Land, Water and Rocks: Earth's Surface

Look at a globe and turn it slowly. Blue spreads across much of it. Earth's surface is the outside of our planet: its land and water.

How much is water?

Water covers about 71 out of every 100 parts of Earth's surface. Land covers about 29 parts. Think of 100 equal floor tiles seen from above. About 71 would show water, and 29 would show land.

You can make a picture of this. Draw 10 rows of 10 boxes. Color 71 boxes blue. Count the boxes left over:

Color those 29 boxes green. About 7 of every 10 parts are water. Your picture shows the outside area of Earth, spread out flat.

Oceans and continents

An ocean is a huge body of salty water. We name five: Pacific, Atlantic, Indian, Arctic, and Southern. The Pacific is the biggest. The five oceans join together, so water can move between them.

A continent is one of Earth's very large land areas. We count seven: Africa, Antarctica, Asia, Australia, Europe, North America, and South America. Find them one at a time on a globe. A continent can have many countries. Africa is a continent; the countries within it are smaller parts.

Land is made of rock and soil. You often see soil on top, with rock below. Rock also lies under the oceans. The water has a rocky floor beneath it.

Try a globe count

With a friend, gently toss a soft globe 10 times. After each catch, check the place under your friend's right thumb. Make a mark for land or water. One class counted 7 water touches and 3 land touches. Your count might be 6 water touches, or 8. Ten catches sample only a few spots, and luck changes the count.

Your town might have streets and fields in every direction. That view shows only one small place. A globe lets you look beyond your town and see the large oceans.

Land can rise, dip, spread out flat, or reach into water. A landform is a natural shape of land. Look at its height, sides, and surroundings to name it.

Look at the shape

A plateau and a plain can both look flat on top. Look at the edges too. A plateau stands high; a plain is a broad, flat area. There is no single height that separates all hills from mountains. Mount Everest in Asia rises about 29,000 feet (about 8,850 meters) above the sea. A mountain can have a bare top without snow.

Where land meets the sea

The coast is the meeting place of land and sea. A beach is a strip of sand or pebbles beside water. A cliff is a very steep rock wall. A headland is high land reaching out into the sea.

An island has water all around it. Ireland and Madagascar are islands. A peninsula has water on most sides but stays joined to bigger land. Florida and Italy are examples. To tell them apart, follow the edge. Can you go all the way around by water? An island's land continues down under the water; it does not float.

More land shapes

Wind piles sand into hills called sand dunes. A volcano is a place where hot melted rock, ash, and gas can come out. Many volcanoes build mountains. A moor is open, high ground with grass and low bushes. A bog is soft, wet ground made from old plants.

Try these clues: “A stream runs along low ground. Steep green hills stand on both sides.” First notice that the ground is low. Next notice the hills around it. Low land between hills is a valley. The stream helps you picture the place, but the low ground between the hills gives it its name.

A body of water is a place where water gathers on Earth's surface. Some water spreads out in a basin. Some travels along a channel. You can use both shape and movement to name it.

Salt water and fresh water

Salt water contains a lot of salt. Oceans and seas hold salt water. Fresh water has very little salt. Streams, rivers, ponds, and most lakes hold fresh water. “Fresh” tells you about salt, not whether water is clean enough to drink. Drink water that a grown-up says is safe; never taste outdoor water to test it.

Oceans are the largest bodies of water. A sea is a smaller area of salt water, usually part of an ocean near land. The Caribbean Sea is one example. A bay is a part of the sea that curves into the land.

Water with land around it

A lake has land all around it. A pond is smaller and usually shallower. Lake Superior is so wide that you cannot see across it, but land still surrounds it. Most lakes hold fresh water. The Great Salt Lake in Utah holds salty water and is still a lake.

Water on the move

A river is a long path of flowing water. A stream is a small river. Water flows from higher ground to lower ground. A river can reach a lake, a bigger river, or the sea. The Amazon, Nile, and Mississippi are large rivers. A waterfall forms where flowing water drops over a rock ledge.

A wetland is land soaked or covered by shallow water for much of the year. Marshes and bogs are wetlands. Reeds, frogs, and birds can live there. Wetlands mix clues from land and water: you might see plants growing right through shallow water.

Put the clues together

Picture this: “A small pool sits in a field. Land surrounds it, and the water stays in the pool.” The small size and enclosed, still water point to a pond. Now picture a narrow ribbon of water traveling down a hillside. Its moving water and channel point to a stream. Size alone is not enough: look at the whole shape and how the water moves.

