1. Summary

The Grand Canyon is an immense canyon in Arizona, USA. The canyon is 446 kilometres long, 29 kilometres wide and 1 mile deep. This video explains how a number of forces created this canyon over millions of years. Erosion, plateaus and the Colorado River all contributed to what we now know as the Grand Canyon.

2. Why Watch This Video?

1. Have you ever wondered how the Grand Canyon got to be so big?
2. Would you like to know how erosion works?
3. Have you ever been confused by plateaus?

3. Key Terms

Erosion: the wearing away of rocks, dirt and other particles from the Earth’s surface.
Tectonic: refers to the structure of the Earth’s surface as well as the changes that it goes through
Plateau: a part of land that is noticeably higher than the land that surrounds it, plateaus usually have steep sides and a flat surface.

4. Loose Ends

Loose end #1: The video did not go into much detail about how the plateau was formed before the Grand canyon was created. This Colorado Plateau was a result of igneous and metamorphic rocks forming almost 2 billion years ago. Between 70 and 30 million years ago, through plate tectonics, the region was uplifted, creating the plateau that the Colorado River would cut through.

Loose end #2: What is happening to the Grand Canyon today? The Grand Canyon is continuously being shaped and moulded by nature. It is continually getting deeper and wider because of the Colorado River and tributary streams.

Loose end #3: How does such a small river carve out such a large canyon? This is called ‘downcutting’. Downcutting is the process of a river carving out a canyon or valley. Downcutting happens as a result of flooding and big rocks and boulders being carried along, chipping off pieces of the riverbed as they are carried down. The Colorado River has a steep slope, high volume of water and flows through an arid climate (which allows for mechanical weathering of the bedrock). All of these factors increase the amount of downcutting, creating a large canyon.

5. Self-Test Questions

Question 1: How many tons of debris is carried by the Colorado River each day?
Answer a: 5,000,000
Answer b: 500,000
Answer c: 500
Answer d: 50

Question 2: Why is the Grand Canyon so deep?
Answer a: The Colorado River drops 10 feet for every mile and carries silt and sand that wore through the plateau.
Answer b: The Colorado River is the widest and fastest rive in North America, so it quickly eroded away the Grand Canyon.
Answer c: Melting snow ran down from the Rocky Mountains and cut through the rocks to create the Grand Canyon.
Answer d: An earthquake occurred millions of years ago and created the Grand Canyon.

Question 3: The Colorado River is…
Answer a: 10 Meters wide.
Answer b: The largest River in North America.
Answer c: 100 Meters wide.
Answer d: 1000 Metres long.

Question 4: Why does the Grand Canyon have so many layers of differently coloured rocks?
Answer a: The sun bleaches the higher rocks so they appear white but the lower rocks are shaded so they are not bleached.
Answer b: Millions of years ago, the Grand Canyon was a mountain range. Erosion wore away the mountain in layers.
Answer c: Due to the rise and fall of water over millions of years, sand, mud and ancient seashells were compressed into colourfully layered rocks.
Answer d: The Grand Canyon is made of multicoloured metamorphic rocks.

Question 5: How was the Colorado River created?
Answer a: After the Ice Age, glaciers melted and flowed together to create the Colorado River.
Answer b: Over millions of years, lakes joined together and flowed through Colorado.
Answer c: Oceans overflowed into the Grand Canyon and the River was born.
Answer d: Melted snow from the Rocky Mountains pooled and joined together.

6. Answers to Self-Test Questions

1. Answer b is correct. The Colorado River drops 10 feet for every mile that it travels and carries 500,000 tons of snd and silt downstream. This makes the river like a ‘liquid sandpaper’ that wears away the plateau.

2. Answer A is correct. The Colorado River does drop 10 feet for every mile that it travels which cut through the plateau similarly to how a waterfall wears away at a cliff.

3. Answer c is correct: The Colorado River is 100 metres wide, much smaller than many other rivers that have not created canyons the size of the Grand Canyon.

4. Answer c is correct. 1 billion years ago, ancient oceans covered the area that is now the Grand Canyon and the rise and fall of these waters deposited different layers of sediment that we see now as differently coloured layers in the canyon.

5. Answer d is correct. 5 or 6 million years ago, the Colorado River was created from melting snow running down from the Rocky Mountains.

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Item Credit: It's OK to Be Smart (PBS)

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