Miss Nelson

SCIENCE ~ CHAPTER 6

VOLCANOES

Volcanic Eruptions

SECTION 2

What comes out of a volcano when it erupts?

ANTICIPATORY SET

S 6.1.d – Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fissures are locations where magma reaches the surface

S 6.2.d –Students know earthquakes, volcanic eruptions, landslides, and floods change human and wildlife habitats

STANDARDS

What happens when a volcano erupts?

What are the two types of volcanic eruptions?

What are a volcano’s stages of activity?

THE BIG IDEA

Magma chamber – the pocket beneath a volcano where magma collects

Pipe – a long tube through which magma moves from the magma chamber to Earth’s surface

Vent – opening through which magma leaves a volcano

Lava flow – area covered by lava from a volcano’s vent

Crater – a bowl-shaped area that forms around a volcano’s central opening

KEY TERMS

Silica – a material found in magma that is formed from the elements oxygen and silicon

Pyroclastic fl ow – an explosive volcanic eruption of ash, cinders, bombs, and gases

Dormant – a volcano that is not currently active, but that may become active in the future

Extinct – a volcano that is no longer active and is unlikely to erupt again

Geyser – a fountain of water and steam that builds up pressure underground and erupts at regular intervals

KEY TERMS

Read “Volcanic Eruptions” on page 221 of your textbook

VOLCANIC ERUPTIONS

A volcano is more than a large, cone-shaped mountain

Inside is a system of passageways through which magma moves

MAGMA REACHES EARTH’S SURFACE

All volcanoes have a pocket of magma beneath the surface and one ore more cracks through which the magma forces its way

View the figure of a volcano on page 223 of your textbook

INSIDE A VOLCANO

The explosion of a volcano is similar to shaking a soda bottle and then opening itYou cannot see the carbon dioxide gas in the soda because it is dissolved in the liquid

When you open the bottle, pressure is releasedThe carbon dioxide expands and forms bubbles, which rush to the surface

Dissolved gases are trapped in magmaAre under tremendous pressure

A VOLCANIC ERUPTION

As magma rises toward the surface, the pressure of the surrounding rock on the magma decreases

The dissolved gases begin to expand, forming bubbles

As pressure falls within the magma, the size of the gas bubbles increases greatly

These expanding gases exert an enormous force

A VOLCANIC ERUPTION

When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent

A VOLCANIC ERUPTION

Read “Magma Reaches Earth’s Surface” on pages 222-223 of your

textbook

MAGMA REACHES EARTH’S SURFACE

Geologists classify volcanic eruptions as quiet or explosive

KINDS OF VOLCANIC ERUPTIONS

The properties of magma determine how a volcano erupts

Whether an eruption is quiet or explosive depends on:The magma’s silica contentWhether the magma is thin and runny or thick and sticky

KINDS OF VOLCANIC ERUPTIONS

A volcano erupts quietly if its magma is low in silica

QUIET ERUPTIONS

Low-silica magma is thin and runny and flows easily

Oozes quietly from the vent and can flow for many kilometers

Can produce two different types of lava that differ in temperature:

Pahoehoe Fast moving, hot lava that is thin and runny Surface looks like wrinkles and ropelike coils

Aa Thicker than pahoehoe Forms a rough surface consisting of jagged lava

chunks

QUIET ERUPTIONS

The Hawaiian islands were formed from quiet eruptions

On the Big Island of Hawaii, lava pours out of the crater on Mount Kilauea

QUIET ERUPTIONS

A volcano erupts explosively if its magma is high in silica

EXPLOSIVE ERUPTIONS

High-silica magma is thick and sticky

It builds up in a volcano’s pipe, plugging it like a cork in a bottle

Dissolved gases, including water vapor, cannot escape

The trapped gases build up pressure until they explode

The erupting gases and steam push the magma out of the volcano with incredible force

EXPLOSIVE ERUPTIONS

An explosive eruptions breaks lava into fragments that quickly cool and harden into pieces of different sizes

Pumice and obsidian form from high-silica lava

EXPLOSIVE ERUPTIONS

Read “Kinds of Volcanic Eruptions” on pages 224-226 of your textbook

KINDS OF VOLCANIC ERUPTIONS

Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of activity

STAGES OF VOLCANIC ACTIVITY

An active, or live, volcano is one that is erupting or has shown signs that it may erupt in the near future

A dormant, or sleeping, volcano is expected to awaked in the future and become active

An extinct, or dead, volcano is unlikely to erupt again

STAGES OF VOLCANIC ACTIVITY

Are often found in areas of present or past volcanic activity

A hot spring forms when water deep underground is heated by a nearby body of magma or by hot rock

A geyser is a fountain of water and steam that erupts from the ground

HOT SPRINGS AND GEYSERS

Geologists use instruments to detect changes in and around a volcanoThese changes may give warning a short time before a volcano erupts Cannot be certain about the type of eruption or how

powerful it will be

Geologists use tiltmeters and other instruments to detect slight surface changes in elevation and tilt cause by magma moving underground

MONITORING VOLCANOES

Read “Stages of Volcanic Activity” on pages 227-228 of your textbook

STAGES OF VOLCANIC ACTIVITY

Describe the order of parts through which magma travels as it moves towards the surface.As magma travels towards the surface it collects in the magma chamber, then moves upward through the pipe, and finally leaves the volcano through the vent.

What are the two main kinds of volcanic eruptions?The two main kinds of volcanic eruptions are quiet and explosive.

CHECK FOR UNDERSTANDING

Describe the order of parts through which magma travels as it moves towards the surface.

What are the two main kinds of volcanic eruptions?

CHECK FOR UNDERSTANDING

What do lava flows made of pahoehoe and aa indicate about the type of volcanic eruption that occured?

GUIDED PRACTICE

What do lava flows made of pahoehoe and aa indicate about the type of volcanic eruption that occured?Lava flows made of pahoehoe and aa indicate that the volcanic eruption was quiet.

GUIDED PRACTICE

Complete Volcanoes 6-2 Independent Practice

INDEPENDENT PRACTICE