Picture of the point taken by a former student. Different colors of the river showing the different types/amounts of sediment the Mon and Allegheny carry.
Pink Sand, BermudaGreen and Black Sand Hawaii
Chapter 9 Erosion PowerPoint
Wearing Down and Building Up•Weathering breaks down earth’s surface into sediments.“Breaks”•Erosion is when sediments are transported. “Takes”•Deposition is when sediments “settle out” or are “dropped”by whatever is carrying them. This can build up landforms orfill in areas. “Makes”•Principle of Uniformitarianism- Weathering, erosion, anddeposition act together in a cycle that wears down and buildsup Earth’s surface today and in the past.
• Have you ever seen a “muddy” stream or river?• The water appears “muddy” because it is carrying small particles of sediment! Erosion from running water is at work!
Runoff and Erosion•After it rains, some of the water that doesnot soak into the ground becomes runoff.•More runoff = more erosion.
Streams and Rivers•Streams and creeks form fromrunoff.•Rivers form when streams cometogether.•A tributary is a stream or river thatflows (along with any sediment it iscarrying) into a larger river.•Exs. Pine creek is a tributary to theAllegheny river.•As water moves through streamsand rivers, it picks up sediment andcarries it! Erosion is happening!
Today you are going on a river trip!You are going to follow a river from start to finish and discover many things along the way!Pick your boat of choice:
River Erosion•You start your journey at the river’s source whichis often a steep mountain stream. You will end yourjourney at the river’s mouth which is when itreaches the sea.
River Erosion•Wow! At the river’s source, younotice the water is fast flowing andfollows a straight and narrowcourse. You barely have to paddle!•It erodes deep into the mountain,eventually forming a V-shapedvalley. As it cuts into the earth, thewater continues to pick upsediment.
River Erosion• You are at the bottom of the mountain and you notice a fan- shaped deposit. You realize that this must be an Alluvial fan.• When a stream comes to the bottom of a mountain water slows down, spreads out, and drops or deposits its sediment in a fan shape.
River Erosion•As the river flows, you notice there are deep canyons,gorges and waterfalls carved by the river.•The “Grand Canyon” was slowly carved out of the ColoradoPlateau by the Colorado River over millions of years. Grand Canyon of PA
River Erosion•Waterfalls are created when softer rock erodesbefore overlying harder rock, creating a steep drop.•Be careful, rocks that have broken off create rapids!World Record Holder-Costa Rica
Erosion by Rivers•The paddling is getting tough!! Nowyou are lower on the river’s course andcloser to the sea, you notice it slowsdown and spreads out. It is no longerfast and narrow. It erodes a wide areaon its sides called a flood plain. This iswhere water goes when it overflows theriver banks.•You notice that the land along the floodplain is flat and you notice a lot of farmsso it must be very fertile.•Although land along a floodplain maybe appealing, what are some potentialproblems with living along it?
Erosion by Rivers• You are moving so slowly and younotice you are no longer taking astraight path! Instead, the rivermeanders or forms loop like bends.•You move to the outside of the curvebecause you notice the water movesfaster on the outside and slower on theinside curve.•Therefore erosion happens along theoutside and where the water is slowingdown, along the inside, sediment thatthe water is carrying gets deposited andforms cute little beaches!•Yum! You stop along one of thesebeaches for a snack!
Erosion by Rivers•As you are moving, you notice a strange C- shaped lake!•When meanders get too curved, the river is able to “breakthrough” and takes a straight course again. The meander getscutoff as sediment builds up. This is called an oxbow lake. The lake looks like a C!
Deposits by Rivers•You made it! You have reached the ocean!•Sediment that is eroded away by rivers and streams isdeposited at the river’s mouth as a river slows down forminga landform called a delta.•Delta -Rivers slow as they meet the sea. The sediment isdeposited which forms new land. They take on a triangularshape and are named after the Greek letter ∆.•The city of New Orleans is built on land created by a river!
Groundwater Erosion•Rain water that does not evaporate or becomerunoff soaks into the ground and is calledgroundwater.•Groundwater can be in the form of underwaterstreams but most of the time it is held in thepore spaces of rocks.
Groundwater Erosion•Carbon dioxide can dissolve in water making it acidic. Thisacidic water can weather and dissolve limestone undergroundforming a cave.•Once the groundwater in the cave lowers, the cave is filled withair.•When water containing dissolved limestone (main mineral iscalcite) drips from the ceiling and hits the air, the waterevaporates and leaves behind the limestone again forming aStalactite.•When this water drips to the floor it builds up a Stalagmite.
Groundwater Erosion• Karst topography is found in many parts of theUnited States where the bedrock is made up of layersof limestone that have been eroded by acidic water.• Karst topography has caves and sinkholes(sinkholes are caves where the roof has collapsed)
Glaciers – “Rivers of Ice”•Glacier-A large, slow moving mass of ice. Glaciersform when more snow falls each year than melts so itbuilds up.
How Glaciers Form and Move•A continental glacier is a glacier that covers muchof a continent or large island. Antarctica andGreenland are large continental glaciers!•Continental glaciers move by spreading outwardsin all directions towards the sea.
How Glaciers Form and Move•The last Ice Age endedaround 12,000 yearsago. Most of Earth’swater was in the form ofice. During the last iceage, a continental glaciercovered most of northernNorth America.•Most of the land andwater were coveredunder an ice sheet thatwas 2.5 miles thick insome places.
