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Tsunami is a Japanese word that means "harbor wave." It is a large wave caused by movements in Earth's outer layer, or crust, which move ocean water. For example, an earthquake or a volcano in the ocean could cause a tsunami.

Earth's crust is made up of pieces called tectonic plates. An earthquake happens when these plates push against each other so hard that one of them slips or breaks. Imagine leaning against a friend. If you push against each other harder and harder, one or both of you will fall over. When Earth's plates push against each other, they can move a lot.

If an earthquake happens in the ocean, a large piece of Earth's crust can be thrust upward or slip from side to side. The movement of a large chunk of Earth displaces the water above it, meaning it takes up the space where the water used to be. So where does the water go? It ripples out from the earthquake in waves.

This can also happen if a volcano erupts in the ocean. The lava flowing out of the volcano displaces the water around it. That water can turn into a large wave.

If the earthquake or volcano is very big, then the wave can be very big, too. Big tsunamis usually begin in the deep ocean, where a large volume of water can be displaced. As the wave moves closer to the shore, it grows taller as the ocean becomes shallower. If the water recedes-or moves away from the coast-after an earthquake, a big tsunami may be on its way.

Tsunamis can be hundreds of feet tall, and they travel very fast. This means they can be dangerous even for people who aren't on the beach. Tsunamis can be extremely destructive and can knock down whole buildings.

But not all earthquakes or volcanic eruptions cause tsunamis. Whether a tsunami forms can depend on a lot of things. The shape of the ocean floor can determine if a tsunami happens. So can the distance and direction of the earthquake.

If there's a chance a tsunami is on its way, you don't want to be near the coast. So how can we tell if a tsunami is coming? We use satellites!

MISR (the Multi-angle Imaging SpectroRadiometer) is a NASA tool that watches for tsunamis from space. It has nine cameras, all pointed in slightly different directions. As the satellite passes overhead, it takes nine pictures of the same spot from different angles.

MISR can see sunlight reflecting off ripples and waves. Satellites that look straight down cannot see these ripples.

These images help scientists understand how tsunamis work. The more we learn about tsunamis, the better we can predict where, when and how strong a tsunami will be. That way, people can have enough warning to get away and stay safe.

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A tsunami is a large wave caused by movements in Earth''s outer layer, or crust. Learn more about these big waves and how NASA monitors them.

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A volcano is an opening on the surface of a planet or moon that allows material warmer than its surroundings to escape from its interior. When this material escapes, it causes an eruption. An eruption can be explosive, sending material high into the sky. Or it can be calmer, with gentle flows of material.

These volcanic areas usually form mountains built from the many layers of rock, ash or other material that collect around them. Volcanoes can be active, dormant, or extinct. Active volcanoes are volcanoes that have had recent eruptions or are expected to have eruptions in the near future. Dormant volcanoes no longer produce eruptions, but might again sometime in the future. Extinct volcanoes will likely never erupt again.

What Causes Volcanoes?

Volcanoes occur when material significantly warmer than its surroundings is erupted onto the surface of a planet or moon from its interior. On Earth, the erupted material can be liquid rock ("lava" when it's on the surface, "magma" when it's underground), ash, cinders, and/or gas. There are three reasons why magma might rise and cause eruptions onto Earth's surface.

Magma can rise when pieces of Earth's crust called tectonic plates slowly move away from each other. The magma rises up to fill in the space. When this happens underwater volcanoes can form.

Magma also rises when these tectonic plates move toward each other. When this happens, part of Earth's crust can be forced deep into its interior. The high heat and pressure cause the crust to melt and rise as magma. A final way that magma rises is over hot spots. Hot spots are exactly what they sound like--hot areas inside of Earth. These areas heat up magma. The magma becomes less dense. When it is less dense it rises. Each of the reasons for rising magma are a bit different, but each can form volcanoes.

Are There Volcanoes Elsewhere In Our Solar System?

Yes! There have been a lot of volcanoes on other planets in the past. Some places in our solar system have active volcanoes erupting right now! Venus and Mars are covered with extinct volcanoes. Some of the moons of Jupiter, Saturn, and Neptune actually have eruptions happening right now. We have taken pictures of these with many different NASA spacecraft!

Check out Space Place's Space Volcano Explorer to learn more!

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Hurricanes are the most violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur. The scientific term for all these storms is tropical cyclone. Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are called "hurricanes."

Whatever they are called, tropical cyclones all form the same way.

Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below.

Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.

Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis.

As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.

When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm." And when the wind speeds reach 74 mph, the storm is officially a "tropical cyclone," or hurricane.

Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.

Tropical cyclone categories:

Category Wind Speed (mph)Damage at Landfall Storm Surge (feet)

The two GOES satellites keep their eyes on hurricanes from far above Earth's surface-22,300 miles above, to be exact! ( Learn more about this kind of orbit.)

These satellites, built by NASA and operated by the National Oceanic and Atmospheric Administration (NOAA), save lives by helping weather forecasters predict and warn people where and when these severe storms will hit land.

Related Resources for Educators

Our World: What is a Hurricane?
Real World: Hurricane Hunters
NASA's Eyes on Extreme Weather

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An earthquake is an intense shaking of Earth's surface. The shaking is caused by movements in Earth's outermost layer.

Why Do Earthquakes Happen?

Although the Earth looks like a pretty solid place from the surface, it's actually extremely active just below the surface. The Earth is made of four basic layers: a solid crust, a hot, nearly solid mantle, a liquid outer core and a solid inner core.

The solid crust and top, stiff layer of the mantle make up a region called the lithosphere. The lithosphere isn't a continuous piece that wraps around the whole Earth like an eggshell. It's actually made up of giant puzzle pieces called tectonic plates. Tectonic plates are constantly shifting as they drift around on the viscous, or slowly flowing, mantle layer below.

This non-stop movement causes stress on Earth's crust. When the stresses get too large, it leads to cracks called faults. When tectonic plates move, it also causes movements at the faults. An earthquake is the sudden movement of Earth's crust at a fault line.

The location where an earthquake begins is called the epicenter. An earthquake's most intense shaking is often felt near the epicenter. However, the vibrations from an earthquake can still be felt and detected hundreds, or even thousands of miles away from the epicenter.

How Do We Measure Earthquakes?

The energy from an earthquake travels through Earth in vibrations called seismic waves. Scientists can measure these seismic waves on instruments called seismometer. A seismometer detects seismic waves below the instrument and records them as a series of zig-zags.

Scientists can determine the time, location and intensity of an earthquake from the information recorded by a seismometer. This record also provides information about the rocks the seismic waves traveled through.

Do Earthquakes Only Happen on Earth?

Earthquake is a name for seismic activity on Earth, but Earth isn't the only place with seismic activity. Scientists have measured quakes on Earth's Moon, and see evidence for seismic activity on Mars, Venus and several moons of Jupiter, too!

NASA's InSight mission took a seismometer to Mars to study seismic activity there, known as marsquakes. On Earth, we know that different materials vibrate in different ways. By studying the vibrations from marsquakes, scientists hope to figure out what materials are found on the inside of Mars.

InSight is collecting tons of information about what Mars is like under the surface. These new discoveries will help us understand more about how planets like Mars-and our home, Earth-came to be.

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Learn more about tremors on Earth-and other planets too!