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Extinct – a volcano that is no longer active and is unlikely to erupt again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