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Earth Science · Unit 6 · Grades 6–8

Earthquakes and Volcanoes

The ground under your feet is not as still as it seems.

Every day, somewhere on Earth, rocks snap and slide along faults and send waves racing through the planet. In other places, melted rock rises from deep underground and bursts out as lava and ash. Earthquakes and volcanoes can be terrifying, but they also built mountains and islands and taught us what is inside our planet.

In this unit you will learn why the ground shakes, how scientists measure earthquakes, what makes some volcanoes ooze while others explode, and how to stay safe. Then you can test yourself in the Volcano Field Guide.

Play Volcano Field Guide Take the self-guided quiz

Inside a volcano, with an earthquake trace on seismograph paper below.

What is an earthquake?

Earth’s outer shell is broken into moving plates (you met them in Plate Tectonics). Where rocks are pushed, pulled, or dragged past each other, they bend a little and store up stress, like a stick you slowly bend. A fault is a crack in the crust where the rocks on each side can move. When the stress gets too great, the rock on one side of the fault suddenly slips. That sudden slip sends out waves of energy, and the ground shakes. That shaking is an earthquake.

Focus (hypocenter)The place underground where the rock first starts to slip.
EpicenterThe spot on Earth’s surface directly above the focus. News reports give the epicenter as the earthquake’s location.
AftershocksSmaller earthquakes that follow a big one as the rocks around the fault settle into place.

Kinds of faults

When a fault is slanted, geologists name the two sides like a miner standing in a tunnel along the fault. The block above the fault is the hanging wall (the miner could hang a lantern on it). The block below is the footwall (the miner’s feet stand on it).

The three main kinds of faults. Arrows show whether the crust is pulled apart, squeezed, or dragged sideways.

Normal faultThe crust is pulled apart (tension), and the hanging wall slides down. Normal faults are common where plates spread apart, such as at mid-ocean ridges and in the Basin and Range of Nevada and Utah.
Reverse faultThe crust is squeezed (compression), and the hanging wall is pushed up. A thrust fault is a reverse fault with a gentle slope. Many big earthquakes in Japan and other subduction zones happen on these faults.
Strike-slip faultBlocks slide past each other sideways, with little up or down motion. The San Andreas Fault in California is a famous example: stand on one side and the other side moves to your right.

Seismic waves

The energy from an earthquake travels as seismic waves. Scientists record them with seismographs, which draw a wiggly line when the ground moves. There are two big groups: body waves, which travel through Earth’s inside, and surface waves, which travel along the top.

How each kind of wave moves the ground. The arrival order is the order they reach a seismograph.

P waves (primary)The fastest waves, so they arrive first. They push and pull the rock in the same direction the wave travels, like a pulse along a stretched-out spring toy. Sound is this kind of wave too. P waves can travel through solids and liquids.
S waves (secondary)About 60 percent as fast as P waves in rock, so they arrive second. They shake the rock up and down or side to side, across the direction of travel, like a wave flicked along a rope. S waves cannot travel through liquids.
Surface wavesThe slowest waves, arriving last, but usually the biggest. They roll and sway the ground and cause most of the damage to buildings.

Did you know?

Seismic waves let scientists “see” inside Earth. Seismographs on the far side of the planet from an earthquake record P waves but no S waves. Because S waves can’t pass through liquid, this “shadow” showed that Earth’s outer core is liquid.

Magnitude and intensity

There are two different ways to describe how big an earthquake is, and they answer different questions.

MagnitudeHow big the earthquake is at its source. Each earthquake has one magnitude. Today scientists report large earthquakes using the moment magnitude scale, which is based on how much the fault slipped and over how big an area. The older Richter scale is rarely used now.
IntensityHow strongly the shaking is felt, and how much damage it does, at one particular place. Intensity changes from place to place, so one earthquake has many intensities. In the United States it is measured on the Modified Mercalli Intensity scale, written in Roman numerals, from shaking nobody feels to total destruction.

The magnitude scale climbs fast. Each step up by one whole number means about 10 times more ground motion on a seismograph and about 32 times more energy. A magnitude 8 releases about 32 × 32, or roughly 1,000 times, as much energy as a magnitude 6.

Real example

The largest earthquake ever recorded struck Chile on May 22, 1960. It was magnitude 9.5 and broke a fault almost 1,000 miles (about 1,600 kilometers) long. A magnitude 10 is not thought to be possible, because no known fault is long enough.