Valley Glaciers•Valley Glaciers form when snow and ice pile up highin the mountains. The ice slowly begins to movedownward because of gravity.•Ex. Alaska and Glacier National Park in Montana
How Glaciers Form and Move•Like the force of gravity and running water, glacierstoo cause both erosion and depositional featureswhich scar and shape the land!
Glacial Erosion Features-How Glaciers shape the land **Remember, erosion is the transportation of sediment**1. As a glacier moves, plucking breaks andpicks up pieces of bedrock from the ground andcarries them.
Glacial Erosion Features2. V-Shaped valleys become U-Shaped asglaciers erode old river valleys. Glaciers canalso carve a mountain peak into a sharp horn orsharp peak and leave bowl shaped depressionsalong mountains called cirques.
Glacial Erosion Features3. Glaciers also carved depressions in the land which canfill with water to create lakes (ex. Great Lakes)
Glacial Erosion Features4. As glaciers move across the land, theyscratch the bedrock (remember they arecarrying rocks). These are known as glacialstriations.
Glacial Deposition Features*Remember, deposition is the dropping of sediment.* 1. As glaciers melt and retreat, they deposit the sediment that they were carrying which is a mixture of clay and boulders known as Till. How did this rock get here? 2. Many big boulders were dropped as the ice melted away at the end of the last ice age. These big, out of place boulders are known as glacial erratics. Glaciers can move huge rocks that wind or water could not move!
Glacial Deposition3. The very end of theglacier pushes along a lot ofrocks, so when it quitsmoving and begins to melt,it leaves the till in a pile.This is known as a terminalmoraine.
Glacial Deposition•Moraine State Park, about 40 minutesNorth of Shaler, is the southernmostpoint in Pennsylvania that the glaciersreached during the last ice age.•The soil in the park is full of bouldersand a mixture of rock that was stuck inthe ice as it moved across Canada!
Glacial Deposition• Lateral moraines are parallel ridges of sediment deposited along the sides of a glacier.
Glacial Deposition Features4. Often when glaciers retreat (begin to melt) they leavebehind chunks of ice. These melt and create lakes calledkettle lakes. Ex. Conneaut Lake in PA & Walden Pond inConcord, Massachusetts!
Erosion by Waves•Many ocean waves are caused by windand can have tremendous power (we willtalk about tides later this year which arecaused by something else!). Some of theworld’s biggest storms happen at sea!
Erosion by Waves•Due to constant erosion by wave action,there are many unique landforms foundalong the edges of the continents.
Erosion by Waves•Waves shape the coast through weathering(abrasion), erosion, and deposition. Wavescarry sand and gravel which wear away rocklike sandpaper breaking it down, transportingthe sediment, and depositing it.
Erosion by Waves •Weathering, erosion, and deposition creates a variety of unique landforms along a coast. 1. Wave-Cut Cliffs 2. Sea Caves 3. Sea Stack 4. Sea Arch 5. Headland-land that stands out from the coast because it is made of erosion- resistant rock.
Deposits by Waves•Waves shape a coast when they deposit sediment, formingcoastal features such as beaches, spits, and barrier beaches.1. Beaches2. Spit3. Sandbar4. Barrier Beach- large sandbar Ocean City MD andOuter Banks, NCand Cape Cod
Outer Banks, NC Barrier BeachOcean City, MD Spit Sandbar
Deposits by Waves•Beaches are constantly being moved around andreshaped by a process known as longshore drift.•Because waves often hit the shore at an angle,then move straight back due to gravity, sand oftentravels down a beach in a zig-zag manner.
Deposits by Waves•Beaches are constantly beingchanged by longshore drift.•Sometimes in heavily developedareas beaches have to have sandadded to them or have structurescalled jetties built to reduce erosion.
Deposits by Waves•You can tell a lot about a beach by examiningits grains of sand.•What do the shape of the sand grains tell you?How much weathering the grains have beenexposed to.•What do the different colors in sand tell you?What minerals/materials the beach is madefrom.
How Wind Causes Erosion•Wind erosion usually occurs in dryareas where there is littlevegetation to keep sediment inplace.•Ex. deserts and recently plowedfarmland.•Wind erosion led to the Dust Bowl!
How Wind Causes Erosion•Wind wears down rock, kind of like a naturalsandblasting. This is known as abrasion.
Wind Deposition•Wind erosion anddeposition may formgiant piles of sanddunes. Erg Chebbi, Morocco
Mass Movement- Gravity •Mass Movement is when rock is eroded by gravity. •Gravity pulls everything toward the center of the earth. •Mass movement can be triggered by earthquakes, volcanoes, heavy rainfall, etc. •The different types of mass movement include landslides, slump, creephttp://www.youtube.com/watch?v= and mudflows.K1qY8nPqcCw http://www.youtube.com/watch?v=mknStAMia0Q
Landslides - Problems Around Pittsburgh•Every year, tumbling rocks calledlandslides along Route 28 nearHarmarville cause nightmares formotorists!•Also a few years ago a landslidecaused Rt 65 to close for weeks afterthe construction of a new Walmartdestabilized the land!•Pittsburgh is especially prone tolandslides and mudslides becauseour local rocks contain a lot of clay sothey easily crumble when wet.
Mudslides•Mudflows or mudslides - can occur whenloose sediment becomes drenched afterheavy rainfall. (Hurricanes)•Can also happen around volcanoes(when snow and ice melt rapidly after avolcanic eruption.)
Mudslide from the May 1980 eruption of Mount St. Helens.