Download Week 10

10-1
Volcanoes
Basaltic Volcanoes
Volcanic eruptions can be quite spectacular:
The main factor which determines a volcano’s
eruptive style is the composition of the lava it erupts.
Mt. St. Helens
The island of Krakatau literally blew itself up.
However, some volcanoes erupt in a quite peaceful
manner, quietly flowing lava out onto the surface.
Mafic (low silica content) magmas are much less
viscous than felsic (high silica content) magmas.
Thus it is relatively easy for gases in such magmas
to escape.
Thus are large pressures likely to develop?
Clearly there are a variety of types of volcanoes.
Why do they differ in size and shape?
Are they likely to be explosive?
Why are some explosive and others not?
Basalt volcanoes are the most common.
Seen in two main geologic settings:
1. Mid-oceanic ridge
Volcanism at Hot Spots
In addition to the mid-ocean ridges, basaltic
volcanoes are seen at intra-plate volcanoes overlying
hot spots.
The Hawaiian Islands are the classic example.
==> Ocean floor almost entirely made of basalt and
gabbro.
2. “Hot Spots”
Because the lava generated is of low viscosity, would
you expect the lava to be able to flow over relatively
large or small distances?
Would you expect such volcanoes to be gently or
steeply sloped?
These are called shield volcanoes.
10-2
Types of Lava
Underwater we often see pillow basalt.
Much of our knowledge of basaltic lava flows come
from the Hawaiian islands.
The water quickly cools the lava as it emerges.
Thus, our names for types of lava come from
Polynesian roots.
Outside layers solidify instantly.
pahoehoe—“ropy” lava.
Lava then continues to ooze out within the solidified
skin.
Low viscosity and thus flows quickly yielding a ropy
appearance.
Typically seen near the eruption site.
aa—
Looks much as if someone were squeezing
toothpaste out of its container.
Looks much like pillows stacked on top of each
other.
Higher viscosity (cooler) lava. Moves more slowly
and has a more blocky jagged look to it.
Andesitic Volcanoes
How do these volcanoes differ from those erupting
more mafic material?
Volcanoes which occur at subduction zone
boundaries tend to exhibit a more felsic composition.
The higher silica content makes the lava more
viscous.
Why?
Will lava flow as far?
As with more mafic volcanism, it is thought that the
magma is created from melted mantle.
What about the slope of the volcano?
What makes it more felsic?
Finally how would you expect the eruptive style to
differ from more mafic volcanoes?
10-3
Some volcanoes are on the border between being
explosive or effusing lava quietly.
Rhyolite Volcanoes
They may do both at various times.
Very felsic volcanoes are rare.
These types of volcanoes create composite cones or
stratovolcanoes.
However, when they do occur the lava is very viscous
and thus they tend to be quite explosive.
Why are they rare?
Volcanoes that are more explosive form pyroclastic
cones or cinder cones.
These types of volcanoes tend to be small.
The unconsolidated pyroclastic material also tends to
be eroded quickly.
Geology of the Olympic Peninsula
We live near a subduction zone boundary between
the Juan de Fuca plate and the North American plate.
What does this mean for earthquakes in our region?
20 to 30 million years ago the olympic peninsula
was not here:
the coastline was near present day Seattle.
Over time material has been brought to the
subduction zone and scraped off forming the
olympic peninsula.
What kind of feature is the olympic peninsula?
Volcanoes?
The subduction zone boundary leads to current uplift
of our region. Evidence for this?
10-4
If the olympic peninsula was created by rocks
scraped off of the oceanic plate, what types of rocks
would you expect to see?
Weather Patterns:
Prevailing winds at our latitude are from the west
(from the Pacific ocean).
As air flows over the olympics it drops the moisture
it is carrying—
lots of precipitation on the westward side of the
mountains.
Rain at lower elevations, snow at higher elevations.
Much less rainfall on the eastern side of the
mountains.
Abundant snowfall at high elevations has maintained
numerous glaciers to this day.
However, the glaciation was much more extensive
during the period of the last glacial advance.
At this time glaciers flowed out of Canada and down
the Strait of Juan de Fuca and Hood canal.
The olympic peninsula was almost an “island” in
the middle of this great glacier.
Glaciers in the mountains themselves reached much
lower elevations than today.
Evidence for this period of glaciation seen in
numerous U-shaped valleys throughout the
olympics.
Lake Quinalt and Crescent lake both currently
occupy such glacial valleys.
U-shaped valleys are quite steep—mass wasting
currently moving material downhill.
Eventually valleys will be “evened out” and take on
a more “V”-shape.