Earthquake safety

Nobody can predict exactly when an earthquake will strike, but you can practice what to do. Earthquake experts agree on three words: Drop, Cover, and Hold On.

Drop to your hands and knees before the shaking knocks you down.
Cover your head and neck. Crawl under a sturdy table or desk if one is close, away from windows.
Hold on to your shelter (or to your head and neck) until the shaking stops.

If you are indoors, stay inside. Don’t run outside during the shaking.
If you are outdoors, move into the open, away from buildings, trees, and power lines.
If you are in a car, the driver should pull over and stop away from bridges, overpasses, and power lines, and everyone stays inside until the shaking stops.
Near a coast, a long or strong earthquake can be a warning that a tsunami may be coming. Move to high ground right away.

How volcanoes form

A volcano is an opening in Earth’s crust where melted rock, gas, and ash escape. Deep underground, melted rock is called magma. Magma is less dense than the solid rock around it, so it rises and collects in a magma chamber. When it reaches the surface it is called lava. Gas dissolved in the magma, mostly water vapor, helps push it up, a bit like the fizz in a shaken soda.

Rock melts to make magma in three main kinds of places: where one plate sinks under another (subduction zones), where plates pull apart, and over hot spots in the middle of plates. About 1,350 volcanoes on land are thought to be potentially active, and about 500 of them have erupted during recorded history.

Kinds of volcanoes

Volcanoes come in a few main shapes, and the shape tells you a lot about the lava that built it.

Three main kinds of volcanoes. They are not drawn to scale: Mauna Loa is far bigger than Mount St. Helens, and a cinder cone is tiny next to both.

Shield volcanoVery wide with gentle slopes, built from thousands of runny basalt lava flows. Hawaii’s Mauna Loa, the largest active volcano on Earth, is a shield volcano.
Composite volcano (stratovolcano)Tall and steep, built from layers of lava, ash, and cinders. Their eruptions can be very explosive. Mount St. Helens, Mount Rainier, and Mount Hood in the Cascade Range are composite volcanoes.
Cinder coneThe simplest kind: a small, steep cone of cinders that piled up around a single vent. In 1943, a cinder cone named Parícutin began erupting in a farmer’s cornfield in Mexico and grew into a cone 167 meters tall in just six days.
Lava domeA rounded, steep-sided mound of lava so thick and sticky it piles up over the vent instead of flowing away. Lassen Peak in California is one of the largest lava domes on Earth.

Runny or sticky magma

Why do some volcanoes ooze while others explode? The answer is viscosity, which means how thick and sticky a liquid is. Honey has high viscosity; water has low viscosity.

The viscosity of magma depends mostly on how much silica (silicon and oxygen) it contains. Silica forms chains that make the magma stiff. Gas bubbles have a hard time escaping from stiff magma, so pressure builds up until the magma blasts apart into ash.

Magma from runny to sticky
Magma	Silica	Flow and eruption
Basalt	Low	Runny; gentle lava flows (shield volcanoes)
Andesite	Medium	Thicker; often explosive (composite volcanoes)
Dacite and rhyolite	High	Very sticky; big explosions or lava domes

Try it

Blow through a straw into a glass of water, then into a glass of thick honey or a milkshake. In the water, bubbles pop right out. In the thick liquid, the bubbles get trapped and burst out all at once. Sticky magma traps gas the same way.

The Ring of Fire and hot spots

Earthquakes and volcanoes are not spread evenly around Earth. Most happen near plate boundaries.

Around the edges of the Pacific Ocean is a horseshoe of trenches, volcanoes, and earthquakes called the Ring of Fire. Along it, ocean plates sink under other plates. About 81 percent of the world’s largest earthquakes happen there, including the 1960 Chile and 1964 Alaska earthquakes. Indonesia, Japan, and the United States have the most potentially active volcanoes, and many of them are on the Ring of Fire.

The next busiest earthquake belt, the Alpide belt, stretches from Indonesia through the Himalaya to the Mediterranean. It has about 17 percent of the largest earthquakes, including the 2004 Indonesia earthquake, whose tsunami killed more than 230,000 people.

Some volcanoes are far from any plate boundary. They sit over hot spots, places where unusually hot rock rises from deep in the mantle. The hot spot stays put while the plate slides over it. The Hawaiian Islands formed this way: the Pacific plate moves northwest about 9 centimeters a year, so the islands get older toward the northwest, and the Big Island, the youngest, sits over the hot spot now. Yellowstone, in the western United States, sits over a hot spot under the North American plate.

