Download map of California's Potential Volcanic hazards map.
1)
What
kind of volcanic landform is Mt. Shasta, and how does it compare in size to
other Cascade volcanoes?
2)
On
a photograph such as Fig. 5-3 (p.68) of you text, be able to identify the
summit cone and Shastina.
3)
Mt.
Shasta is the summit cone, but it has a different geographical name as well.
What is it?
4)
Know
the names and relative ages (youngest to oldest) of the four different
cone-building events that define the current Mt. Shasta (in last 10,000 years).
5)
About
how often does Shasta erupt?
6)
Based
on the past history of Mt. Shasta, what are the chances, very approximately, of
Shasta erupting in your lifetime?
7)
A
truly remarkable eruption took place at Mt. Shasta about 300,000 to 380,000
years ago (see Harden p. 83). Briefly describe it. What does �hummocky�
topography look like (see Fig. 5-21 in Harden)? [Definition: �hummocky� = a rounded or conical mound, generally
equidimensional shape and not ridge like, a slight rise of ground above a level
surface.]
8)
How
does the volume of the catastrophic, 300,000-year-old event at Mt. Shasta
compare to the 1980 rockslide, debris avalanche at Mt. St. Helens?
9)
When
was the last eruption of Mt. Shasta?
10)
What
is the volume of Mt. Shasta and how does it compare to the other Cascade
Volcanoes?
Background:�� Formation of a stratovolcano caldera�see
figure.
1)
What
is a Caldera?� Be able to sketch the
development of a caldera-forming event such as occurred at the ancestral stratovolcano,
Mt. Tehama.
2)
What
is Brokeoff Mountain a remnant of, and when did this event occur (your book
gives us two different ages)?
3)
Which
of the two ages for the destruction of ancient Mt. Tehema would more accurately
indicate the caldera-forming event?
4)
How
was Lassen Peak formed (i.e. what type of volcano is it)?
5)
When
were the most recent eruptions at Lassen Peak?
6)
Caldera
forming events deposit large pyroclasitic ash flow sheets of tuff.� What geological formation is believed to
have been erupted due to the caldera-forming event at ancestral Mt. Tehama?
7)
Imagine
you are standing on Lassen Peak on a clear morning.� Looking to the north and northwest you see two large volcanoes
about 50 miles away.� One is high and
snowcapped, and in comparison, the other is low and dark.� Name these volcanoes.� What two types of volcanoes are they,
approximately how old are they, what natural province are they in, what rock
type is most common in each, and what are their approximate volumes?
8)
What
is the significance of the Chaos Crags and the Chaos Jumbles?
9)
What
is the significance of the Fantastic Lava Beds and Painted Dunes?� What is the age of the Cinder Cone at
Lassen?
Medicine
Lake Volcano and Lava Beds National Monument
1)
What
natural province is Medicine Lake volcano in?
2)
How
old are the oldest rocks at the volcano?
3)
What
type of rock dominates Medicine Lake volcano to give it its shield like shape?
4)
When
were the most eruptions at Medicine Lake volcano?
5)
Medicine
Lake sits in a natural depression at the summit of Medicine Lake volcano. Why
is that depression there and what is it called?
6)
What
type of rock underlies Lava Beds National Monument?
7)
What
is the Lava Beds Nat. Monument most famous for?
8)
Briefly
discuss how lava tubes form.
1)
Be
able to locate Long Valley Caldera on a map (e.g. see fig. 5-5, p. 69 and 5-31,
p. 91 of text), and describe its location in California in words.
2)
Be
able to discuss the development of Long Valley Caldera using a simple sketch.
3)
What
are the compositions and significance of the Glass Mountains of Long Valley and
the Bishop Tuff?
4)
When
did the Bishop Tuff erupt?
5)
How
does the volume of the Bishop Tuff compare to the 1980 eruption of Mt. St.
Helens and the 300,000 eruption of Mt. Shasta?
6)
What
is the geological relationship of Mammoth Mountain and Devil�s Postpile to the
development of Long Valley caldera?
7)
Since
the 1980 earthquakes, how much has the caldera floor/resurgent dome risen?
8)
What
is the current rate of bulging for the resurgent dome/caldera floor (page 93)?
9)
What
are the approximate ages of Mammoth Mountain and the basalts of Devil�s
Postpile?