You can learn about land without traveling far. A schoolyard, park, or view from a window gives you clues. Look for high ground, low ground, water, and things people have built.

Make a little place record

Choose a familiar place to look at with a grown-up. Near water or a steep edge, stay with them and keep back from the edge. Draw what you can see. Add names beside the main parts: “hill,” “pond,” “stream,” or “flat land.” You can take a photo and add labels later. Describe the ground too. Is it rocky, sandy, or covered with soil?

Add the living things you notice. Trees may cover a hill. Reeds may edge a pond. Heather, a low plant with small flowers, may grow on a moor. These details make your description more useful than a list of names alone.

Natural or built?

A natural feature comes from nature. Hills, rivers, and cliffs are natural features. A built feature was made by people. Bridges, roads, dams, and canals are built features.

A canal may look like a river, but people dug its channel. A reservoir is a lake people made to store water. To sort a feature, ask how it got there. Having water does not make every feature natural.

Compare two shores

Imagine standing on a sandy beach, then looking at a rocky shore.

Barnacles and seaweed cling to shore rocks. When the sea moves back at low tide, water can stay in hollows as rock pools. Both places are coasts. Their ground and small homes for living things differ.

Compare near and far

Try describing a flat town beside a river and a village in mountains. Start with one shared feature: “Both places have fresh water.” Then name a difference: “The town is flat, but the village has steep slopes.” Match the same kind of clue in both places. Comparing their ground tells you more than saying one place has a shop and the other has a stream.

A map is a flat drawing that shows where things are. Imagine looking straight down from above a place. A map keeps the main shapes and leaves out small details, such as moving cars.

Find what the marks mean

A map key, or legend, explains the colors and symbols. Read it before naming a shape. Many maps use blue for water and green or brown for land. A triangle might stand for a mountain. Another map can choose different marks, so its own key matters.

Use these steps to read a land-and-water map:

1. Read the key to find the water marks.
2. Find water shapes and water lines on the map.
3. Look at what surrounds each shape.
4. Use the key and the shape together to name it.

A blue line may show a river. A blue shape inside land may show a lake. Land surrounded by water is an island. Follow the boundary with your finger so you can see what surrounds what.

Read an island map

Picture a key that says blue means water, green means land, and brown triangles mean mountains. A green shape sits inside blue water. It is an island. Two triangles sit on the island, so it has mountains. A blue line runs from the mountains to the coast: a river. A small blue shape lies inside the island: a lake.

The blue line stands for real water. You would not see a painted blue line if you visited the place. The map uses simple marks to help you find things.

A globe and a flat map

A globe is a round model of Earth. It shows Earth as a ball. A world map shows Earth flat, letting you see all its regions at once. Making a round surface flat stretches some places. Greenland looks too large on many world maps. Use a globe when comparing the sizes of large places.

The top of a map does not mean higher ground. A north arrow can show north at the top. Rivers flow downhill, not toward the bottom of every page. The Nile flows north toward the sea and runs up the page on many maps.

You can show the same place in two ways. Draw its shapes from above to make a map. Build its hills and hollows to make a model you can touch. A model is a simpler version that shows what something is like.

Draw a park

Imagine this place: “A hill stands on the left. A stream runs down to a pond on the right. A path goes around the pond.”

1. Choose a mark for the hill, such as a brown triangle. Put it on the left.
2. Draw a blue pond shape on the right.
3. Join them with a blue stream line.
4. Add a dotted path around the pond.
5. Draw a key: triangle means hill, blue means water, dots mean path.

Keep the key beside the map so your friend can read it. A symbol means what your key says. The triangle here means hill; on another map it may mean mountain.

See from above

A pond seen from above has a shape with an edge. A line shows the narrow route of a stream. A house seen from above shows its roof, so a simple square can stand for it. Drawing its door and front windows would show a side view instead.

You do not need exact scale. Put features in the right places and show their main shapes. A large pond should look bigger than a small puddle. A path beside water should stay beside it on your drawing.

Build a tray model

Shape clay or damp sand on a tray. Make the hill higher than the pond area. Press a groove from the hill toward the low ground. Lay blue paper in the groove and pond hollow. If you use a little water instead, watch it collect in low places. Add small labels for the hill, stream, and pond.

