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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?
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?
Silica content:
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?
Silica content:
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? ________________
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: ____________________________
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: ________________________
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:
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.
What is the most active volcano in the Cascades?