weathering (teach)

Destructive Forces in Nature..Those that destroy landforms.

By Moira Whitehouse PhD

WEATHERING

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?

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

http://www.flickr.com Wolfgang Staudt

Why?

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

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.

How about a little review.

Do you remember what the word weathering of rock means?

Yes, it means breaking rock into smaller and smaller pieces.

But, with weathering, it is not a hammer or man who breaks the rock. It is forces in nature.

What are the main forces in nature that break rock into smaller and smaller pieces?

Yes, there is:

• fast running water

• wind carrying sand

• water in the cracks freezing, melting and freezing

• acid in water “eating” holes in rock and forming caves

• plant roots

• burrowing animals

• rocks heating up in daytime and cooling down at night

What force in nature is causing the rock to weather in the pictures on the left forming a deeper and deeper valley?

In the desert, there is very little running water. What force in nature causes rock formations like the one seen in the pictures on the left?

What force in nature has caused the rock in the picture on the left to break into two pieces?

What will happen to this rock if water gets into the cracks and freezes, melts, freezes and melts over and over again?

After millions and millions of years the mountains on the left will finally look like the ones shown below.

Weathering is constantly changing the surface of the Earth.