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go in your notes!
Yellow is just to read, not
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Danger on the Edge of a
Plate
Cascade Stratovolcanoes
6th Grade Science
SMS
The edges of plate
boundaries are where
exciting and sometimes
dangerous things happen!
Volcanoes
Earthquakes
Tsunamis
Rift Zones
….. These all occur near the edge of plate
boundaries.
Its where all the action is!
The first subject we are
going to discuss is
volcanoes…
And the first type of
volcano we will discuss
is the stratovolcano.
All the volcanoes in the
Cascade Range are
stratovolcanoes.
This is a special type of
volcano. There are other
types of volcanoes…..
But no other type is as explosive
and DANGEROUS as the
stratovolcano!
Formation of Cascade
Stratovolcanoes
• All stratovolcanoes
are the result of
two converging
plates.
• The Juan de Fuca
Plate subducts
beneath the North
American Plate to
create the Cascade
volcanoes.
Here is a better visual
Even better!
The result is a line of composite
volcanoes that stretches from
Canada to Northern California
All stratovolcanoes form
as a result of subducting
oceanic crust under
continental crust!
Here is the geologic
setting that creates the
stratovolcanoes of South
America
Here, the Nazca Plate subducts
beneath the South American Plate
Stratovolcanoes
1. Large
2. Steep-sided, symmetrical cones
3. Built of alternating layers of lava flows,
volcanic ash, and cinders
4. Conduit system (like pipes) through which
magma rises from a reservoir that is deep
in the Earth's crust
5. EXPLOSIVE ERUPTIONS!
Draw a sketch of the
stratovolcano on the next
slide in your notes!
Eruption Style
• In recorded history, explosive eruptions at
subduction zone (convergent-boundary) volcanoes
have posed the greatest hazard to civilizations.
Subduction-zone stratovolcanoes like Mount St.
Helens typically erupt with explosive force,
because the magma is too stiff to allow easy
escape of volcanic gases. As a consequence,
tremendous internal pressures mount as the
trapped gases expand during ascent, before the
pent-up pressure is suddenly released in a violent
eruption. Such an explosive process can be
compared to putting your thumb over an opened
bottle of a carbonated drink, shaking it vigorously,
and then quickly removing the thumb. The shaking
action separates the gases from the liquid to form
bubbles, increasing the internal pressure. Quick
release of the thumb allows the gases and liquid to
gush out with explosive speed and force.
Cont…
• Since the year A.D. 1600, nearly 300,000 people
have been killed by volcanic eruptions. Most deaths
were caused by pyroclastic flows and mudflows,
deadly hazards which often accompany explosive
eruptions of subduction-zone stratovolcanoes.
Pyroclastic flows are fast-moving, avalanche-like,
ground-hugging incandescent mixtures of hot
volcanic debris, ash, and gases that can travel at
speeds in excess of 150 km per hour.
Approximately 30,000 people were killed by
pyroclastic flows during the 1902 eruption of Mont
Pelée on the island of Martinique in the Caribbean.
In March-April 1982, three explosive eruptions of El
Chichón Volcano in the State of Chiapas,
southeastern Mexico, caused the worst volcanic
disaster in that country's history. Villages within 8
km of the volcano were destroyed by pyroclastic
flows, killing more than 2,000 people.
So Basically,
1. Trapped gases in sticky
magma lead to violent
eruptions!
2. Like opening a can of shaken
carbonated soda (maybe you
can try this at home… or in Mrs.
Roby’s classroom
)
…just kidding
Cascade Stratovolcano examples:
Mt. Jefferson
Mt. Hood
Mt. Rainier
Mt. Saint Helens
Mt. Saint Helens
In early 1980, Mount St. Helens started to
experience shallow earthquake swarms and
a bulge appeared on its northern slope.
These are both evidence that magma is
moving upward and collecting within the
volcano. So, geologists everywhere were in
a frenzy, trying to monitor everything,
because it was the first time that such a
thing has happened where we (humans) had
sophisticated analytical equipment to
quantify the growth and activity preceeding
an eruption. Little was known about
eruptive processes, because relatively few
eruptions had been witnessed in recorded
history.
Cont….
• Back in 1980, there was no Mount St.
Helens observatory there like there is
now. So, the US Geological Survey
stationed geologists at a "safe"
position away from the mountain, to
observe its growth and activity. In the
days preceeding the eruption, these
geologist were Harry Glicken and
David Johnston. Here's a picture
Harry took of Mount St. Helens the
day before it erupted
Mt. Saint Helens the day before it erupted
•
Back in 1980, there was no Mount St.
Helens observatory there like there
is now. So, the US Geological Survey
stationed geologists at a "safe"
position away from the mountain, to
observe its growth and activity. In
the days preceeding the eruption,
these geologist were Harry Glicken
and David Johnston. Here's a picture
Harry took of Mount St. Helens the
day before it erupted
Cont..
• You can sort of see the bulge--it is
basically pointed right at the viewer. Now,
one might intuitively think, “Jinkies, that
bulge is pointing right at us, ain't it? Maybe
we shouldn't be here", but this wasn't
really the thinking of the day. People were
predicting a "classic" eruption where the
ash and ejecta goes up. This did happen at
Mount St. Helens, but what happened first
was this: An earthquake triggered a
landslide, which decapitated the mountain
along with most of the northern flank,
resulting in a "lateral blast" which just
torched and leveled everything in its path.
Cont…
• Luckily enough, geologist Harry Glicken (a
grad student at the time) had a committee
meeting he had to attend ON May 18, 1980,
so he left...the day before the eruption. 30
year old David Johnston, the USGS
geologist with Harry, stayed behind and
was killed during the eruption. His last
words over the transmitter were to the
USGS base in Vancouver, WA, and are now
'famous' amongst volcanologists:
"Vancouver, Vancouver, this is it!" The
ridge David Johnston was on has since
been renamed Johnston Ridge and the
Johnston Ridge Observatory sits there
now.
"Vancouver! Vancouver!
This is it!"
David Johnston's voice crackled
over the radio link, on the clear
Sunday morning of May 18,
1980. Seconds later, the
government volcanologist was
engulfed in the volcano's
gigantic lateral blast.
His body was never
found….
Side note of interest…..
David Johnston was Mrs. Clark’s
Uncle’s dormitory roommate in
college at the University of
Washington.
Cont..
•
Now, you might be thinking,
"wow, that Harry Glicken was
one lucky dude." No so. He was
killed during a volcanic eruption
at Mt. Unzen, Japan in 1991
along with famed French
volcanologists Maurice and
Katia Kraft.
Possible video links…