Look at your model from above and compare it with your map. Both should show the hill on the left and pond on the right. The model also lets you see their height difference from the side.

Try it with a friend

Ask a friend to find the pond using your map. If your friend points to the path, check your key. Maybe the water mark is missing, or the lines look too alike. Change the confusing mark, then try again.

Pick up a small rock and look closely. You might see shiny specks, stripes, tiny holes, or a smooth edge. Rocks have many clues you can compare.

What makes it a rock?

A rock is a solid, natural material from Earth. Rock lies under soil and under the sea. Mountains and cliffs let you see large areas of it. Rocks are not alive. They do not eat, breathe, or have young.

People make bricks and concrete using materials from Earth. A brick is made by people, so it is different from a natural rock. Shape alone will not tell you: some natural rocks have flat faces too.

Look, feel, and test

Compare two small rocks. Use a magnifier to look at their color, specks, and stripes. Feel each surface with your fingers. Hold one in each hand and notice how heavy it feels for its size. Then gently rub a coin across each surface. Look for a real scratch in the rock, not just a mark left by the coin.

Hardness tells how easily a material scratches. Texture tells how its surface feels. A smooth rock can be hard or soft. Feeling smooth does not settle the scratch test.

Here are two example rocks:

Both are solid, natural materials. Rock B is softer in this test because the same coin scratches it. Saying “A is rough” describes texture, not the result of the hardness test. Rock B also leaves a white mark when rubbed on dark paper.

Choose a sorting rule

Make one pile for rough rocks and one for smooth rocks. A friend might sort those same rocks into light and dark piles. You can both sort correctly: you used different rules. Say your rule before you sort, then use it for every rock.

A boulder is a very big rock. A pebble is a small, smooth stone. These names tell you size. If a boulder breaks, its pieces are still made from the same rock. Rocks vary in weight and hardness; some are soft enough for a coin to scratch, and some can even float.

A small grain of sand can begin as part of a much bigger rock. Water, wind, and ice break rocks over time. Pieces can become smaller as they knock and rub together.

Compare the sizes

Put these pieces in order from biggest to smallest: boulder, pebble, sand grain, silt grain, clay grain. Gravel is a loose mix of pieces bigger than sand. Pebbles can be part of gravel.

A sand grain can be as wide as about 2 millimeters, less than 1/10 of an inch. A magnifier helps you see the separate grains and compare their shapes.

Sand has different beginnings

Most sand grains are bits of rock. Many beaches have tan or white sand. Some Hawaiian beaches have black sand from dark rock. Other beach grains come from shells or coral. The color gives a clue about what the sand is made from.

Sand is found in deserts and riverbeds as well as at the beach. A handful of sand is loose grains, while a larger rock is one solid piece.

Separate a mixture

Try a sieve, a tool with small holes, over a bowl. Use holes that let sand through but hold back gravel. Put a spoonful of the mixture in the sieve and shake gently. Sand falls into the bowl. Gravel stays above because its pieces do not fit through the holes.

You can also watch grains settle in water. Put soil and water in a lidded jar. Close it, shake, then leave it still. Larger grains such as sand settle near the bottom first, often within about a minute. Silt settles later. Tiny clay grains can keep the water cloudy for hours or longer.

The jar separates grains that were already there. It does not turn sand into clay. In nature, breaking larger pieces into finer ones takes much longer than shaking a jar.

Look under fallen leaves in a garden. You might find crumbly soil, roots, a worm, and bits of old leaves. Soil is the loose top layer of ground where plants grow.

What is in soil?

Soil mixes tiny rock pieces with remains of plants and animals. It also holds water and air. In healthy garden soil, about half the space is solid bits. The other half is small gaps that hold air and water. Roots and worms use those gaps.

Humus is the dark, rotted material from once-living things. It adds nutrients plants need. Worms dig through soil and mix it as they move. Many smaller living things are there too, even when you cannot see them.

Feel three soils

Loam mixes sand, silt, clay, and humus. Many plants grow well in it. Mud is soil with lots of water. Clay soil can become hard as it dries, so compare samples while they are equally damp.

Watch water pass through

Put sandy soil, loam, and clay soil in three cups with holes below. Use equal amounts of soil. Pour the same amount of water into each cup and catch what comes out.