10)
Devil�s
Postpile is famous for columnar jointing. What is columnar jointing and how
does it form?
Mono
Domes-Craters and Inyo
Volcanic Chain
1)
Be
able to locate the Mono Craters and Inyo Craters volcanic chain, and be able to
describe their locations relative to Mono Lake and the Long Valley Caldera.
2)
What
is the dominate rock type of the Mono Craters, and what are the common volcanic
landforms there?
3)
When
were the most recent voluminous eruptions?
4)
What
might be the significance of the Mono Craters chain (compare this to the Glass
Mountain Rhyolites of Long Valley)?
5)
When
were the most recent volcanic eruptions in the Inyo Craters chain?
6)
When
was the last volcanic activity in the Mono-Inyo volcanic chain (Mono Lake)?
Volcanic
unrest (i.e. activity associated with magma) was first recognized following a
sequence of four magnitude 6 earthquakes in May 1980, and the activity reached
a new phase in 1989.� Incidentally, the
May 1980 earthquakes followed the eruption of Mt. St. Helens by only two weeks,
and caused great alarm among geophysicists and volcanologists at the USGS.� Evidence for magmatic unrest includes three
types of activities.
(1)
Earthquakes
of various sorts.� Swarms have been
concentrated in the south moat, and since 1989, there have been persistent
swarms of small (all but a few M<4), shallow (10 km) quakes beneath Mammoth
Mt.
(2)
Bulging
of the caldera floor (Harden Fig. 5-35).�
Since 1970 there has been 70+ cm (i.e. 2.5 ft.) of uplift, mainly of the
resurgent dome.� The uplift of the
resurgent dome seems to be driving at least some of the earthquake activity in
the southern moat.� Since May 1997 the
uplift has accelerated.
(3)
CO2
gas emissions from the ground have killed stands of trees in several areas near
Mammoth Mt. and the southern moat.
What
kind of eruption is likely?� Over the
past 5000 years there have been about 20 eruptions in the area (including Mono
and Inyo chains) at intervals of about 200 to 700 years.� The probability of an eruption here is about
the same as that for a M8 earthquake on the San Andreas Fault or the eruption
of a large Cascade volcano such as Mt. Shasta.
Color Coded Volcanic Hazards Conditions for Long Valley:
Green
� no immediate risk (weak to moderate unrest).
Yellow
Watch � intense unrest � this level has never been reached, but was nearly
declared in November, 1997.
Orange Warning � eruption likely within hours or days.� May occur every few hundred years.
Red
Alert � eruption underway.� May occur
every few hundred years.
Look
carefully at your textbook cover. You should be able to pick out many of the
young volcanic features of California that we have discussed. Look for the
following.
1)
Shasta
Valley (site of the catastrophic 300,000-380,000 year old debris avalanche
deposit).
2)
Mt.
Shasta�white dot at the southeast edge of Shasta Valley.
3)
Medicine
Lake Highlands�dark area east of Mt. Shasta at the edge of the Modoc Pleateau.
4)
Lassen
Peak�white dot northwest of Lake Almanor, both are labeled on the geologic map
inside the front cover.
5)
Mono
Lake�the white dot in Mono Lake is Paoha Island, which is composed of lake sediments
that were pushed up by an underlying magmatic intrusion about 250 years ago.
6)
Mono
Craters�immediately south of Mono Lake.�
Look carefully an you can see the concave westward shape of the chain.
7)
Long
Valley Caldera�Northeast wall of the caldera is in shadow.� Slightly larger than Mono Lake.
Look
at the map explanation and note that the pink color is used for Cenozoic
volcanic rocks (rocks younger than 66 million years).� Note that the Cenozoic volcanic rocks are concentrated in the
northeastern part of the state, but they also occur elsewhere, principally in
the northern Sierra Nevada, Basin and Range, Mojave, and northern Coast
Ranges.�
On
this geologic map, find Mt. Shasta, Lassen Peak, Mono Lake, the Long Valley
Caldera, and the Volcanic Tableland underlain by the Bishop Tuff.
Mojave
and Basin and Range Province Volcanoes
Read about Amboy Crater in the Mojave province and the Coso Volcanic Field in the Basin and Range province.
Colorado Desert Province Volcanoes
Read about the Salton
Buttes Lava Domes in the Salton Trough.
