Photograph from the top of Amboy Crater.

Panoramic view from the top of Amboy Crater.

Overview

This field trip will take you to the closest volcanoes to Los Angeles and will provide an opportunity to observe firsthand cinder cone volcanoes as well as other volcanic features, such as lava flows, a crater and vent, lava tubes, and more.

Pisgah and Amboy craters are two of four volcanoes in the Cima Volcanic Field and constitute just a couple of the approximately 40 young cinder cone volcanoes in the Mojave Desert and greater Basin and Range Geologic province. The Basin and Range is a region of continental rifting, extending from the eastern front of the Sierra Nevada Mountains to western Utah and as far south as Mexico. The following video, courtesy of Robert McMillian, provides a nice visual overview of some volcanoes in the Mojave Desert: drone video of volcanoes in the Mojave Desert. Both volcanoes are on the North American Plate, with Pisgah Crater being about a half-hour closer to Los Angeles than Amboy Crater, making it the more practical destination of the two. On the other hand, Amboy Crater has been developed for visitors, having recently been designated a Natural Landmark; it has a paved road and parking area, restrooms, and a picnic area. It also has retained a nearly perfect conical shape, while the configuration of Pisgah Crater has been degraded by mining operations.

Map showing location of Mojave Desert.

Map of Mojave Desert, Basin and Range, Pisgah Crater (P), and Amboy Crater (A). –Modified image from CC.

Special Safety considerations at Pisgah and Amboy craters

Rattlesnakes have been observed during a past field trip so exercise extra caution if your students walk off of the road or trails.
If you opt to explore the lava tubes then encourage students to bring gloves for some hand protection.
As for any field trip, students should bring plenty of water, but visiting either of these volcanoes will require walking a few miles, meaning several hours away from vehicles, under potentially very hot and dry conditions.
It can be dangerously warm during the summer and early fall. Visits should be made during the late fall, winter, or early spring months.

Geology

Cinder cones are small, steep-sided, cone-shaped piles of cinders - small blobs of lava that erupt from a central vent to build up a volcano. Cinder cone eruptions are not explosive but can be spectacular to behold, with lava fountaining out of the vent hundreds of feet into the air. To get a sense of such an eruption access the following video, where lava is being ejected from a vent near Kilauea, resulting in constructing a cinder cone: volcanic eruption.

From the time a cinder cone starts erupting, it will continue to do so regularly for months to decades, building its cone through the accumulation of lapilli-sized pyroclasts, (cinders). These are ejected from the vent as a pea to golf-ball-sized blobs of lava that quickly solidify into rocks of basalt (cinders) after landing. Eventually, the volcano may emit basaltic lava flows from its base, which typically signals that the eruptive cycle is coming to an end; for once the lava flows stop, these volcanoes likely will go extinct and never erupt again.

Cinder cones usually form over hot spots or within continental rift zones, like the Basin and Range. They can also form near convergent boundaries, in instances where the magma is mafic in composition.

When Pisgah Crater and Amboy Crater last erupted is not known for certain. Lava flows interbedded with lakebed sediments constrain a maximum age of about 100,000 years (Sharp and Glazner, 1993), but most studies hypothesize an age for these two cones in the range of 20,000 to 5,000 years old; perhaps even younger. Certainly, the flows and Amboy Crater look quite young. Pisgah crater may look older, but this is because mining operations have degraded the shape of the cone. Pisgah and Amboy craters rise 300 and 250 feet high above the surrounding plain, respectively, and were built from basaltic pyroclastic material called cinders. Each has extensive basaltic lava flows emanating from its base. Geographically they roughly align with other volcanic features in the area, suggesting that they may have formed along a fault (Sylvester and Gans, 2016). Certainly, earthquake faults exist in the area, as evidenced by the 1999, magnitude 7.1 Hector Mine earthquake.

The Cady and Bristol mountains that make up the skyline north of Pisgah and Amboy craters are composed of rock that provides a notable contrast to the very young basalt flows associated with these volcanoes. While these mountains are also made up of igneous rock, they are significantly older and more felsic in composition, including intrusive, Mesozoic-age granitic rock, and Tertiary (~ 20 million years) lava and pyroclastic flows. The Bristol Mountains also contain Proterozoic metamorphic rocks.