In one class's test, water first came through the sand after 10 seconds. It came through loam after 30 seconds and clay after 90 seconds. The sand had the shortest wait. Big sand grains leave larger gaps for water to pass through. Tiny clay grains leave smaller gaps. These were that class's results; your test gives you your own observations.

Rock, sand, and soil

A rock is one solid piece. Sand is loose grains, many made from rock. Soil holds rock bits plus once-living material, air, water, and life. It is more than crushed rock.

Worms and beetles make homes in soil. Crusty patches called lichens grow on rocks. Barnacles cling to seaside rocks. New soil forms very slowly; an inch often takes hundreds of years. Plants that hold soil in place help protect this slow-growing layer.

Look at a brick wall, a window, and a garden. Their materials have links to the ground. People use rock, sand, clay, and soil in different ways.

Where materials come from

A natural resource is something from nature that people use. Rock, water, soil, and wood are resources. People cut or blast rock from a quarry, a large open pit. A mine is a place where people take useful materials from Earth. Some mines are open pits; others have tunnels underground. People also collect sand and gravel from riverbeds, the bottoms of rivers.

Match the material to its job

Follow the materials in a house. A brick begins as clay. People shape it and bake it until hard. A glass window begins with sand heated until it melts. A concrete path mixes sand, gravel, cement, and water. Garden vegetables grow in soil. Each material helps with a different job.

Think about the place left behind

The environment is the land, water, air, and living things around us. Digging changes it. A quarry leaves a big hole and brings noise and dust. Taking gravel from a riverbed can change the river and harm animals' homes. Removing trees leaves bare soil for wind and rain to carry away.

People can help too. They can reuse old stones and bricks, plant trees, and care for land after digging ends. Some old quarries fill with water and become lakes for birds and fish.

Imagine that a wall comes down. The bricks are still sound. Reusing them in another wall means those bricks do another job. Fewer new bricks need to be made from fresh clay.

A planted quarry can become a useful home for wildlife. The missing rock still took a very long time to form. It does not grow back like grass after mowing.

Ocean waves, a snowy mountain, and water under your feet all belong to Earth's water. Some water is liquid. Some is solid ice. Much of it is hidden from view.

Liquid water and ice

Liquid water fills oceans, seas, rivers, lakes, and ponds. It also fills tiny spaces underground. Rain can soak through soil and enter gaps in the rock below. This hidden water is groundwater. Wells reach down to it.

A glacier is a huge mass of ice on land that slowly moves. An ice sheet is a giant glacier spread over a very wide area. Earth's two ice sheets cover Antarctica and Greenland. Snow on high mountains and ice on frozen lakes are water too.

Where most water is

Imagine collecting all of Earth's water into 100 equal cups. Nearly 97 cups would be ocean water. Frozen water would fill a little less than 2 cups. Groundwater would fill most of the small amount left. Lakes and rivers together would hold less than one teaspoon in this picture.

The cups give a rough picture of amounts. Almost all the water is in oceans; the lakes and rivers we see hold only a tiny share. Do not confuse this picture with the 100-box surface picture. The boxes compare how much outside area is covered. The cups compare amounts of water.

Most of Earth's fresh water is frozen. About 2 of every 3 parts of fresh water are ice. That compares fresh water alone. Ocean water is salty and is not included in those three parts.

Two icy places

The Arctic Ocean in the far north has floating sea ice over part of it. Antarctica in the far south is land under a thick ice sheet. Both look icy, but the ground beneath is different: ocean in one case, land in the other.

Picture a list that names only oceans, ponds, and rivers. Add a glacier, groundwater, and mountain snow. Now the list includes solid water and water you cannot see on the surface. Earth's large water supply includes salty water and ice, so only a small share is easily available as fresh liquid water.

An ice cube keeps its shape when you put it on a plate. Liquid water spreads out and flows. Both are water, but they are in different forms.

Solid or liquid?

A solid keeps its own shape. A liquid flows and takes the shape of its container. Ice is solid water. Freezing changes liquid water into ice. Melting changes ice into liquid water.

Fresh water freezes at 32°F (0°C). Ice melts when it warms above that temperature. At 25°F, it is cold enough for fresh water to freeze. At 40°F, ice can melt. Freezing takes time; water does not become a block of ice the moment the air turns cold.

Snow is tiny pieces of ice. When mountain snow melts in spring, the liquid water enters streams. That extra water can make rivers rise.

