ShareChanging Earth's Surface - It's Only a Matter of Time...!
Slow changes
So, you've decided that a canyon would be an interesting landscape feature for your backyard. Choose a path, take the garden hose to simulate a river, turn it on and wait. How long? You'll wait a long, long time. Weathering and eroding a canyon, butte, valley or arch takes more than years, more than decades or even centuries. You can visit a favorite geological location as an old person and it will look essentially the same as it does today. The rate of these changes is imperceptible; meaning you can't perceive or see it. We simply don't live long enough.
Mountains can be growing as you will see later. Here we watch them shrink by weathering and erosion - slowly, imperceptibly, over time.
- Young mountains are steep. Their tops can be above the timberline and have snow year round. The Himalayas are young mountains.
- Middle-aged mountains take on a rounded shape. Trees may go right up to the top.
- Old mountains are well-worn. They look more like hills. The Appalachians are old mountains.
How old is that stream?
Streams
also tell their age. A young stream runs quickly and in a straighter path.
Old streams meander, or wind. They run more slowly. What about the image
above and to the left? Notice the twists and turns. You don't see rushing
water. The canyon walls are steep. This took some time. What's your verdict?
Other changes are sudden. Avalanches, landslides, and flash floods can erode, or move land very quickly.
- Avalanche - a waterfall of snow and ice down a hill. They can travel over 100 mph effectively moving anything in their path: trees, rocks, soil, cabins, skiers, or cars.
Landslide - is essentially
the same as an avalanche but instead of snow and ice it is composed of
soil and rock. Landslides often happen after a fire. The fire destroys
the plants. Plant's roots had helped hold the soil and rock on the hill.
Without the roots gravity and spring rains pull the hill down.
Flash
flood - if more rain falls than can be soaked into the soil,
absorbed by plants or channeled through rivers and creeks, it floods.
Flash floods can erode more than 25 cm. of soil in a few hours.
