How do Rocks Change?

How do Rocks Change?

Have you ever seen waves pounding against the coastline?
The force of the water moves the sand and pebbles on the beach.
Sea and river water are always moving sand, soil and rocks from one place to another.
This process is called erosion. 
Wind and ice also wear away, or erode, rocks and soil. Over thousands of years, erosion can move mountains, dig out or fill in valleys, and change the direction of rivers.

How are rocks broken down?

Rocks feel hard and solid. But they can be broken down.
For example, water which seeps into cracks in rocks may freeze and expand into ice.
The ice splits the rock and breaks it into smaller pieces.
These pieces may be washed away by seas and rivers, blown away by the wind, or moved down a mountainside by a frozen river of ice called a glacier.

How are canyons formed?

Over millions of years, loose stones carried along by large, fast-flowing rivers can erode a passage through solid rock.
The huge valleys they make are called canyons. 
In Arizona, the Colorado River has carved out a deep canyon called the Grand Canyon.
Chemicals in water can dissolve some rocks, leaving behind large caves.
Rain water often mixes with carbon dioxide gas, making a weak acid that eats away some types of rock.
Have you ever seen an archway of rock jutting out into the sea? 
The moving water has worn a hole through the weaker middle parts of this rock.
You might see a rock arch inland, too.

What are sedimentary rocks?

Strong winds blow sand against the rock, wearing away the weaker parts but leaving the firm parts still standing.
Small pieces of rock that are washed or blown away are called sediment.
The sediment will settle somewhere else in layers.
Eventually, these layers are pressed together to make new rocks, called sedimentary rocks.
Flowing river water washes sand, clay and soil down towards the sea.
In some places, where a river meets the sea, this sediment piles up in layers that form a piece of new land called a delta.

In the Arches National Park in Utah, the center of this rock has been worn away to form a large, arched opening.
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