Learning Objectives

Through participation in this field trip students should be able to:

Name the type and the specific name of the plate boundary that was crossed in order to get to Pisgah/Amboy Crater
Identify aphanitic (fine-grained) igneous rock texture
Identify basalt, 2 types of lava flows, pahoehoe or aa lava, and lava flow features such as pressure ridges and squeeze-ups
Describe the evolution and life cycle of cinder cone volcanoes
Identify different components of a volcano, such as a crater and central vent
Describe why volcanoes have formed in the Mojave Desert

Key Vocabulary

Aa – basaltic lava flows with a jagged surface, representing a more viscous part of the flow
Basalt – Most common volcanic rock; mafic in composition
Cinders – small, basaltic pyroclastic material ejected during the eruption of cinder cones that solidify into small, pea to golf-ball-sized rocks
Continental rifting – stretching of continental crust by tectonic forces; could be in the initial stages of making a divergent plate boundary
Crater – closed depression at the summit of a volcano
Decompression melting – melting of rock in the upper asthenosphere triggered by a release of pressure through sea-floor spreading or continental rifting
Mafic – Term used to describe the composition of igneous rocks that contain abundant Iron and Magnesium and lower amounts of silica, ~ 50%
Pahoehoe – fluid basaltic lava flows with a smooth, “ropy” surface
Pressure ridge – an elongated, uplifted portion of a solidified lava flow, formed while the lava was flowing
San Andreas fault – Major right-lateral strike-slip fault and transform plate boundary between the Pacific and North American plates
Squeeze-up – small extrusion of solidified lava from a fracture in a lava flow
Vent – an opening or fissure at the bottom of the crater, or a fracture on or near the volcanic cone from which the volcano releases gas, pyroclastic material, and lava

Drawing of the anatomy of a Volcano.

The anatomy of a volcano.

Pre-Field Trip Questions

Can a volcano form anywhere?
Why do volcanoes form where they do?
What is needed to make a volcano?
Can magma be made anywhere?
Compare and contrast a cinder cone to a shield volcano and a composite cone volcano. Answer as a short essay or illustratively, using labeled sketches of each of volcano or by constructing a Venn Diagram.
Describe how the color of mafic rocks would be different than the color of intermediate or felsic igneous rocks.
Quickly sketch or describe in words the difference in texture between an aphanitic and phaneritic igneous rock.

Directions to Pisgah Crater

From Cerritos College, take the 605 north to the I-10 east, I-15 north, and I-40 east. Continue on I-40 eastbound for just over 3.5 miles to Hector Road. Exit the freeway turn right and immediately left on National Trails Highway (CA-66); proceed about 4.5 miles and turn right onto Pisgah Road (look for a beat-up sign).

Pisgah and Amboy Crater Field Trip Stops

All stops listed in this chapter are accessible by large charter bus, van, or 2-wheel drive car. However, some charter bus drivers may be reluctant to drive the charter bus on the unimproved Pisgah Road. It would be wise to inform the charter bus representative ahead of the field trip of the road condition and to give the bus driver a heads-up upon loading the bus at the start of the trip. If the driver chooses not to drive the road upon arrival to Pisgah Crater then Plan B would be to continue on to Amboy Crater, which has a paved road and parking area, but, as mentioned in the introduction, is about a half-hour beyond Pisgah Crater.

Photograph of Pisgah Crater.

Pisgah Crater and Pisgah Road.

Pisgah Crater

Stop 1 – Observe aa flows

Addresses learning objective:
3. Identify basalt, and pahoehoe and aa lava flows in the field

Drive up to the gate and park, exit, and inspect the lava flow just past the gate on the east side of the road to observe a good example of aa basalt flows.

Activity 1: Ask students to describe the surface of the aa flow as smooth or jagged.