Watch a change go back

Put some water in a freezer-safe open cup. Mark its water level. Leave room above the mark, because water takes up more space as it freezes. An adult can help place it in a freezer. After it freezes, the ice reaches above the old line.

Starting with 100 milliliters of water gives about 109 milliliters of ice. The water's weight stays the same; its space increases. Let the ice melt in the same cup without spilling any. The water returns to the original level. You changed its form and then changed it back.

Why ice floats

Compare a piece of ice with liquid water filling exactly the same space. The ice weighs less. That is why ice floats on liquid water. The ice did not lose water when it froze: the same water spread into more space.

You can see this in a drink. Ice cubes float at the top. A pond also freezes first at its surface. Liquid water can remain below the ice, where fish can live.

A glass holding only melting ice can gain liquid water while the ice shrinks. The ice is turning into that water. Freezing and melting can happen outdoors as well as in a kitchen.

After rain, look for puddles in low places. Water gathers there, runs along gutters, or soaks into soil. A drop can move from your neighborhood to a river far away.

Downhill and into the ground

Rain and snow fall from clouds. Liquid water runs downhill into streams and rivers. Many rivers carry it to the sea. Some rain soaks into soil and joins groundwater.

Follow this example drop: it lands on a street and joins a puddle. The puddle spills into a drain that leads to a stream. That stream reaches a river, which flows to the sea. The route joins small nearby places to bigger bodies of water.

Up into the air and back

The Sun warms water in oceans, lakes, and puddles. Some water enters the air as water vapor, a gas you cannot see. Higher up, water vapor cools and water forms clouds. Rain or snow can fall again.

A journey that goes around and around is a cycle. Water can go through the cycle again. When a puddle dries, its water has moved into the air. It has not stopped being part of Earth's water.

You can notice clues after rain. A puddle may grow, then shrink later. A stream may run higher and faster than before. Different drops can take different routes: some soak in, while others run over the ground.

People and living things need water

We drink water, wash with it, and use it to grow crops. Factories use water, and boats travel on it. Plants and animals need it too.

What gets into flowing water can travel with it. Imagine someone drops oil beside a street drain. Rain can wash it into a stream, then a river. Water farther along the route can become dirty too. Keep trash and oil out of drains and streams.

You can use water carefully by turning off a tap when you finish. Earth's water keeps moving through its cycle, but one place can still lack clean water. A nearby river needs care even when a map shows big oceans elsewhere.

A brown stream after rain carries tiny bits of land. Moving water can loosen pieces, carry them away, and drop them somewhere else. Both the starting place and the landing place can change shape.

Wearing away and carrying

Erosion is the wearing away and moving of rock and soil by water, wind, or ice. A river's water rubs against its bed. Sand and pebbles in the water scrape the rock too, like sandpaper. Over many years, the channel can grow deeper and wider, making a valley or canyon.

Rain washes soil down bare slopes. Waves strike the coast and wear cliffs away. Pebbles roll and bump in water, gradually becoming rounder and smoother.

What happens when water slows?

Fast water can carry bigger pieces than slow water. A gentle stream may carry sand and silt. Stronger flow after rain can move pebbles too. When water slows, it drops bigger pieces first. Finer pieces can travel farther.

Picture a river reaching the sea. The flow slows, and sand and mud settle. They can build new land called a delta. The Nile Delta in Africa is an example. Waves can also move sand onto a shore and build a beach.

Ice can break rock in place

Water can enter a crack in a rock. When it freezes, it takes up more space and pushes on the sides. Repeated freezing can widen the crack until pieces break off. Breaking rock where it is is called weathering.

Follow the two parts of this example. Ice first breaks a chip from a rock in place: weathering. Meltwater later carries the chip downhill: erosion. Breaking and moving are linked, but they name different parts of the change.

Moving ice scrapes land

A glacier slowly slides over land. Rocks carried with the ice scrape the ground below it. Over time, this can carve a wide valley. The ice moves slowly, but it carries hard pieces that do the scraping.

Look at where material went, as well as where it came from. A worn bank and a new pile of sand can be parts of the same journey.

On a windy beach, sand can hop across the ground. Wind is moving air. It can pick up loose bits of land and carry them to a new place.

Which ground moves easily?

Dry, bare ground is easy for wind to change. Tiny dust can rise high into the air. Heavier sand grains hop and bounce near the ground. Wet ground holds together more, so wind cannot lift it as easily.

