volcanoes
TRANSCRIPT
Volcano:
an openingopening in the earth’s surface through which lava, hot gases, and rock fragments erupterupt
Introduction to volcanoes
1. Magma 50-100 miles below the earth’s surface slowly begins to rise to the surface
2. As the magma rises it melts gaps in the surrounding rock
3. As more magma rises a large reservoir forms as close as 2 miles below the surface (magma chamber)
Origin of Volcanoes
4. Pressure from the surrounding rock causes the magma to blast or melt a conduit (channel) to the surface where magma erupts onto the surface through a vent (opening)
Origin of Volcanoes
5. The magma, now called lava, builds up at the vent forming a volcano
Origin of Volcanoes
6. Often the volcano sides will be higher than the vent forming a depression called a crater
Origin of Volcanoes
Crater:
Caldera: an unusually large crater or the remains when the cone collapses into its own magma chamber
Anatomy of a VolcanoCone:
the above ground structure built from lava and/or tephra
Conduit: the path that magma takes from the magma chamber to the vent
Magma Chamber: the reservoir located under the volcano where magma collects and becomes the supply of magma/lava to build the volcano
Parasitic Cone: a smaller secondary volcano
built on the side of or near the main volcano, but sharing the same conduit to the magma chamber
Fumarole: a secondary vent that emits only gases
Fissure: a long fissure (crack) from which lava flows
Vent: opening of the volcano, through which lava, ash and gases flow
Take a minute to label the parts on the diagram (not all parts are shown)
Magma chamber
conduitmantle
Parasitic Cone
Ash Cloud/Gases Vent
Lava Flow
Crater
Lava— There is 3 kinds:
Pahoehoe lava:
Hot, thin, fast flowing
harden with a relatively smooth surface
Often has a ropy or wrinkled appearance
Aa lava: • Cooler,
thicker, slow moving
• Hardens with a rough, jagged, sharp edge surface
Pillow Lava: Lava suddenly cooled
by water shows sack-like
segments (stuffed pillows)
Can you identify the kinds of lavalava from the pictures? Circle
your choice.
Tephra (pyroclastic, rock fragments)
Volcanic Dust: Smallest particles and carried by atmosphere circulation
Volcanic Ash: 0.25-0.5 cm diameter Generally settles out within miles
of the cone but can be carried greater distances by stronger winds.
Forms a mudflow when mixed with water
Bomb: Smaller bombs (gravel, pea size) are called
cinders. Walnut size bombs are called lapilli. Larger fragments up to 4+ feet in diameter are
called bombs.
Lahar (mudflow): mixture of ash, eroded land,
and water flowing down river valleys
Lahar (mudflow):
Gases: water vapor, carbon dioxide, nitrogen, sulfur dioxide, hydrogen sulfide, chlorine
Divergent Boundaries: Divergent Boundaries: As the plate move long
cracks (rifts) form and lava builds up forming volcanoes.
Locations of Volcanoes
If the boundary is on the ocean floor, volcanoes can grow tall enough to break the surface of the ocean and become islands (Iceland)
Convergent Boundaries:Convergent Boundaries: Places where plates are
moving toward each other forming a subduction zone.
One plate melts under the other and the magma moves upward to form volcanoes.
Example: Pacific Ring of Fire
Example: Cascade Volcanoes
Magma that may originate in the mantle or outer core will move upward, breaking the surface and forming a volcano, they are independent of plate boundaries and a chain of volcanoes may form as the plate moves across a hot spot.
Hot Spots
(Examples: Hawaiian Islands and Yellowstone National Park)
Hot Spots
Two factors determine the type of eruption:
Amount of water vapor & other gases in the magma
The chemical composition of the magma
Types of Volcanic Eruptions
Trapped gases under high pressure will violently explode when the magma reaches the lower pressure of the surface.
Has granitic magma is very thick and plugs the vent causing the pressure to build until it blows violently out the vent
The high water content of the magma produces more water vapor which when mixed in granitic magma produces explosive eruptions
Explosive Eruptions
Explosive Eruptions
Mt. St. HelensMont serrat
Mt. Pinatubo
Low pressure gas Has basaltic magma (is
more fluid and will flow instead of explode)
And has low water content
Quiet Eruptions
Cinder Cones: Small base, steep-sided, loosely
consolidated Up to 1000 feet tall Life span of a few years Commonly built from gravel size
lava rock fragments call cinders Has violent eruptions, dangerous
when close.
Types of Volcano Mountains
Cinder Cones:
Types of Volcano Mountains
High pressure gas bubbles causes thick lava to explode into the air, lava begins to cool as it rises and falls becoming very sticky
When lava hits the ground it sticks rather than flows
This builds a steep cone with a small base
Sketch a Cinder Cone Volcano:
Types of Volcano Mountains
Shield Volcanoes: Large base, gentle slope, lava
rock layers A few miles wide Life span of a million years or
more The lava is hot, thin, very fluid,
often basaltic. Example: Hawaiian Islands
Shield volcano on Mars; Taken from space
Take a look at these examples:http://www.volcano.si.edu/world/tpgallery.cfm?category=Shield%20Volcanoes
The Mauna Loa volcano in Hawaii—the largest volcano on Earth—has the broad expanse characteristic of shield volcanoes. It spreads across half the island of Hawaii.
Shield Volcanoes
Mauna Kea
Sketch a sheild Volcano:
Composite (strato) Volcanoes: Composite (strato) Volcanoes: Large mountain volcano often snow
capped, a few miles high Life span of million years or more Have alternating eruptions of tephra (air-
borne) and lava. The tephra adds height to the volcano and the lava cements the tephra together and adds to the base.
Found mostly in subduction zones and have violent eruptions.
Examples: Mt Rainier, Mt Fuji, Mt Kilimanjaro
Types of Volcano Mountains
Composite (strato) Volcanoes: Composite (strato) Volcanoes:
Mt. Rainier
Mt. Fuji
Mt. Kilimanjaro
Sketch a composite Volcano:
Active (awake): Has erupted within recent time
and can erupt again at any time. Pre-eruption activities:
Increase in earthquake activity under the cone
increase in temperature of cone, melting of ice/snow in the crater swelling of the cone steam eruptions minor ash eruptions
Volcano Activity Levels (Stages)
Mt St. Helens
Dormant (sleeping):
No eruption within recent times, but there is record of past eruptions
Can become active and erupt again after a “wake up” period
ExampleExample: Mt. Rainier
Extinct: No eruption within recorded
history Not expected to ever erupt
againExample: Mount Mazama (Crater
Lake)
Crater Lake
Mount RainierMount Rainier• The most dangerousdangerous volcano in the US• The danger is mostly from laharslahars traveling
down river valleys at a speed of 25mph and destroying everything in its path
• 100,000 people live on the solidified mudflows of previous eruptions
Mount Rainier• The mountain is dangerously unstableunstable, a
tall, steep heap of loose rock held together by the force of gravity and a cubic mile of glacier ice that could be melted or shaken loose
• Lahar flows average every 500 years500 years and have gone as far as the Puget Sound lowlands (1 in 7 chance of it happening during your lifetime)
• Mount Rainier has erupted 4 times4 times in the last 4000 years with the last eruption 200 years ago