Volcanic hazards and safety

Lava usually moves slowly enough for people to walk away. The most dangerous hazards are often faster.

Pyroclastic flowsAvalanches of hot rock, ash, and gas that race down a volcano at more than 80 kilometers (50 miles) per hour and destroy everything in their path. In 1902, pyroclastic flows from Mont Pelée on Martinique killed about 30,000 people.
LaharsVolcanic mudflows of water mixed with rock and ash. They rush down river valleys like wet concrete, sometimes faster than 200 kilometers per hour on steep slopes. In 1985, lahars from Nevado del Ruiz in Colombia killed about 25,000 people.
AshTiny pieces of rock and glass. Ash can collapse roofs, damage airplane engines, and make breathing hard. Heavy wet ash on roofs caused many of the roughly 350 deaths at Mount Pinatubo in the Philippines in 1991.
Lava flowsBurn or bury buildings and roads in their path, but rarely kill people.
Volcanic gasesGases such as carbon dioxide and sulfur dioxide can collect in low areas or make acid rain and smog.

Real example

On May 18, 1980, a magnitude 5.1 earthquake shook Mount St. Helens in Washington. The volcano’s bulging north side collapsed in a giant landslide, and the trapped gas and magma burst out sideways in a blast. The eruption killed 57 people. Scientists had warned of the danger, and the area had been closed, which saved many lives.

Scientists at volcano observatories watch for warning signs, such as swarms of small earthquakes, the ground swelling, and changes in gas. If you live or travel near a volcano, learn the evacuation routes, follow warnings from officials, stay out of river valleys where lahars can travel, and keep a mask or damp cloth handy to breathe through if ash falls.

Try the interactive version at jugami.com/earth-science/earthquakes-volcanoes/

Game: Volcano Field Guide

Six mystery volcanoes are waiting in your field guide. Look at their shape, their lava, how they erupt, and where they are. When you think you know what kind of volcano it is, choose its type. The real volcano’s name is revealed after you answer.

This game needs JavaScript. With JavaScript turned off, the six volcanoes and their clues are listed below.

The six volcanoes and their clues:

Mauna Loa, Hawaii: shield volcano. Shape: Very wide with gentle slopes, like a warrior’s shield lying on the ground. Lava: Runny, low-silica basalt that pours out in long rivers of lava. Eruptions: Mostly gentle: lava fountains and long lava flows, not huge explosions. Setting: In the middle of the Pacific plate, over a hot spot.
Kīlauea, Hawaii: shield volcano. Shape: Gentle slopes that spread out for many kilometers. Lava: Hot, runny basalt. Gas bubbles escape easily. Eruptions: Lava lakes, lava fountains, and flows that can reach the sea. Setting: On the Big Island of Hawaii, which sits over the hot spot right now.
Mount St. Helens, Washington: composite volcano. Shape: Before 1980 it was a steep, nearly perfect cone built of layers. Lava: Thick, sticky, gas-rich magma (dacite). Eruptions: Explosive. In 1980 one side collapsed and a sideways blast and ash cloud followed. Setting: Above a subduction zone, part of the Cascade Range on the Ring of Fire.
Mount Rainier, Washington: composite volcano. Shape: Steep and tall, built from layer after layer of lava and ash. Lava: Thick, sticky andesite. Eruptions: Can be explosive. Melting glaciers could send lahars (volcanic mudflows) down the valleys. Setting: Above a subduction zone in the Cascade Range, on the Ring of Fire.
Parícutin, Mexico: cinder cone. Shape: A small, steep cone with a bowl-shaped crater on top. Lava: Gas-rich blobs of lava that cool into bubbly cinders as they fly through the air. Eruptions: Fountains of glowing cinders that fall back around the vent and pile up. Setting: In a volcanic field in central Mexico, inland from a subduction zone.
Lassen Peak, California: lava dome. Shape: A rounded mound with very steep sides, not a pointed cone. Lava: Very thick, sticky, high-silica lava (dacite) that piles up instead of flowing. Eruptions: The lava oozes out and piles up. Its last eruptions were from 1914 to 1917. Setting: Above a subduction zone at the south end of the Cascade Range.

Self-guided quiz

Nine questions. Choose one answer for each, then press Check my answers.