Water and wind can both carry land. Moving water can carry heavier pieces, such as pebbles. Wind mostly carries sand and dust. When either slows, it drops pieces.

Build a dune

Imagine wind blowing sand along a beach. A bush slows the moving air. Some sand drops beside it. More grains arrive and pile up. A hill of sand made by wind is a sand dune.

The wind can shift a dune little by little. It takes grains from one place and drops them farther along. The whole hill can slowly change its position. Dunes stand behind many beaches and in deserts such as the Sahara in Africa.

Blowing sand can change rock too. Grains strike the surface again and again. Each hit is small, but many hits can smooth or carve rock over a long time.

Compare two fields

Picture two dry fields on a windy day. One field is bare. The other has growing plants. Dust rises from the bare field and lands far away. The field loses soil that crops need.

Plants help in two ways. Stems and leaves slow air near the ground. Roots hold the soil together below. The planted field can lose much less soil than the bare field.

A quiet-looking dune may still be moving slowly. One windy day might change it too little for you to notice. Repeated winds can add those small changes together over years.

A tray of sand can show how water cuts a channel and carries grains. A model is smaller and simpler than real land. It helps you see a change closely.

Pour water down a slope

Put damp sand in a tray. Raise one end on books. Slowly pour a cup of water at the high end. Watch for three things: a channel forming, sand traveling downhill, and a pile forming where water slows.

To test the slope, try again with the tray raised higher. Rebuild the same sand shape and use the same amount of water poured the same way. Change just the slope. If you change the water too, either change might explain the new result.

Compare bare soil and grass

Use two trays with equal soil and the same slope. Leave one bare and cover the other with growing grass. Pour equal amounts of water at the same speed. Catch the water and soil leaving each tray.

One class collected 6 spoonfuls of soil from the bare tray and 1 from the grass tray. Compare the amounts:

The grass tray lost 5 fewer spoonfuls in that test. Roots held soil, and grass slowed the water. Write your own results when you try it. Repeating a test helps you check whether the same pattern happens again.

Watch a wind model

Outdoors, put a low pile of dry sand on a tray with a block nearby. With a grown-up, wear safety glasses and use a fan on low, keeping blowing sand away from faces. Keep the fan and its cord dry and away from the water test. Watch grains move and pile against the block where the air slows.

You can test the strength of flowing water too. Place sand, pebbles, and a small stone on a tray. A slow pour may move sand first. A faster pour can move pebbles. Keep the slope and starting pieces the same.

Say what the result means

Try this report: “We poured equal water over bare soil and grass. More soil left the bare tray. Growing plants can help hold soil on a hillside.” A sandy model can change in minutes. Real rock is harder, and a real hill is much bigger. The tray shows how a change happens; it does not tell how long a canyon takes.

A river can wash soil from a park. Wind can carry soil from a field. People compare different ways to slow the movement and keep useful land in place.

Plants and wind barriers

Roots hold soil together. Leaves and stems slow wind and rain. Grass can protect slopes; shrubs can protect riverbanks. A windbreak is a row of trees or bushes that slows wind across a field.

A sand fence slows wind on a beach. The slowed air drops sand beside it, helping a dune grow. The fence works by changing the air's movement.

Ways to hold water back

A dike, also called a levee, is a long earth bank that holds water away from low land. A seawall is a strong coastal wall that blocks waves. Large rocks along a shore or riverbank can help keep water from washing land away.

These do different jobs. A field losing dry soil needs help with wind. Low land beside a river may need help holding floodwater back. Start by naming the problem at the place.

Compare two plans

A river is wearing away a park bank. One plan uses big rocks. Another uses willow trees.

The park needs quick help at a sharp river bend. Rocks can protect that spot now. Trees can grow along the rest. A mixed plan can meet different needs in different parts of the park.

Test and keep checking

Try small designs in a tray: a clay wall, a row of pebbles, and a strip of grass. Give each the same slope, sand, and water. Compare how much sand remains or washes out.

Ask how well each design works, how much it costs, how long it takes, and how it affects living things. People also inspect and repair real barriers. A large storm can damage a dike. A seawall can change where waves hit nearby beaches. A design that works today may need care later.

A hillside can look different by the end of one morning. Some Earth events happen fast enough for people to watch them change the land.

