Erosion is the way moving water, wind, ice and gravity pick up bits of rock and soil and carry them somewhere else. It is how nature digs. A river carved the Grand Canyon in Arizona, which is 446 kilometres long and more than 1,800 metres deep. Ocean waves cut the Twelve Apostles, a row of rock towers off the coast of Australia. Giant glaciers helped shape Yosemite Valley in California. In the 1930s, wind blew away so much farm soil on the American Great Plains that the time is remembered as the Dust Bowl. Erosion is usually slow, but over thousands and millions of years it can move whole mountains, one grain at a time.
First, one useful word. Weathering is when rock breaks or crumbles but stays where it is. Erosion is when those broken pieces get carried away. The two often work as a team: weathering cracks the rock, and erosion hauls the pieces off.
Water Erosion: The Colorado River and the Twelve Apostles
Water is a powerful digger. Rain washes soil down hills. Rivers roll pebbles and sand along their beds, and those pebbles scrape the riverbed like sandpaper. Waves smash against the coast again and again.
How a river carved the Grand Canyon
The Grand Canyon is in Arizona in the United States. It is 446 kilometres long and up to 29 kilometres wide. At its deepest it drops 1,857 metres, more than a mile, from the rim to the river.
The Colorado River did the digging. Many geologists think the river took its course through the area about 5 to 6 million years ago. At the same time, the land around it, called the Colorado Plateau, was slowly pushed upward. As the land rose, the river kept cutting down. Its smaller side streams cut too, and the cliffs crumbled back. Bit by bit, the canyon grew deeper and wider.
The canyon walls are like a giant layer cake of rock. Some of the layers at the bottom are close to two billion years old. Each stripe of colour is a different kind of rock laid down at a different time.
How waves made the Twelve Apostles
The Twelve Apostles stand in the sea off Port Campbell National Park, beside the Great Ocean Road in Victoria, Australia. They are tall stacks of limestone, a fairly soft rock. That limestone formed on the seabed between about 15 and 5 million years ago.
Storms from the Southern Ocean keep pounding the cliffs. First the waves hollow out caves. Then a cave is worn right through a headland to make an arch. Finally the top of the arch falls in, leaving a lonely tower in the water. Some of these stacks are up to 50 metres tall, about as high as a 15-storey building.
Here is a surprise. There were never twelve of them. People think there were nine stacks at most. The name was picked to attract visitors. The sea is still at work, too. In July 2005, a 50-metre stack fell into the ocean. Another collapsed in 2009, so seven are left. Scientists expect the waves to cut new stacks out of today’s cliffs in the future.
Wind Erosion: The Dust Bowl of the 1930s
Wind cannot lift a boulder, but it can lift dry, loose soil and sand. Wind erodes fastest where the ground is bare, with no plants to hold it down. The Dust Bowl on the American Great Plains shows how that works.
The Great Plains are a huge, flat, dry region in the middle of North America. They were once covered in deep-rooted prairie grass. Those roots held the soil together, even in dry years. Then, from the late 1800s, settlers ploughed up the grass to grow wheat. New tractors made it possible to plough much more land, much faster.
In the 1930s, severe drought struck in three waves, in 1934, in 1936 and in 1939–1940. With the grass gone, nothing held the dry soil. The wind lifted it into giant clouds called “black blizzards”. Some were so thick that people could see only about one metre in front of them. Dust from the Plains blew all the way to New York City and Washington, D.C.
The worst day was 14 April 1935, now called Black Sunday. That day a cold front from Canada whipped up a dust cloud that raced south across the Plains at about 97 kilometres an hour. A news editor named Edward Stanley used the words “Dust Bowl” in a story about that storm, and the name stuck.
The drought and erosion hit about 400,000 square kilometres of land. That is bigger than the whole country of Germany. The Texas and Oklahoma panhandles suffered most. Tens of thousands of families lost their farms, and many moved west to California looking for work.
Ice Erosion: Yosemite Valley and the Hoodoos of Bryce Canyon
Ice erodes in two ways. Huge glaciers grind and scrape across the land like bulldozers. And tiny bits of ice in cracks can split rock apart.
Glaciers in Yosemite Valley
Yosemite Valley is in the Sierra Nevada mountains of California. It is about 12 kilometres long and 910 to 1,070 metres deep. Granite giants surround it, including Half Dome and El Capitan, a granite cliff about 914 metres tall.
