Volcanoes!
Volcanism - the formation
of volcanoes
A. Molten rock
1. magma – molten rock below the Earth’s surface
2. Where rock will melt
- rock melts in the asthenosphere
- rock also melts at the plate boundaries
due to friction created during plate
movements
B. Movement of magma
1. magma is less dense than solid rock
therefore it will rise
2. magma will push thru cracks in crust pushed
upwards
3. when it reaches the surface – a volcano forms
Volcanoes - magma affects
volcano shape
Shield cone volcano
▲ Low, wide based volcanoes
▲ Example : Hawaiian volcanoes
▲ Mafic magma
• thinner in viscosity, flows easily
• oozes out, forming a shield shape as each layer
flows and hardens
• fewer gases in this form
• eruptions not very explosive
▲ Mafic lava erupts and then flows easily
▲ Produces pahoehoe features (ropey, mafic
lava)
Volcanoes – how magma
affects volcano shape
Cinder cone volcano
▲ Tall, steep, narrow based volcanoes
▲ Example : Mt. St. Helens
▲ Felsic magma
• thicker viscosity, slower moving
• eruptions are often violent
• lava is blocky (aa) features, felsic in nature
▲ Large amounts of gas trapped in magma
•
•
•
•
carbon dioxide, carbon monoxide, sulfur oxide
All dangerous, poisonous in some cases
Even superheated steam can be formed
Sudden expansion of these gases similar to a soda
bottle being shaken!!!
Volcanoes – how magma
affects volcano shape
When volcanoes erupt violently
▲Lava fragments called tephra thrown out
• ash - less than 2 mm in size - smallest kind
• lapilli - 2 mm to 64 mm in size
• over 64 mm in size – two kinds
 blocks – launched and land as solid chunks
 bombs – launched as liquid, solidifies as it falls
▲Pyroclastic cloud
• tephra mixed in with volcanic,
superheated gases
• can move at very high speeds (200 mph!)
Why Volcanoes are Hazardous!
Volcanoes – how magma
affects volcano shape
▲Examples
▲Mt. Vesuvius and the end of Pompeii
▲Krakatoa, Indonesia
▲Mt. St. Helens, Washington State
▲Mt. Pinatuba, Philipines
Cities in Dust" by Siouxsie
and the Banshees
• Water was running;
children were running
You were running out
of time
Under the mountain, a
golden fountain
Were you praying at
the Lares shrine?
• We found you hiding, we
found you lying
Choking on the dirt and
sand
Your former glories and
all the stories
Dragged and washed with
eager hands
Cities in Dust" by Siouxsie and the Banshees
• Water was running; children were running
We found you hiding, we found you lying
Water was running; children were running
We found you hiding, we found you lying
your city lies in dust
• Hot and burning in your nostrils
Pouring down your gaping mouth
Your molten bodies blanket of cinders
Caught in the throes .......
• But ohh oh your city lies in dust, my friend
ohh oh your city lies in dust, my friend
your city lies in dust
Plutonic Activity – when magma
doesn’t reach the surface
Trivia - Who was Pluto? What was his name in
Greek mythology?
• Plutons – igneous intrusions below the surface
• Sills – igneous intrusion that are parallel to
existing rock layers
• Dikes – igneous intrusions that cut across rock
layers
• Laccolith – igneous intrusion that pushes up
existing layers to form a dome shape
• Neck – remains of the center of an extinct
volcano after outside edges are eroded
• Batholiths – massive underground intrusions
• Stocks – less massive underground intrusions
(under a hundred square miles)
Plutonic Activity – when magma
doesn’t reach the surface
Trivia - Who was Pluto? What was his name in
Greek mythology?
• Plutons – igneous intrusions below the surface
• Sills – igneous intrusion that are parallel to
existing rock layers
• Dikes – igneous intrusions that cut across rock
layers
• Laccolith – igneous intrusion that pushes up
existing layers to form a dome shape
• Neck – remains of the center of an extinct
volcano after outside edges are eroded
• Batholiths – massive underground intrusions
• Stocks – less massive underground intrusions
(under a hundred square miles)
Plutonic Activity – when magma
doesn’t reach the surface
Trivia - Who was Pluto? What was his name in
Greek mythology?
• Plutons – igneous intrusions below the surface
• Sills – igneous intrusion that are parallel to
existing rock layers
• Dikes – igneous intrusions that cut across rock
layers
• Laccolith – igneous intrusion that pushes up
existing layers to form a dome shape
• Neck – remains of the center of an extinct
volcano after outside edges are eroded
• Batholiths – massive underground intrusions
• Stocks – less massive underground intrusions
(under a hundred square miles)