Nasa

WeatheringErosionDeposition

How are Landforms Made?

• The forces that create the different landforms are, broadly speaking:

• Constructive forces

• Destructive forces

• Are those that build up the land. Constructive forces

• Landforms such as deltas, plains and sand dunes are created when rocks and soil resulting from weathering and erosion are carried away and deposited in new areas.

• Landforms such as mountains ranges, volcanoes and plateaus are built by the movement of the Earth’s plates

• Those that wear down the land, like weathering and erosion.

Destructive forces

(Don’t let the name “destructive forces” mislead you. Destructive forces create landforms like canyons, valleys, etc. but to do so they first had to destroy some other landforms, mountains, plateaus, etc.)

The two players in the destructive process are:

Weathering

Erosionand

Weathering is the breaking down of rocks into smaller pieces.

All rocks do not weather at the same rate.

Whatever the reason, one can find some very odd looking weathered rock:

Further we will find that parts of some rocks weather faster than other parts of the same rock. Why?

Image courtesy of National Geographic

Grand canyon

Why?

http://www.flickr.com Wolfgang Staudt

Why?

Is it because the rock on top is harder than the rock below?

Yes, the rock on the bottom is softer than the rock on top of this formation. The top rock weathers more slowly.

What causes weathering; that is what causes rocks to break into smaller and smaller pieces?

• Nonliving things and living things can break bigger rocks into smaller pieces.

We will look first at non living things that break up rock.

1. Water running over the rock

4. The abrasion of rock by the blowing wind carrying sand

3. The temperature of rocks changing from hot to cold

2. Water freezing in cracks in the rocks

5. Water with acid in it

Nonliving things that break rocks into pieces.

Running water

http://www.ngu.no/en-gb/hm/

Rapidly moving water particularly high up in the mountains or a canyon can break off pieces of rock.

Seen here, a small, fast running stream of water is dramatically cutting through the rock in the side of this mountain.

http://www.flickr.com kia4067

Rocks carried by fast moving water hit other rocks breaking them into pieces. Moving sand acts like sandpaper on the larger rocks in the river bed rubbing off pieces of rock.

These smaller pieces are then carried downstream by the fast moving river.

http://www.flickr.com Randy OHC

Here you can see pieces of rock created by fast moving water.

http://www.flickr.comDawn

Rocks that have been tumbled for a long time in rivers and streams become smooth and rounded.

Freezing water

http://www.ct.gov/

Here we can see cracks in large mountain rocks.

Copyright 2004 by Andrew Alden, geology.about.com, reproduced under educational fair use

Rain fills these cracks with water.

When the water freezes, it expands within the crack pushing the rock apart.

Image courtesy of the geology department umd

As the ice melts, the water flows deeper into the crack.

Then the water freezes again, further cracking the rock.

This repeated of freezing and thawing forces the crack open more and more, eventually shattering the large rock into smaller pieces.

Changing temperature

3. Changing temperature also causes weathering. As rocks heat up, they expand. As they cool, they contract. This process is repeated over and over again in nature. Eventually this process causes them to break apart.

http://www.flickr.comHoggheff aka Hank Ashby aka Mr. Freshtags'

Here we see large rock in a desert environment that has probably been exposed to the freeze-melt cycle.

Abrasion

4. Abrasion by windblown sand also weathers rocks particularly in deserts. It is similar to rubbing sandpaper over a piece of wood or sandblasting concrete.

http://www.flickr.com beige alert

http://www.flickr.com nukeit1

Frequent sandstorms in the desert weather exposed rocks.

http://www.flickr.com lumierefl

The scouring of the rock by the wind carrying sand wears off fragments of rock.

Water with acid in it

Carbonic acid is very common in nature. It is produced when carbon dioxide combines with water.

When this weak carbonic acid trickles into cracks in limestone, it dissolves the rock and eats “holes” in it.

The mildly acidic rain water flows into cracks in the ground.

http://www.esi.utexas.edu

Sometimes it eats huge holes in the rock--caves.

The same acid that made this rock “holy” when it was buried in the ground, also works to make caves

This is a picture of a cave with stalactites and stalagmites.

stalactites

stalagmites

When the acid water dissolved the rock evaporates, crystals of calcite are left behind.

