Landslides: Fast Changes on Earth

Unlike the slow changes you learned about in Chapter 4, some changes to Earth's surface happen very quickly — in just hours or even minutes! Gravity is a pulling force that acts on all objects. It is always pulling materials from high places to low places.

Sometimes gravity moves materials very slowly. Soil on a hillside might creep downhill just 1 to 2 centimeters per year — so slowly you can't even see it happening. But other times, gravity moves materials very fast.

A landslide is the rapid downhill movement of a large amount of loose rock and soil. During a landslide, huge amounts of earth can rush down a slope in just seconds. Landslides can move millions of kilograms of material, carrying trees, homes, and even cars along with them.

The key difference between slow changes and fast changes is time. Slow changes like weathering and erosion take thousands or millions of years. Fast changes like landslides can reshape the land in minutes!

Many things can trigger a landslide, but gravity always plays the main role. Without gravity pulling materials downhill, landslides wouldn't happen. Here are some of the most common triggers:

Natural Triggers

Earthquakes shake the ground and loosen rock and soil on slopes. The 1989 Loma Prieta earthquake in California triggered many landslides across the region. Volcanic eruptions can also send massive amounts of rock and ash sliding down mountainsides.

Heavy storms are one of the biggest triggers. Rain soaks into the ground, making soil heavy and slippery. When the ground can't hold any more water, the soil loses its grip and slides downhill. Fires can also set the stage for landslides — after a wildfire burns away plants and trees, there are no roots to hold the soil in place.

Human-Caused Triggers

People can accidentally cause landslides too. Building on slopes adds weight to hillsides. Clearing land by removing trees and plants takes away the roots that hold soil together. Even the natural freeze-thaw cycle can cause landslides — water seeps into cracks in rock, freezes and expands, splitting the rock apart over time until it breaks loose.

Landslides can be incredibly destructive. They can move millions of kilograms of soil and rock in just minutes. Everything in their path — trees, homes, cars, roads — gets swept away or buried.

La Conchita, California (2005)

In 2005, a massive landslide struck the small town of La Conchita, California. A huge section of hillside broke loose and crashed into the neighborhood below. The landslide damaged or destroyed more than 35 homes and tragically killed 10 people.

Why California?

Landslides are especially common in California because of the state's steep coastal cliffs and mountain ranges. The famous Highway 1 along the Big Sur coast has been closed many times due to landslides that sent parts of the cliff crashing into the Pacific Ocean.

Landslides don't just happen in California, though. They occur anywhere there are steep slopes — in mountains, along riverbanks, and near coastlines all around the world. The damage from landslides costs billions of dollars every year.

A flood happens when water overflows the sides of a stream, river, or drain. Like landslides, floods can change the land very quickly — sometimes in just a few hours.

How Floods Happen

When heavy rains fall, the ground absorbs as much water as it can. But when the rain is too heavy or lasts too long, the ground gets saturated — it simply can't hold any more water. The extra water runs off into streams and rivers, which quickly rise and overflow their banks.

Flooding in Cities

In cities, the problem can be even worse. Concrete, asphalt, and buildings don't absorb water at all. When heavy rain falls, all the water rushes into storm drains. If the drains can't handle the flow, streets and buildings flood.

Flood Damage

Floods are powerful land changers. Rushing water erodes soil quickly, carving new channels and washing away riverbanks. Floods can destroy bridges, roads, and trees. The Mississippi River flooding in the 1990s was so powerful it changed the course of the river in some places, washing away farmland that had been there for generations.

Sometimes floods and landslides work together to cause even more damage. When heavy rains soak sloped land, the water mixes with soil to create a mudslide — a fast-moving river of mud and rock that flows downhill like thick, heavy liquid.

How Mudslides Flow

Mudslides are different from regular landslides because the material is mixed with so much water that it flows like a river rather than sliding as a solid mass. A mudslide can travel at speeds of 20 miles per hour or faster, carrying boulders, trees, and debris with it.

A Dangerous Cycle

The materials left behind by mudslides can block drainage channels and streams. When these natural drains get blocked, water backs up and causes even more flooding. It's a dangerous cycle — the mudslide causes more flooding, which can cause more mudslides.

San Bernardino, California (2003)

In 2003, a devastating mudslide struck San Bernardino, California after heavy rains soaked hillsides that had been stripped bare by recent wildfires. Without trees and plants to hold the soil, the rain turned the slopes into rivers of mud that poured through neighborhoods, burying homes and blocking roads.