Web Pages:
Check
out the U.S. Geological Survey Volcano Hazards Program on the Long Valley
Caldera at�
http://quake.wr.usgs.gov/volcanoes/longvalley/
Below
are my suggestions for where to go and what not to miss at our state�s volcanic
hot spots.� Note, I will discuss Amboy
and Pisgah craters found in the Mojave province when we discuss the Basin and
Range and Mojave provinces.
Take
the Everitt Memorial Highway up the mountain to the area once occupied by the
Ski Lodge, at a elevation of 7,700 ft.�
Debris avalanches as well as snow avalanches destroyed this Ski Lodge.� You will see a variety of andesitic to
dacitic volcanic rock, often in a jumbled, �hummochy� landscape.� Also, look for glacial features on the
mountain as well as on the rocks (we will discuss these when we talk about
Yosemite).
Hike
to the top of Lassen Peak for great views of Mt. Shasta and Medicine Lake
volcano.� Best to do this in the
morning.� Take the � mile trail to Bumpass Hell and notice the altered
rock due to hydrothermal circulation.�
This rock was once hard andesite and dacite lava (this is a form of
metamorphism).� On your way out through
the northern park entrance, stop and look at the famous �1915� Hot Rock and Devastated Area. How has the forest rebounded since 1915?� Continue north and stop when you get a clear
view of Chaos Crags (recall, it�s a
group of dacite domes).� The area where
you are standing is known as Chaos Jumbles.�
Chaos Jumbles is a Rock
Avalanche deposit that slid off of Chaos Crags.� The large extent and travel of the Jumbles is interpreted to have
been aided by trapped air beneath the Rock Avalanche as is slid off the
mountain. Air cushioned the debris slide and propelled it to farther distances
from the Chaos Crags.� Finally, go to
the northeastern area of the park and hike to the top of Cinder Cone.� From the
summit of Cinder Cone, look at the Fantastic
Lava Beds immediately below the Cone to the southwest.� The red and yellow colors are due to iron
oxidation of a pyroclastic tephra erupted onto a still hot basaltic lava flow.
You
get great views of Medicine Lake volcano, the highlands, and Glass Mountain
rhyolite lava flow from Highway 139 in the morning.� The highlands are heavily forested; however, drive to the top of
little Mt. Hoffman for a panoramic view of Mt. Shasta, Lassen Peak, Mt.
McLaughlin in Oregon, and the little Glass Mountain rhyolite lava flow.� When visiting the Lava Beds National
Monument, go into the Lava Tubes and see Captain Jack�s Stronghold.
Take
the Volcanic Auto Tour (see attached brochure cover, purchase from US Forrest
Service Mono Lake Visitors Center or at the Mono Lake Committee bookstore in
Lee Vining).� Don�t miss the Devil�s
Postpile National Monument, the Earthquake fault, and the Inyo Craters.� Visit the Rhyolite domes of the Inyo and
Mono Craters.� Obsidian dome and Panum
crater are easy hikes.� Visit the
Mammoth Lakes, in particular Horseshoe Lake.�
See the Tree-Kill area due to CO2 gasses.� For exposures of the Bishop Tuff, take the Little Round Valley
loop near Crowley Lake.� A note about
hot springs.� There are a variety of hot
springs in Long Valley caldera.� They
range from 80�F to ~130�F, and can scald your skin.� The best pool for wading is the Neon Church hot spring.� Take the Benton crossing road (where the
Neon Church is) from Highway 395 just south of the Mammoth Lakes turn off.� Count three cattle guards, beginning with
the one right as you turn on to the Benton crossing road. At the third cattle
guard, make a hard, hairpin turn to the right.�
Follow this dirt, bumpy road for about one mile.� Park at the designated parking lot and walk
down to the pools.� Best to enjoy the
hot springs in the evening�lots of shooting stars in the eastern Sierrra.
Amboy
and Pisgah craters are basaltic lava domes.�
They can not really be called shield volcanoes because they are far too
small.� They are best classified as
basaltic lava, cinder cones (the lava has overwhelmed the cinder cones).� Amboy and Pisgah are in the Mojave�s Devils
Playground.� The craters are on
Interstate 40 with Amboy to the south and Pisgah on the north.� Good samples of vesicular basalt (scoria).