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Physical Geology 101
10. Volcanoes I
(p. 126-149)
Volcano Statistics
How many volcanoes erupt on land or in shallow water every year? ____________
How many volcanoes have erupted in the USA in the past 200 years? ___________
How many volcanoes in the world emit rocks, ash, gas or lava every day? ___________
Examples of 5 volcanoes that have shown eruptive activity in the past week:
_______________________
_______________________
_______________________
_______________________
_______________________
How many active volcanoes are there in the world? ___________
How many people live on or near these volcanoes? _______________
In the past 500 years, there have been 7 catastrophic eruptions that have killed more than ____________
people at a time. Since 1800, there have been 19 eruptions that have killed more than 1,000 people.
Potential volcanic hazards:

__________________ – e.g. Hawaii, Iceland, Democratic Republic of Congo, Mexico

__________________ – e.g. Galeras, Colombia

__________________ – e.g. Mt. Vesuvius, AD79; Mt. St. Helens, 1980; Pinatubo, 1991

__________________ – effects: respiratory ailments and collapsed roofs; e.g. Pinatubo

__________________ – e.g. Lake Nyos, Cameroon, 1986 – 1,700 people killed

__________________ – e.g. Armero, Colombia, 1985 – 25,000 killed; Mt. Rainier, WA.

__________________ – pyroclastics, rocks, trees, etc. - e.g. Nevado del Ruiz, Colombia

__________________ – e.g. Krakatau, Indonesia, 1883 – eruption created a tsunami that
killed >35,000 people.

__________________ – e.g. Kalapana, Hawaii, 1975 (also caused a tsunami)
Additional causes of death in the long-term: _________________________________
The most destructive known volcanic episode happened about 251 million years ago in Russia, forming a large
region of basalt lava flows called the _______________________. The lava from that eruption covered a million
square miles and resulted in the death of 90% of all sea life and 70% of all land creatures- an extinction level
event of global proportions.
Since then, there have been 9 other enormous floods of basalt onto the Earth's surface.
Which flood basalt event happened about 16 million years ago in the Pacific NW?
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Physical Geology 101
____________________________________________
Types of Eruptions
The two types of eruptions are ______________________ and ____________________.
Nonexplosive Eruptions
What location typically experiences nonexplosive eruptions? _________________
What is the largest volcano on Earth? _____________________
What are the viscosity and silica content characteristics of lava that generally produces nonexplosive eruptions?
Viscosity:
HIGH or LOW ?
Silica content:
HIGH or
LOW ?
What type of lava would have these characteristics? ______________________
Gases spattering out of a volcanic vent produce a ____________________.
The most active volcano on Earth is __________________, with >50 eruptions since 1823.
Mafic lava that erupts nonexplosively can flow great distances, and eventually builds up a type of volcano called
a _____________________.
Such volcanoes typically have very wide bases and gently sloping sides.
Examples of shield volcano locations:
___________________
____________________ ____________________..
Where are fissure eruptions common (also nonexplosive)?
______________________
______________________
______________________
______________________
Explosive Eruptions
What are the viscosity and silica content characteristics of lava that generally produces explosive eruptions?
Viscosity:
HIGH or LOW ?
Silica content:
HIGH or
LOW ?
What type of lava would have these characteristics? ___________________________
Explosive eruptions results in huge volumes of pyroclastic material and gases being forced up into the
atmosphere in the shape of a huge column called a ______________________.
How high can these columns reach? ________________
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Physical Geology 101
Examples: ______________________ and _______________________.
When the pyroclastic material first gets ejected sideways from the volcano, the type of eruption is called a
_____________________ (happened at Mt. St. Helens in 1980).
Sometimes, volcanic material can rush down the slopes of the volcano just like an avalanche, and is
called a _________________________. It contains extremely hot bits of semi-molten rock, called
___________, as well as huge volumes of poisonous gases.
What are the three types of tephra (from biggest to smallest particles)?

______________________________

______________________________

______________________________
The lava from explosive eruptions is silica-rich and very viscous so it doesn’t flow far. Along with pyroclastic
flows, it builds up a multilayered, steep-sided volcano called a:
__________________________ ; also called a _____________________________
Examples of stratovolcanoes: ___________________ and _____________________
Volcanic Features
There are a number of types of features on Earth that formed through volcanic processes.
Cinder Cones
Cinder cones are built from blobs of congealed lava and lapilli size pyroclastic material ejected from a volcanic
vent that fall to the ground as cinders or clinkers. This material collects in a heap around the vent, forming a
circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a
thousand feet or so above their surroundings. They form due to a single, short-lived eruptive event.
Example from Idaho: ________________________________
Craters and Calderas
A crater is a funnel-shaped depression that exists at the top of most volcanoes.
What is a caldera? _______________________________________________________
Example of a caldera in the Cascades: ______________________________
Tuff Rings and Maar Craters
Explosive eruptions of pyroclastic material may leave a ring of debris around a vent, called a tuff ring.
Example: ____________________________
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Physical Geology 101
Similar explosive eruptions may occur when lava flows cover over a lake, forming a circular depression at the
surface called a maar crater, surrounded by ejecta and which may fill with water to form a circular lake.
Lava Domes
After an eruption, a volcano doesn't necessarily become inactive. It may start to push up the crater or caldera,
forming a mound called a lava dome.
Examples: ________________________
and
________________________
What is the world’s largest lava dome? __________________________
Predicting Eruptions
What are the three categories for volcano activity?
1. __________________ (is erupting or has erupted in recent history)
2. __________________ (has not erupted in recent history but has the potential to erupt)
3. __________________ (shows no signs of activity)
Some tell-tale signs of an impending eruption include:
1.
2.
3.
4.
Sudden bulging inside the crater due to the magma chamber filling up (measured with a tiltmeter).
Increased minor earthquake activity.
Increased water temperature in the crater lake.
Increased gaseous emissions.
FINAL QUESTION:
What is the most active volcano in the Cascades?
__________________________________
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