Stop 2 – Observe pahoehoe flows and discuss the geology of Pisgah Crater and cinder cone volcanoes

Addresses learning objectives:
2. Identify aphanitic (fine-grained) igneous rock texture in the field
3. Identify basalt, and pahoehoe and aa lava flows in the field
4. Describe the evolution and life cycle of cinder cone volcanoes
6. Describe why volcanoes have formed in the Mojave Desert

Hike (if the gate is locked shut) or drive up haul road to the wide, flat abandoned mining operations area on the western flank of the volcano. Park on the western side. Exit vehicles and assemble along the top of the western edge, so you’re atop the slope looking down upon the extensive pahoehoe basalt flows.

Photograph of pahoehoe lava flows.

Pahoehoe flows.

Activity 2: Ask students to describe the surface of the aa flow as smooth or jagged. Do these flows exhibit aa or pahoehoe characteristics? – pahoehoe

Discuss the presence of cinder cones in the Mojave Desert
- About 40 young cinder cone volcanoes and extensive lava flows are found in the Mojave Desert
- The age of the oldest lava flows indicates the volcanic activity started about 7.5 million years ago
- Considering the timing and geographic location, it is feasible that the concentration of young volcanoes in Mojave is a consequence of decompression melting in the upper mantle, caused by crustal extension from the continental rifting of the Basin and Range.
- Cinder Cone volcanoes
  - Typically erupt cinders over the course of months, years, or decades building up their cone
  - Lava flows often signal the end of the eruptive cycle and break through the base or side of a volcano, as the loose pile of cinders is not strong enough to contain the lava all the way from the magma chamber up to the crater
  - They have effusive, quiet eruptions – throwing out small, fragmented basaltic lava that hardens into small rocks (cinders) that pile up around a central vent, building up the cone over time
  - Once the eruptive cycle has completed they go extinct

Activity 3: Ask students to inspect lava flows and cinders, noting, in particular, the texture and composition. If these rocks were felsic in composition and formed intrusively, how would the color and texture be different?

Stop 3 – Inside crater

Addresses learning objective:
5. Identify different components of a volcano, such as a crater and central vent

Walk uphill, around the south flank of the volcano to a trail leading into the crater on the east side of the cone.

Once everyone is inside the crater, which is about where the trail levels off, explain that you’re now inside the crater
Point out the central vent, which should be visible at the low point of the crater depression

Activity 4: Ask students to answer the question: Considering what is typical of cinder cone volcanoes, how likely is it that Pisgah Crater will erupt while we are visiting it today? How about during our lifetime? If Basin and Range extension is still happening in the Mojave Desert region, is possible that another cinder cone volcano could form nearby in our lifetime?

From here you can head down to the bottom of the crater and into the vent, which forms a cave-like feature that can fit a few students at a time – students love caves!
Additionally, you can take the trail to the top of the volcano, which provides outstanding views of the surrounding landscape – again, remind students to be careful as they walk up and down the trail. Even a small stumble could result in significant skin loss on hands and knees from the sharp basalt.

Stop 4 – Lava Tubes

Addresses learning objective:
3. Identify basalt, 2 types of lava flows, pahoehoe or aa lava, and lava flow features such as pressure ridges and squeeze-ups
5. Identify different components of a volcano, such as a crater and central vent

If you feel comfortable with your student’s ability to manage their own safety, you might want to explore one or more of the lava tubes. Lava tubes form when the top of a fluid lava flow that is contained in a channel solidifies, making a roof over the rest of the flow. This happens because the flow below is insulated; while the top of the flow loses heat the fastest. Eventually, the lava empties out of the “roofed” channel, leaving behind a tunnel or lava tube. There are dozens of lava tubes in the lava fields around Pisgah, many with collapsed roofs; several can be found near the base of the eastern side of the volcano. If you choose to venture out into the lava field, exercise caution. The ground is uneven, unstable, and sharp. Watch for rattlesnakes. Be sure to have a safety chat with the students before allowing any of them to explore. If students are uncomfortable then propose that they just observe from the edge of the “parking area” on the eastern flank of the cone.

Directions to Amboy Crater

From Barstow head east on I-40 for 52 miles to Ludlow. Turn right (south) from the off-ramp then immediately left (east) and continue about 26 miles to the road leading to Amboy Crater National Natural Landmark and park in the parking lot.