When the water from many, many drips at the top of a cave evaporates, a stalactite forms. (the one on the ceiling stuck tight ...stalactite)

Drips that fall on the cave floor cause stalagmites to grow. (The stalagmites might have stuck to the ceiling but they didn’t.)

More pictures of stalactites and stalagmites, do you remember which is which?

Sometime other minerals in rocks react with the weak acid in water to form other weaker substances. These weaker substances are then more easily worn away by weathering.

Feldspar changes to clay.

Living things that causeweathering

These are some of the living things that break rocks into smaller pieces:

1. Plant roots—particularly tree roots

2. Lichen growing on rocks

3. Burrowing animals

Plant roots

The roots of plants, particularly tree roots, are amazingly strong. When they start growing as tiny root hairs they can fit into the smallest of cracks.

As these tree roots continue to grow, cause the cracks to get bigger and bigger breaking the rock apart.

http://www.flickr.comChazz Layne

Here the roots of the tree are growing in the cracks in the rocks making the cracks larger.

http://media.photobucket.com/ city bumpkins

Lichen growth

Lichens appear in the form of small patchy crusty colors of green, brown, and orange patches. They often grow on rocks and break them apart.

http://www.flickr.com brian http://www.flickr.comSeaDavid

Burrowing animals

3. Burrowing animals

When animals burrow in rocks or between the rocks, they carry seeds which germinate in the cracks in the rocks.

They will be considered together because it is hard to separate them; they occur at the same time.

So far we have examined weathering Now we will move to the concepts of erosion and deposition.

Erosion and Deposition

Erosion—the movement of rocks and sediment from one place to another. The main agents are:

Deposition—the dropping of sediment and/or rocks in another place follows weathering and erosion. Deposition occurs when:

1. Water2. Wind 4. Gravity

3. Ice

1. Water carrying the sediment slows down.2. The wind carries the sediment dies down.3. The glacier carrying the sediment melts.

Here we see the processes acting of weathering, erosion and deposition working together.

Deposition

Gravity

Agents of erosion: water.

Water is the main agent of erosion.

Running water carries weathered pieces of rock from one location to another.

It can carry big pieces of rock as well as smaller rock pieces and soil.

It can also weather rock at the same time as it is eroding it (carrying it away).

Rock is worn away (weathered) at the same time that is carried away (eroded) by fast moving water.

Rocks hit one another causing them to break.

http://www.flickr.com Hamed Saberhttp://www.flickr.com Diego

Fast moving water can move (erode) very large rocks transporting them downhill along with smaller rocks.

http://www.flickr.com watchsmart

A slow moving river carries mainly pebbles, sand, silt and clay (sediment. The slower water cannot carry bigger rocks.

http://www.flickr.com traveling lunas

Photo courtesy daneen_vol of Flickr under Creative Commons license

As soon as the rushing water slows down, the larger pieces of weathered material it had been carrying is deposited.

As can be seen by this diagram, as the water slows down, first the larger pieces fall out. And then as it flows slower and slower, smaller and smaller pieces are deposited.

Soil and pebbles may be carried for a great distance as sediments in the river. As we said before, a river carrying a lot of sediment looks muddy. Shown here, sediments are being deposited at the mouth of a river in Lake Genoa.

Wikipedia Commons

http://www.arthursclipart.org/

When sediments (gravel and soil) are deposited at the mouth of a river, a delta may be formed. It becomes a fertile area for to grow crops such as rice.

Sediment deposited at the mouth of a river also may build a sandy beach.

• Most of the sediment becomes suspended in sea water and is carried along the coast by the longshore current, a stream of water flowing parallel to the beach. This current is created by waves breaking at an angle to shore.

• Some of the sediment is deposited immediately at the mouth of the river.

Sediment deposited at the mouth of a river also may build a sandy beach.

Agents of erosion: Wind

Wind picks up small pieces of rock or soil and transports them from their source to another location where they are deposited. http://www.flickr.com nukeit1

Wind, the second agent of erosion.

When the wind blows in the desert, sand is continuously deposited in a different places. When the wind stops blowing, new sanddunes may havebeenformed.

In this section we see will see mesas, buttes, arches, canyons and and other strange rock formations that were created through both weathering and erosion.