1. What is the epicenter of an earthquake? The point underground where the rock first slips The surface spot right above where the rock first slips The place on the map where the shaking is felt the very strongest

2. At a fault, the hanging wall has slid DOWN compared with the footwall. What kind of fault is it? Reverse fault Strike-slip fault Normal fault

3. Which seismic wave arrives at a seismograph first? P wave S wave Surface wave, because it travels along the ground

4. S waves cannot travel through liquid. What did scientists learn from the fact that S waves do not pass through Earth’s outer core? The outer core is liquid The outer core is made of ice The outer core is hollow and full of air

5. One earthquake is magnitude 7.0 and another is 6.0. About how much more energy does the 7.0 release? Exactly 2 times as much About 32 times as much About 1.1 times as much

6. Why can one earthquake have many different intensity values but only one magnitude? Magnitude is only a rough guess, so it gets changed every single day Each city uses its own magnitude scale, so the numbers all disagree Intensity describes the shaking at each place, which varies

7. What should you do indoors when the ground starts shaking hard? Drop, cover under a sturdy table, and hold on Run outside as fast as you can, before the building falls Stand in the nearest doorway, which is the strongest part of a building

8. Magma with a lot of silica is thick and sticky. How does that usually change an eruption? Gas escapes easily through thick magma, so the eruption is very quiet Gas gets trapped, so an explosive eruption is likelier The magma always stays underground and never erupts at all

9. The Hawaiian Islands get older toward the northwest. Why? Each island formed at a different subduction zone, one after another The islands are slowly drifting back together toward the oldest one The Pacific plate moves northwest over a fixed hot spot

Answer key: 1. The surface spot right above where the rock first slips. 2. Normal fault. 3. P wave. 4. The outer core is liquid. 5. About 32 times as much. 6. Intensity describes the shaking at each place, which varies. 7. Drop, cover under a sturdy table, and hold on. 8. Gas gets trapped, so an explosive eruption is likelier. 9. The Pacific plate moves northwest over a fixed hot spot.

Talk about it

Your school is planning an earthquake drill. What are the safest places to Drop, Cover, and Hold On in your classroom, and which places should you stay away from?
Why do you think so many more people died from lahars at Nevado del Ruiz than from the lava at Kīlauea?
Millions of people live near active volcanoes like Mount Rainier and Mount Vesuvius. Why might people choose to live there, and what would help keep them safe?
Seismic waves let scientists learn about Earth’s core without ever going there. Can you think of other ways people learn about things they can’t see directly?

Sources

U.S. Geological Survey (USGS), What is a fault and what are the different types? (normal, reverse or thrust, and strike-slip faults; hanging wall and footwall).
USGS, Earthquake waves (P waves fastest, S waves about 60 percent of P-wave speed, surface waves slowest and largest), Earthquakes: measuring, and P-wave and S-wave paths through Earth (S waves do not pass through the liquid outer core).
USGS, Earthquake magnitude, energy release, and shaking intensity (10 times the amplitude and about 32 times the energy per whole number), How much bigger?, and The Modified Mercalli Intensity scale.
USGS, Can megaquakes really happen? (1960 Chile, M9.5, on a fault almost 1,000 miles long) and Where do earthquakes occur? (Ring of Fire about 81 percent and Alpide belt about 17 percent of the largest earthquakes).
USGS, What should I do during an earthquake?; Earthquake Country Alliance, Drop, Cover, and Hold On; and Ready.gov, Earthquakes.
USGS, Principal types of volcanoes and Volcanoes of the United States: lava domes; U.S. National Park Service, Types of volcanoes (Lassen Peak as a lava dome).
USGS Volcano Hazards Program, Silica (silica raises viscosity; basalt to rhyolite), Pyroclastic flows, and Lahars.
USGS, How many active volcanoes are there on Earth? and Which volcanic eruptions were the deadliest? (Mont Pelée, Nevado del Ruiz, Pinatubo, St. Helens, Mauna Loa, Lassen Peak).
USGS, 1980 cataclysmic eruption of Mount St. Helens.
USGS, Volcanoes of the United States: Hawaii (hot spot, Pacific plate moving northwest about 9 centimeters a year) and Track of the Yellowstone hotspot.
USGS, Birth and development of Parícutin volcano (Bulletin 965-D; the cone reached 167 meters in six days) and Smithsonian Global Volcanism Program, Michoacán-Guanajuato volcanic field (includes Parícutin).
USGS Lassen Volcano Observatory, Volcano Hazards of the Lassen Volcanic National Park Area (Fact Sheet 022-00): Lassen Peak is one of the largest lava domes on Earth.

Credits

Written by Mary, Jugami’s Earth science teacher. The volcano, fault, and seismic wave drawings are original, hand-coded SVG made for Jugami. They are simplified sketches, not drawn to scale.