Photograph of Amboy Crater.

Amboy Crater with aspiring geologists in the foreground.

Amboy Crater

Stop 1 – Overview

Addresses learning objectives:
2. Identify aphanitic (fine-grained) igneous rock texture in the field
3. Identify basalt, and pahoehoe and aa lava flows in the field
4. Describe the evolution and life cycle of cinder cone volcanoes
6. Describe why volcanoes have formed in the Mojave Desert

From the parking lot walk up to the gazebo-type observation area to provide students with a geologic overview:

Cinder cones in the Mojave Desert
- Pisgah and Amboy craters are two of four volcanoes in the Cima Volcanic Field and constitute just a couple of the approximately 40 young cinder cone volcanoes in the greater Mojave Desert region.
- About 40 young cinder cone volcanoes and extensive lava flows are found in the Mojave Desert
- The age of the oldest lava flows indicates the volcanic activity started about 7.5 million years ago
- Considering the timing and geographic location, it is feasible that the concentration of young volcanoes in Mojave is a consequence of decompression melting in the upper mantle, caused by crustal extension from continental rifting of the Basin and Range.

Activity 1: Ask students to answer the question: Considering what is typical of cinder cone volcanoes, how likely is it that Amboy Crater will erupt while we are visiting it today? How about during our lifetime? If Basin and Range extension is still happening in the Mojave Desert region, is possible that another cinder cone volcano could form nearby in our lifetime?

Cinder Cone volcanoes
- Typically erupt cinders over the course of months, years, or decades building up their cone
- Lava flows often signal the end of the eruptive cycle and break through the base or side of the volcano, as the loose pile of cinders is not strong enough to contain the lava all the way from the magma chamber up to the crater
- They have effusive, quiet eruptions – throwing out small, fragmented basaltic lava that hardens into small rocks (cinders) that pile up around a central vent, building up the cone over time
- Once the eruptive cycle has completed they go extinct

Stop 2 – anywhere along the trail to the volcano

Addresses learning objectives:
2. Identify aphanitic (fine-grained) igneous rock texture in the field
3. Identify basalt, and pahoehoe and aa lava flows in the field
4. Describe the evolution and life cycle of cinder cone volcanoes

It’s best to use the trail to access the volcano, as opposed to going overland and running the risk of twisted ankles, snake encounters, or losing students. Along the way, there are many opportunities to discuss lava features, such as pahoehoe vs. aa lava, pressure ridges, and squeeze-ups. One could also discuss the gradient or slope of the flank of the volcano and relate this to interpreting topographic maps.

Photograph of a pressure ridge.

Pressure Ridge. - CC

Activity 2: Ask students to inspect lava flows and cinders, noting, in particular, the texture and composition. If these rocks were felsic in composition and formed intrusively, how would the color and texture be different?

Activity 3: Ask students to visually identify and draw pressure ridges and squeeze-ups.

Stop 3 – Inside crater

Addresses learning objective:
5. Identify different components of a volcano, such as a crater and central vent

Walk counterclockwise around to the west side of the volcano and ascend the steep trail up into the crater through a breach in the side of the volcano. It’s likely that this side of the volcano was carried away by a lava flow bursting through the base of the cone and flowing off towards the south.
Once inside, you’ll notice (especially if you walk up onto the rim of the crater) that there are two small roughly shaped “rings” of cinders that represent two subsequent eruptions that happened after the principal, volcano-building eruption.
Climbing up on the rim offers excellent views of the Bristol Mountains off to the north and the Marine Corps Base off to the south.

Follow-up Activities and Questions

Ask students to reflect on and write about how observing a cinder cone volcano first-hand was different and the same as their expectations.
Have students summarize the eruptive history of Pisgah or Amboy crater, including why the volcanoes exist in the Mojave Desert, the timing of the eruptions, and the chemistry of the lava flows.
What is a “water gap” (like the Whittier Narrows that contains the 605 freeway) and how does one form?
Should scientists be surprised if the Pisgah/Amboy crater were to erupt again? Explain.
Considering that the Mojave Desert lies within the Basin and Range, should we expect more volcanoes to form in the Mojave in the future? Explain.