris found at the base of mountains which is the result of frost wedging is called talus
Abrasion
o Grinding rocks down through friction created by rocks rubbing/smashing against each other
o Causes rocks to become smaller, smoother, and rounder
o Occurs in:
Streams
Oceans pounding along the coast creates beaches
Wind picks up sand. This sand slowly wears away the base of rocks creating rocks
that look like mushrooms mushroom rocks
This occurs more in arid (dry) environments because the is little moisture and
vegetation to hold the sand down
Biological Activity
o As trees grow, the roots will split rocks apart
Unloading or Exfoliation
o Intrusive igneous rocks, like granite, form deep inside the earth. As a result they are under
tremendous pressure
o Uplift and the erosion of the overlying layers causes these rocks to become exposed to the
surface.
o As a result there is little to no pressure pushing down on the rock and in response the rock
expands. This causes the outer layers to peel off in layers, like onions.
Chemical Weathering
Occurs on when rocks are exposed to the elements (hydrosphere and atmosphere)
o So only of the surface of rocks
Water is the primary agent of chemical weathering for a couple of reasons
o Many minerals dissolve in water which removes materials and changes the chemistry of the
rock in the process
o Gases also dissolve in the water and those dissolved gases can chemically react with certain
minerals in the rock
Dissolved oxygen causes the oxidation of certain minerals
Ex: Rust
Dissolved CO2 (CO2 + H2O) creates a weak acid called carbonic acid
This acid will dissolve rocks that are composed of calcite (like marble and
limestone)
o The dissolve effect of limestone leads to the formation of caves
Cave Formation
o Water absorbs CO2 from the atmosphere and/or soil making it mildly
acidic
o Groundwater will slowly dissolve the limestone
o If the land is lifted up, or the groundwater lowers, the water‐filled
caverns empty creating caves.
o Water containing dissolved limestone drips from the ceiling. As it
drips it leaves behind (precipitates) a little bit of the calcite.
Overtime the calcite builds up creating stalactites (on the
ceiling) or if the water drips fast and hits the ground a
stalagmite
Biological Activity
When plants die and decompose acids are created.
o These acids can chemically weather rocks
Some plants grow on the rock, lichen, and actually secrete acid which weathers the rock
Weathering Rates
3 major factors
o Climate
o Surface area
o Rock composition
Climate
This is the most influential factor of how fast a rock will weather
Temperate or cool wet climates favor physical weathering (mainly frost wedging)
Tropical climates favor chemical weathering mainly because of abundant rainfall and higher
temperatures
o More vegetation = more decomposition = more acids
o Chemical reactions speed up in warmer temperatures
Surface Area
Because chemical weathering only occurs on the surface, if more surface is exposed the rock will
weather faster
This is a good example of how physical and chemical weathering interact.
o In the winter a rock my split into smaller pieces through frost wedging. This will open up
new areas for chemical weathering to occur in the summer.