Four fast events

A volcano erupts when hot melted rock, ash, and gas come out of the ground. Melted rock flowing outside is called lava. Lava cools into new rock. An eruption can build land or remove part of a mountain. A volcano can also stay quiet for many years between eruptions.

An earthquake is sudden ground shaking. Shaking may last seconds, or minutes for a very big event. It can crack the ground and start rocks sliding.

A landslide happens when rock and soil slide down a slope. Heavy rain or an earthquake can start one. A hillside can change within a minute.

A flood covers land that is usually dry. Heavy rain or quickly melting snow can make a river spill over. A flash flood can begin within hours of heavy rain. Floodwater can remove soil and leave mud elsewhere. Storm waves can change a beach overnight too.

A mountain changed one morning

Mount St. Helens is a volcano in Washington State. On May 18, 1980, a huge landslide and blast removed its top. Before the eruption, its height was 9,677 feet. Afterward, it was 8,363 feet.

Find how much height was lost:

The mountain lost 1,314 feet, about 400 meters, in height. That change happened in one morning. It is a clear example of a fast event changing a large landform.

Fast describes time

“Fast” can mean seconds, minutes, hours, or days when you compare Earth changes. A tiny patch of bank sliding in seconds is fast. A whole mountain changing in a morning is fast too. Size does not decide the word.

Most days in a place pass without a sudden land change. Scientists watch rivers, storms, and volcanoes for warning signs. Follow a grown-up's directions and official warnings to stay safe during dangerous events.

A river pebble can become smooth through many small knocks. A mountain can lose tiny pieces over many years. Slow changes add up even when you cannot see them happen during one visit.

Rock wears away slowly

Water, waves, wind, and ice usually wear rock away bit by bit. Very old mountains are often lower and rounder than younger ones because material has been removed over a long time.

The Colorado River has carved the Grand Canyon in Arizona for about 5 to 6 million years. Along the river, the canyon is about 277 miles (446 kilometers) long. It is more than a mile deep in places. This enormous shape did not come from one flood.

Millions of years are far longer than one person's life. You do not need to watch the whole change happen. The canyon, rock layers, and what scientists find provide clues about its long history.

Look for smaller clues

A dune can move a little farther each year. A beach can slowly gain or lose sand. A river's bends can shift, and a glacier can slowly slide downhill.

Look at old stone steps. Many footsteps can wear a dip in the middle. Letters carved in old stone can become faint as rain wears the surface. Rounded river stones show the effect of repeated rubbing. Each small action may do little; many actions together can leave a clear mark.

The cause alone is not enough

Water often changes hard rock slowly, but a powerful flood can cut rock quickly. In Texas in 2002, a flood cut a canyon about 23 feet (7 meters) deep in three days.

Compare that with the Grand Canyon. Both involve moving water and worn rock. The evidence about time is different: three days in one case, millions of years in the other. “Water did it” is not enough to sort a change as fast or slow.

For a slow change, look for records across many years. Repeated photos of a river bend can show a little more land lost each year. One quiet day beside the river shows only a small part of that story.

A photo shows what a place looked like at one moment. To tell how quickly it changed, you need more of its story. Evidence is information you find that helps show what happened.

Ask what changed and when

Start with four steps:

1. Name the change you can see.
2. Find out how long it took.
3. Check another source of information.
4. Use the time to call the change fast or slow.

Photos, videos, maps, reports, books, and scientists' measurements can give evidence. People who saw an event can describe it too. Look for dates and times, because a picture alone may not tell them.

Work through a fast example

One photo shows a green hillside with a road. A second photo, taken the next day, shows mud and rock across it. First name the change: material has moved from the slope onto the road.

Next check a report. It says heavy rain caused a landslide that lasted less than a minute. The photos show the moved material, and the report supplies the time. Together, they support calling this a fast event.

Look across many years

A ranger photographs a dune from the same spot every year for 10 years. Each photo shows it a little nearer a path. The ranger's notes describe small shifts during those years. The repeated photos and notes support a slow change.

Now imagine only two cliff photos, taken 50 years apart. The later cliff edge is farther back. You know the cliff changed between the photos. You still need information about what happened in the gap: small losses over time, sudden falls, or both. A dated report or more photos can help.

Keep size and time separate

A huge canyon may form slowly. A small bank collapse may happen in seconds. Wind can move a dune slowly or flatten one during a storm. Water can carve rock over millions of years or wash a bank away in a day. Choose your answer from evidence about the time, then explain how your sources support it.