Starting about 2 to 3 million years ago, the Sierra Nevada went through a series of ice ages. At least four big rounds of glaciers flowed through the mountains. The largest, called the Sherwin, filled Yosemite Valley and ended about 1 million years ago. Scientists think a Sherwin glacier did much of the big scraping and shaping of the valley. Some glaciers in the area were up to 1,200 metres thick.
Glaciers left clues behind. Side valleys that the big glacier did not cut so deeply were left hanging high above the main valley floor. Today their streams pour over the edge as waterfalls. Yosemite Falls drops 739 metres in three steps, making it one of the highest waterfalls in North America.
Frost wedging at Bryce Canyon
Bryce Canyon National Park is in southern Utah. Despite its name, it is not really a canyon. It is a row of giant bowls, called amphitheaters, eaten into the edge of a high plateau. The biggest, Bryce Amphitheater, is 19 kilometres long, 5 kilometres wide and 240 metres deep. It is packed with thin orange and white rock spires called hoodoos. Some are 60 metres tall.
The main tool here is ice. Bryce sits more than 2,400 metres above sea level, so it is cold at night. Melting snow trickles into cracks by day. At night it freezes, and freezing water swells by almost 10 percent. That pushes the crack a tiny bit wider. Bryce gets more than 200 of these freeze-and-thaw cycles every year. Then summer rainstorms wash the loose bits away. Many hoodoos wear a cap of tougher rock called dolomite, which protects the softer rock beneath like a hat.
The Paiute people have lived near Bryce for a long time. One Paiute story tells that the hoodoos were the Legend People, turned to stone by the trickster Coyote for behaving badly.
Gravity Erosion: Rockfalls in Yosemite and the 1903 Frank Slide
Gravity is the quiet partner in every kind of erosion. It pulls rivers downhill and makes glaciers slide. But sometimes gravity works on its own. Rock and soil simply fall, slide or creep down a slope. Scientists call this mass wasting.
Back in Yosemite Valley, gravity turns out to be a bigger digger than ice. Geologists think only about 10 percent of the valley’s widening, and about 12 percent of its digging out, came from glaciers. Most of the widening came from rockfalls. The granite walls are full of long, straight cracks called joints. Chunks of rock break off along those cracks and crash down to the valley floor. In 1996 a rockslide struck near Happy Isles, at the eastern end of the valley.
Sometimes gravity acts in seconds. On 29 April 1903, at 4:10 in the morning, about 110 million tonnes of limestone broke away from Turtle Mountain in Alberta, Canada. Witnesses said the rock crossed the valley in about 100 seconds and buried part of the coal-mining town of Frank. It is still the deadliest landslide in Canadian history. Between 70 and 90 people died.
Even the Twelve Apostles show gravity at work. Waves undercut each stack at the bottom. Gravity does the final job when the heavy top crashes down.
Terraces and Windbreaks: How People Slow Erosion Down
People cannot stop erosion, but they can slow it a lot. The trick is to keep soil covered and to stop water and wind from moving too fast.
Trees against the wind on the Great Plains
After the Dust Bowl, the United States government tried something big. In 1934 President Franklin D. Roosevelt began the Great Plains Shelterbelt. Workers planted long rows of trees along the edges of farms. The trees acted as windbreaks. They slowed the wind and kept moisture in the soil. By 1942, 220 million trees had been planted. They stretched in a belt about 160 kilometres wide, from North Dakota near the Canadian border all the way to the Brazos River in Texas.
Farmers also learned new ways to plough and plant. These included contour ploughing, where the furrows curve around a hill instead of running straight down it. They also rotated crops and planted in strips. By 1938, these efforts had cut the amount of blowing soil by 65 percent. A new government office, the Soil Conservation Service, helped spread the ideas. Today it is called the Natural Resources Conservation Service.
Steps on the mountains of the Philippines
On a steep slope, rain rushes downhill and takes soil with it. Terraces turn one steep slope into many flat steps, so the water slows down and the soil stays put.
Some of the most amazing terraces in the world are on the island of Luzon in the Philippines. The Ifugao people built rice terraces into the mountains around Banaue. They built stone or mud walls and carved the steps to follow the natural curves of the hills. Water from the forests on the mountaintops runs down through the terraces to the rice fields. In 1995 the Rice Terraces of the Philippine Cordilleras became a UNESCO World Heritage Site.
The terraces need care. When the walls are not repaired, the steps themselves wear away.
Next time you see a muddy river after a storm, you are watching erosion carry the land downhill. To see where all that digging can lead, explore the geography of the Grand Canyon, walk among the hoodoos in our Bryce Canyon National Park guide, or find out how ice moves mountains in glaciers for kids.