The wind’s remarkable ability to sculpt such odd and beautiful landforms is explained by the hardness of the rock involved. Some rocks are softer and weather faster than others.

http://www.flickr.com/puroticorico

Plateau with mesas, butte and gully

http://pics4learning.com/

Photo of a mesa—this is what is left of the plateau that made up this entire landform before weathering and erosion carried much of it away.

http://pics4learning.com/

Photo of buttes

What could the red line be depicting?

Photo courtesy of USGS

Photo of arch

Why was the middle of the arch eroded away and not the supporting sides?.

http://pics4learning.com/

a hoodoo.

The difference in hardness of the rock composing the “cap” and the rock below the cap explains why the under rock weathered faster than the cap itself.

Here we can see that the weathering and erosion from these boulders is filling in the area beneath them.

Over hundreds, maybe thousands of years, it could have happened something like this.

On their way to the sea, some rivers wind across plateaus, carving deep valleys and taking sediment down stream with them.

The Grand Canyon is just such a canyon, it is continually being carved by the Colorado River.

Over millions of year, these valleys can become giant, majestic chasms called canyons.

Once the valley walls become exposed, these rocks are further weathered by the wind, rain and changing temperatures.

http://www.pdphoto.org

The Grand Canyon

Wikipedia commons

Palo Duro Canyon

Agents of erosion, weathering, and deposition: Ice--Glaciers

Glaciers—great sheets of ice-- create landforms through both erosion and deposition.

Glaciers form when, over many years more snow falls each year than melts. As a result, a deep layer of compacted snow accumulates. This layer of snow becomes compressed into a thick sheet of ice.

What makes glaciers unique is that they move. Due to their mass and the force of gravity, glaciers flow down hill a few inches or feet per year.

2. Mountain glaciers--relatively small glaciers that form near the tops

of mountains.

There are two types of glaciers:

1. Continental glaciers--glaciers that form over large areas of continents close to the North and South Poles.

http://www.coolantarctica.com/gallery2/glaciers/Glacier-10.html

Continental glacier in Antarctica

http://pics4learning.com

Mountain glacier in the Rocky Mountains

During the past Ice Ages, very thick continental ice sheets overlaid much of the continent of North America as shown in this diagram. These very thick glaciers covered all but the highest mountains and resulted in significant erosion. Grey shaded area—continental glaciers

Many lakes in North America including the Great Lakes, were created by glacial moving over the rock and gouging out deep “holes” which filled with water when the glaciers melted.

Wikipedia Commons

In the last glaciation period, which ended approximately 10,000 years ago, 32 percent of Earth's land area was covered with glaciers.

Glaciers now cover only about 10 percent of the land area.

That glacial ice is found mainly over Antarctica.

Most of the other glaciers overlie Greenland; the remaining small percentage are mountain glaciers found in places such as Alaska, the Canadian Arctic, Patagonia, New Zealand, the Himalayan Mountains, the Rocky Mountains, and the Alps.

That glacial ice is found mainly over Antarctica.Most of the other glaciers cover Greenland; the remaining small percentage are mountain glaciers found in places such as Alaska, the Canadian Arctic, New Zealand, the Himalayan Mountains, the Rocky Mountains and the Alps.

Glaciers are not landforms. The action of glaciers, however, creates landforms.

Glaciers move, and as they do, they scour the landscape, "carving" out landforms and eroding material.

This happens because friction breaks the bedrock into pieces of smaller rock and soil. This debris becomes embedded in the bottom of the moving glacier and is carried downhill.

Like a big bull dozer, glaciers drag and push rocky debris downhill to the end of their travel for that winter (or for that ice age).

As they move, the rocks and soil stuck in the bottom of the glacier scrape and scratch the land underneath them.

In the photo above, you can see these “scratches”.

Imagine that scrapping continuing for millions of years and you can see how glaciers turn V shaped valleys into U shaped valleys.

As the glacier moves through a V-shaped valley, it sculpts a “U”.

http://www.flickr.com/ theslowlane

A horn is a pyramid-shaped mountain peak created by several glaciers eroding away at different sides of the same mountain.

http://www.geology.wisc.edu

As the weather warms, glaciers pull back, leaving deposits of the rocky material along the sides and at the end of their travels.

This deposition creates landform features called moraines.

Glaciers leave a predictable mark on the landscape through their deposition.

