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When waves attack: what creates these extreme swells?

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On March 11, 2011, a wall of water 33 feet high hurtled toward the island nation of Japan. The monster wave swept up everything in its path, including cars, homes, and people. Thousands died and many more were left homeless.

A year earlier, a cruise ship sailing on the Mediterranean Sea was struck by three towering waves. Each was higher than a two-story building. They appeared without warning on a clear day. The foaming giants knocked passengers to their feet, injuring 14 people and killing two.

What causes these extreme waves to form? Besides their size and ability to do extreme damage, these sets of giant waves are actually quite different. The waves that struck Japan are known as a tsu-nami (soo-NA-mee). The whoppers that struck the cruise ship in March 2010 were called rogue waves. Let's find out how these killer waves compare.

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There are several kinds of ocean waves, some of which can rise higher than 30 meters (100 feet). It might seem like only strong storms, such as hurricanes, could cause giant waves. Yet super-size swells form for many reasons.

Undersea earthquakes usually cause tsunamis. These marine monsters start out small and get larger as they approach shore.

Rogue waves, on the other hand, appear out of nowhere in the open ocean. Scientists aren't entirely sure of their cause.

Researchers studying giant waves hope to figure out when, where, and how they form.

As of yet, scientists have no way of predicting when a rogue wave will strike. In 2004, the European Space Agency used satellites to record when these ocean waves occurred. The agency's latest project is to track the waves over time. This data will tell scientists where rogue waves happen most often. Then people could be warned to avoid high-risk areas during certain conditions.

For tsunamis, this task is slightly easier. Researchers use sensors scattered throughout the ocean. The sensors measure earthquake activity and changes in sea level. They report the information back to stations on land. This allows scientists to issue warnings when a tsunami is detected. Sometimes, though, people are too close to an earthquake's starting point for a warning system to be useful.

Scientists warn that it may never be possible to know exactly where and when massive waves will strike. Still, improving the ability to predict killer waves is worth working for, they say. It could save hundreds of lives each year.

HOW THEY FORM

TSUNAMI: Tsunamis are usually triggered by deep-ocean earthquakes. Earth's outer layer; or, crust, is made up of rock slabs called tectonic plates. When these tight-fitting plates move against one another, they build up stress. Eventually, earthquakes can result. In some cases, the ocean floor is pushed upward, and the water above is pushed up too. This creates giant waves that travel toward the shore.

ROGUE WAVE: Scientists don't know what causes every rogue wave, but they have ideas about how some of them form. One possibility is that multiple waves are interacting. If a 20-foot wave passes over a 30-foot wave, they could briefly combine into a 50-foot wave. Scientists also think that under specific circumstances, several waves might join together to form one huge wave in an otherwise calm sea.

WHERE THEY FORM

TSUNAMI: A tsunami can occur anywhere in the ocean. However, the Pacific Ocean is the most active tsunami zone. That's because about 90 percent of the world's earthquakes occur around the edge of the Pacific, in a region known as the Ring of Fire.

ROGUE WAVE: Rogue waves occur in open oceans around the world. They tend to occur more frequently in areas with strong currents. For example, off the southern tip of Africa, waves regularly hit the current head-on, making the location a hot spot for rogue waves.

words to know

hurricane--a large spiraling storm system that swirls around a calm, middle area called an "eye"

tsunami--a series of ocean waves that travel across the ocean at high speeds

rogue wave--a giant wave that forms suddenly in the open ocean

tectonic plate--one of the slow-moving rock slabs that make up Earth's outer layer

current--a stream of water moving within another body of water

What's a Wave?

A wave is a motion that carries energy from one place to another. For example, wind energy can be transfered to the ocean's surface. The enegery creates waves that travel over the top of the water. While waves differ in hights, they share the same basic structure.

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HOW HIGH THEY RISE

TSUNAMI: In the open ocean, a tsunami is typically very small. But as these waves zoom toward shore (at speeds up to 966 kilometers [600 miles] per hour), they swell in size. Tsunamis can reach heights of up to 30 meters (100 feet) by the time they reach the coastline.

ROGUE WAVE: Rogue waves can tower up to 58 meters (190 feet) above sea level. Unlike tsunamis, they can reach great heights in the open ocean.

WHEN WAVES ATTACK

BEFORE READING

Set a Purpose

Learn about the different types of massive waves that can form in the ocean.

Background

* Tsunami is a Japanese word. It combines two characters: tsu, meaning "harbor," and nami meaning "wave."

* Rogue wave is a term sometimes incorrectly used to mean any big wave. The correct definition, however, is a wave whose height is more than twice the mean wave height of the largest third of all waves. In other words, a true rogue wave towers above other waves near it.

* When a rogue wave hit the Draupner oil platform in 1995, it was the first one ever to be confirmed with scientific instruments. The wave was taller than three double-decker buses stacked on top of each other.

Discussion Question

* Water waves can be a powerful force. What kinds of things can waves do? (Possible answer: Waves erode, or break down, shell and rock to create sand. Waves can swamp boats and wear down docks and other human-made structures.)

AFTER READING

Discussion Questions

* What are some ways to warn and protect people in tsunami-prone coastal areas? (Answers will vary but could include sirens, radio and television alerts, text message alerts, etc.)

* Do you think it's necessary to have a different strategy for educating tourists who visit areas at high risk for tsunamis? (Answers will vary.)

RESOURCES

* For tips on staying safe in the event of a tsunami, visit: http://www.fema.gov/kifls/tsunami.htm

* To learn more about all types of waves, visit: http.//missionscience.nasa.gov/ems/02_anatomy.html

When Waves Attack, pages 10-13

Which variety of giant wave is likely to increase in size as it gets closer to shore?

(A) tsunamis (C) rogue waves (B) riptides (D) all of the above

(2) Waves are a motion that carries _.

(A) debris (C) currents (B) energy (D) ships

(3) In which area(s) are you most likely to find a rogue wave?

(A) the Ring of Fire (C) areas with strong currents (B) shallow waters (D) underwater mountain ranges

ANSWERS

When Waves Attack: 1. a 2. b 3. c
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Title Annotation:earth science
Author:Smith, Natalie
Publication:SuperScience
Geographic Code:1USA
Date:Sep 1, 2011
Words:1181
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