All writing in ‘white’ should go in your notes! Yellow is just to read, not copy down.

All writing in ‘white’ should go in your notes!

Yellow is just to read, not copy down

Danger on the Edge of a Plate

Cascade Stratovolcanoes

6th Grade ScienceSMS

The edges of plate boundaries are where exciting and sometimes dangerous things happen!

VolcanoesEarthquakesTsunamisRift 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…