Geology 15                                                                                                     

West Valley College                                                                                         R. Lopez


Study Questions 6

California’s Young Volcanoes


  1. Read Harden p. 80-100.
  2. Describe the two part process of forming a lava dome (Plinian Eruption).  Also describe part three – coulee flow.
  3. What are the three forms of calderas?

Download map of California's Potential Volcanic hazards map.

Mt. Shasta

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?


Lassen Peak

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.


Long Valley Caldera

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)?


Recent Volcanic Unrest in the Long Valley Area

  1. What are three lines of evidence that a magma chamber (reservoir) presently underlies Long Valley? (see below)
  2. What has happened at the Horseshoe Lakes (Fig. 5-33, p. 92)?


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.


California Young Volcanoes Wrap-up


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.


Geologic Map Inside Front Cover of Your Textbook


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



California’s Young Volcanoes, What not to Miss


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.


Mt. Shasta


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).


Lassen Peak


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 ½ 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.


Medicine Lake Highlands and Lava Beds Nat. Monument


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.


Long Valley and the Eastern Sierra Nevada


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


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).