Rock Composition
The composition (mineral make up) and structure of a rock will determine how fast a rock will
weather
o Composition
Rocks that contain a lot of iron will oxidize faster than those that don’t
This is why mafic igneous rocks weather quickly compared to felsic igneous
rocks
Rocks that contain calcite will weather faster than those that don’t
Rocks that contain minerals that dissolve are soluble in water (dissolve) will weather
faster than those that don’t
o Structure
Igneous and metamorphic rocks tend to be stronger than sedimentary rocks
Igneous rocks are made of interlocking crystals, making it strong
Metamorphic rocks are formed under extreme heat and pressure making the
rock more dense and the minerals under such pressure grow into each other
locking them together
Many sedimentary rocks are made of sediments that are cemented together
by minerals that are weak and dissolve in water. As a result the cement isn’t
that strong
o Also these rocks form in layers which creates weak bedding planes
where water can get between the layers of rocks and weather them
faster
Soil
Soil is the final product of weathering
It is a combination of weathered material and organic material (top soil or humus which is black in
color)
Parent material – original rock that is the source of the soil
Residual Soil – soil that comes from the bedrock below it
Transported Soil – soil that formed in a different location and was later transported and deposited
in a new location
o NYS has a lot of transported soil that was deposited by glaciers at the end of the last ice age
Different types of soil are found in layers (called horizons)
o Top is rich in organic material
o Middle contains more clays and silt
o Bottom contains larger sediment that hasn’t completely weathered
MassWastingNotesMassWasting
masswastingiserosionwhengravityisthedirectcause
Causes
wateristheprimarycauseofmasswastingevents
o whenthegroundbecomessaturatedthewaterreducesthefrictionbetweenthesedimentallowingittoflow
lossofvegetationo leavesactlikeumbrellas,rootssoakupwaterandanchorsoilinplace
oversteepenedslopes
o riversorroadcreationcanundercutslopesleadingtofuturecollapses
Examples
rocksfalls slumps mud/earthflows soilcreep
WindErosionandDepositionNotes
WindErosion&Deposition
occursmoreinaridregions
lackofvegetationtoholdsedimentdownanddrysoilallowssmallersedimenttoberemovedbythewind
Thiscanleadtothetotalremovalofsand‐sizedsedimentandsmallerleavingbehindthelargersediment.Thisprocessresultsindesertpavement.
o Thisairbornsedimentcanabradethesurface
Eventuallythesedimentisdepositedcreatingdunes.
How Glaciers Modify the Landscape Notes
Types of Glaciers
Valley Glaciers
o aka alpine glaciers
o smaller, confined to mountain valleys at high elevations
o flow down due to gravity – like rivers of ice
Glacial Erosion
glaciers erode in two ways
o plucking
o abrasion
Continental Glaciers
o aka ice sheets
o huge, currently found in Greenland and 2 in
Antarctica
western ice sheet in Antarctica is ~2.8
miles thick!
o Like a pancake, thickest at center and flow
outwards due to weight at center pushing
outwards
Movement of Continental Glaciers
U‐Shaped Valleys
o Rivers cut
downward
creating V‐
shaped valleys
o When glaciers
flow down
existing river
valleys they
straighten,
deepen, and
widen the
valleys creating
a U‐shape
Glacial Lakes
o When glaciers move over soft
bedrock they cut deep into the
land forming deep U‐shaped
valleys or large basins
(depressions).
o When the ice melts those u‐
shaped valleys/basins will fill with
water creating lakes
The Finger Lakes and Lake
Champlain are occupy U‐
Shaped Valleys
The Great Lakes are the
large depressions
o Kettle Lakes
As glaciers retreat (melt) sometimes large blocks of ice break off
Later this ice is buried by sediment from outwash plains
Outwash plains from in front of a retreating glacier when glacial meltwater
carrying sediment is deposited
When the ice that is now buried melts, the ground caves in creating a kettle hole
which can fill up with water creating a kettle lake
Hanging Valleys
o Hanging valleys are
typically formed when the
main valley has been widened and
deepened by glacial erosion,
leaving the side valley cut off
abruptly from the
main valley below. The steep drop
from the hanging valley to the
main valley floor usually creates
cascading waterfalls.
