volcanoes. a few volcano basics… active – a volcano that has erupted recently (geologically...
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A few volcano basics…• Active – a volcano that has erupted
recently (geologically speaking)
• Dormant – (sleeping) has erupted within the past few thousand years – will likely erupt again
• Extinct – hasn’t erupted in 10’s of thousands of years; unlikely to erupt again
viscosity: a fluid’s resistance to flow
high viscosity = high resistance to flow
ex. honey is more viscous than water
•Low viscosity magmas allow gases to escape easily (they are hotter!)
•In higher viscosity lavas, gas pressures build up and erupt explosively
The higher the silica content, the higher the
viscosity.
The hotter the magma, the less viscous it is.
Shield Volcanoes
- largest volcanoes (tallest when measured from the ocean floor)- shaped like a gentle arch or shield-basaltic lavas (low silica content)
Composite Volcanoes- coned shaped; sometimes called stratovolcanoes- lava contains mostly andesite or rhyolite (higher silica content)- more viscous
-combination of explosive activity (pyroclastic) and lava flows- responsible for most deaths- ex. Mount Saint HelensMt. PinatuboMt. FujiMt. Vesuvius
Cinder Cones
- smaller volcanoes- form quickly -are active for a few years and then usually go dormant
ex. Paracutin, Mexico
- 1943, hole in ground- 2 weeks prior, small tremors- 1st day grew 10 m- By 5th day it was 100 m tall- After 2 yrs, it was 400 m tall- went quiet after 9 years
Ring of Fire•Area of the world where volcanoes are common
•Here is what a volcano on the ring of fire looks like:
Features of Volcanoes
crater - depression found at the top of a volcano; formed by the explosion of the upper portion of the cone
hot springs: water is heated by the hot rock
and reaches Earth’s surface
geyser: heating and circulation of water forms a pattern (RARE!)ex. Old Faithful
caldera: a giant crater that can be more than 12.5 miles in diameter
- formed by rare, very violent eruptions- none have occurred in recorded history (they are 1000 times more violent than Mt. St. Helens!)- Yellowstone caldera was formed 600,000 years ago!
Lava flows: eruption of magma at Earth’s
surface
Lava flows vary based on the composition of the
magma. We are specifically going to look at
basaltic lava flows.
pahoehoe: fast moving; low viscosity
- smooth ropy texture when it hardens- move at a rate of up to 20 mph
aa: slow moving; higher viscosity
- blocky texture when hardens- move at rates of a few meters per day-found further away from volcano
Pyroclastic Activity: explosive volcanism where tephra is physically blown into the atmosphere
tephra: any material that is blown out of a volcano (mostly ash)
ash fall: huge quantities of rock, glass and gas are blown high into the air
- kills vegetation- contaminate water- structural damage- jet engine “flame out” - respiratory irritation
lateral blast: explosions of gas and ash from the side of the volcano; destroys part of the mountainex. Mt. St. Helens
Poisonous Gases
- CO2, carbon monoxide, sulfur dioxide, and hydrogen sulfide- can cause acid rain; contaminates water supply and vegetation
Secondary Effects
lahar: debris flows and mudflows
-volcanic debris becomes saturated with water-ex. Mt. Pinatubo, Philippines
Predicting Volcanoes
1. Monitoring of Seismic Activity- often the earliest sign2. Thermal monitoring - measuring ground temperatures
3. Topographic Monitoring - mountain may tilt and swell before an eruption
4. Volcanic Gas Emissions - changes in composition of gases may indicates rising of magma