Wikipedia Commons

Wikipedia Commons

Rocks and sediment deposited on the sides or the ends of a glacier create a landform called a moraine.

Formation of sedimentary rock

When sediments are deposited by a river over a long period of time into a shallow ocean or lake, layers of sedimentary rock may be formed.

http://www.texasbeyondhistory.net/

Just what is sedimentary rock?

One of the three major rock types, sedimentary rock is formed when sediments such as tiny fragments of rock including pebbles, gravel, sand, silt and clay are naturally glued together under great pressure.

The next few slides will help us better understand just how sedimentary rock is formed.

The first steps in the formation of sedimentary rock are weathering, erosion and deposition of fragments of rock in a river.

The fast moving river carries the pieces of rock. In the process, the rocks hit one another and are weathered further and further, forming tiny pieces of sediment.

Here you can see the pieces of rock becoming smaller and smaller as the river flows towards the sea.

As it nears the sea the river gradually slows down, depositing bigger pieces of rock. By the time it reaches the sea it is mainly carrying small pebbles and soil. As it enters the sea it slows down even more and deposits the sediment it is carrying.

The river deposits the larger heavier pebbles first and then gradually the smaller and lighter sediments. In this way layers of sediment are deposited on the floor of the sea. The upper layers of sediment are heavy and push the sediments beneath closer together.

Then the process of lithification occurs. The seawater between the grains of sediment evaporates leaving behind the minerals that were dissolved in the water. These minerals glue the tiny fragments of sediment together to form a type of rock called sedimentary rock.

Over millions of years, the sea fills up with sediment and the water evaporates. The result--layers of sedimentary rock.

http://www.knockan-crag.co.uk/ written permission to use

Five types of sedimentary rock are formed in this way:

• conglomerate

The type of rock depends on what type of sediment is cemented together.

• sandstone

• shale

• limestone• siltstone

Particles of sand cemented together become sandstone.

Particles of pebbles or gravel cemented together become conglomerate.

Particles of mud cemented together becomes shale or mudstone.

Particles of silt cemented together becomes siltstone.

Particles ground up sea shells cemented together become limestone.

A layer of limestone on top of shale.Wikipedia Commons

http://rst.gsfc.nasa.gov/

Most rock on the Earth’s surface is Sedimentary Rock

Formation of Beaches

There are two main types of beaches

1. Rocky beaches

2. Sandy beaches

Rocky Beaches (most with cliffs)

• Are often made of volcanic rock.

• Include four kinds of landforms: sea cliffs, sea arches, sea stacks and sea caves.

Sea Cliffs

Sea Caves

Sea Arches

Sea Stacks

Rocky beaches are shaped by the weathering, erosion and desposition by the of waves (water) and wind.

Eroded material is then deposited becoming available to form sand bars and barrier islands.

The constant beating of the cliffs and sea stacks weather and erode the rock.

Because of the severe pounding of both wind and waves, these formations erode relatively quickly.

Over a long period of time sea stack sometimes completely weather and erode away.

The following series of pictures shows the same sea stack photographed over a 100 year period.

1890

1910

http://walrus.wr.usgs.gov/pubinfo/jump.html

1920

http://walrus.wr.usgs.gov/pubinfo/jump.html

1910

1990

1970

Sea arches and sea caves are created when part of the rock making up a cliff is harder than other parts. The softer section can not stand the erosive powers of the waves for as long as the harder section and erodes faster.

http://www.flickr.commikebaird

The softer material in this span has already eroded, leaving the harder rock spanning the arch.

The softer rock in this cliff has already eroded, leaving a sea cave.

Sandy Beaches• Are deposition landforms, meaning that they are formed by deposits of sediment.

•The sediments deposited are sand, silt and pebbles– materials carried by a river or stream from the inland into the ocean;

•River sediments are the source of most of the sand on beaches.

Washed to sea by streams and rivers, two separate processes result in the deposit of this sand and sediment on the shore.

• Most of the sediment becomes suspended in sea water and is carried along the coast by the longshore current, a stream of water flowing parallel to the beach. This current is created by waves breaking at an angle to shore.

• Some of the sediment is deposited immediately at the mouth of the river often forming a delta.

Rocks and sediment deposited at the foot of a glacier where the glacier melts create a landform called a moraine.

THE END