Glacial Deposition
When a glacier melts it drops all of the sediment embedded in the ice
These glacial deposits are called glacial till
o Unlike sediment deposited by wind and water, which is sorted vertically by size, glacial till
is unsorted
Moraines
o at the front of a glacier till can pile
up into ridges (hills) called
moraines
o moraines can help scientists
determine the furthest extent of
the ice sheet from the last ice age
erratic – a boulder deposited
from a glacier
drumlin – small tear‐drop shaped
hill formed from glacial
deposition
o orientated in same
direction so like striations
they show direction of
glacial movement
Sediment Deposited by Water or Wind Sediment Deposited by Glaciers
Cobbles and
boulders
randomly mixed
together with
smaller
sediment
StreamErosionandDepositionStreamErosionSedimentTransportation
1. Solution2. Suspension
a. Smallersedimentsfloatincurrent3. BedloadorSaltation
a. Largersedimentsslide,roll,orbouncealongthebottomofthechannelUpstreamvs.DownstreamUpstream
Knownasthesourceorheadwaters Highergradient Smaller/narrowerstreamchannels Tendtobemorestraight Lessdischarge
o Dischargeisthetotalvolumeofwatermovingthroughastreamatagivenpointinagivenamountoftime Usuallymeasuredincubicfeetpersec
Downcuttingleadstonarrowv‐shapedvalleys,gorges,orcanyons
Downstream
Lowergradient Higherdischarge
o Becausethereismorewaterflowingintoitduetotributariesupstream Tributariesarestreamsthatflowintootherstreams
Wideranddeeperchannels Meanderingchannel
o Outsidebend erosionisgreaterherebecausewatermovementisfaster knownasthecutbank channelisdeepesthere
o Insidebend
depositionisgreaterherebecausewatermovementisslower asaresultsandbarsform channelisnotasdeephere
Floodplains
o duetostreamsmeanderingovertime
WhyLowGradientStreamsMeanderOverTime
Duetoerosionoccurringmoreontheoutsideofthemeanderwhiledepositionoccursontheinside,somestreamsbegintomeander(wanderorcurve)more.
Eventuallythemeanderbecomessomuchthatanewchannelcutsthrough,cuttingoffa
meandercreatinganoxbowlake.FormationofaFloodPlain
Lowgradientstreamscontinualmeanderingresultinhorizontalerosionofthelandscapemorethandownward.
Thiscreatesbroadflatareasonthesideoftheriverthatcouldfloodduringperiodsofrapidsnowmeltorexcessiverainknownasfloodplains.
FormationofNaturalLevees
StreamDeposition
whenwatervelocitydecreasesthesedimentthewaterwascarryingisdeposited
watersortssedimentsbothverticallyandhorizontally
atthemouthoftheriverdepositionofsandandsiltformdeltaso themouthiswhenastreamentersalargebodyofwater
DrainageBasinorDivide
ariverandallofitstributariesemptyitswaterintoabasin(lakeorocean)
thetotalareaoflandthatdrainsintoariversystemisknownasawatershedordrainage
basin theborder,aridgeorhill,thatdividestwoormorewatershedsisknownasadrainagedivide
theContinentalDividerunsthroughtheRockyMountainsanddividesthecontinentalUnited
Statesintotwowatersheds.
Groundwater Notes
Groundwater
Rain water seeps down into the ground in a process called infiltration. Water will then move
through the joints/cracks found in some rocks or through the pore spaces that exists between
sediments and becomes groundwater.
Porosity
Porosity is the percentage of pore space in sediment or rock
Porosity remains the same even if sediment size changes
o Smaller sediment has smaller pore spaces but there are more of them, while larger
sediment has bigger spaces but there are fewer of them
Poorly sorted sediment (big and small mixed together) has a lower porosity than well sorted
sediment because the small sediment fills in the gaps of the larger sediment
Permeability
Permeability refers to how easy water can move through the ground
When sediment gets smaller or less sorted permeability decreases
o Ex: gravel is used in landscaping and construction for drainage
o Clay, because it is small and flat, is impermeable (meaning water basically can’t move
through it), which is why clay is used to line the bottom of ponds or why you see wetlands.
The water won’t drain their
Aquifer
Rock or sediment that is saturated and water can easily be pumped from
o High porosity and permeability
The boundary between the ground that is unsaturated and saturated is called the water table
Spring Formation
When groundwater hits an impermeable layer it is forced sideways.
Eventually the water will exist on the side of a hill and when water flows out of the ground it is
called a spring
Springs can also form when the water table intersects the ground
o This is how many rivers get their water