The Medicine Lake Highlands are best understood as the broad volcanic highland surrounding Medicine Lake volcano in northeastern California. They are part of the Modoc Plateau, but geologically they sit in an interesting transition zone between the Cascade volcanic arc and the extensional tectonics of the Basin-and-Range/Walker Lane region.
A gigantic, broad shield volcano
Medicine Lake volcano is enormous—about 2,200 km² (850 mi²) in surface extent—and is the largest volcano by volume in the Cascade Range. Unlike the steep, symmetrical cone of nearby Mount Shasta, it is a low, very broad shield-like volcanic edifice. Its summit is about 2,412 m (7,913 ft), while its lava flows extend far out onto the surrounding Modoc Plateau. (USGS)
The volcano has been built over roughly the last half-million years or more by repeated eruptions. Rather than one enormous volcanic event, its history consists of many episodes of lava emplacement, cinder-cone construction, explosive eruptions, and periods of dormancy. A particularly important characteristic is its extraordinary compositional range:
basalt → basaltic andesite → andesite → dacite/rhyolite
The surface of the volcano is dominated by mafic rocks, especially basalt and basaltic andesite, but drilling indicates that substantial rhyolite exists beneath the surface.
The tectonic setting is unusual
This is probably the most interesting part of the geology. Medicine Lake is east of the main axis of the Cascade volcanic arc. Instead of sitting directly above the principal Cascades volcanic zone, it occupies a region undergoing significant east-west crustal extension. Imagine pulling the crust apart from east to west. Faults and fractures develop, producing pathways through which basaltic magma can rise from depth. Medicine Lake sits at an especially complicated structural location where several fault systems interact. USGS describes it as being near the northwestern extension of the Walker Lane fault zone, where northwest-trending faults change orientation around the volcano. These structures appear to have strongly influenced where volcanic vents developed. So the volcano isn’t simply a product of the Cascades subduction system. It is essentially:
Cascade volcanism + crustal extension + major fault zones.
That helps explain its unusual size, broad shape, abundant basaltic volcanism, and large number of vents.
The volcano is actually a giant shield with a caldera
At the center is Medicine Lake caldera, approximately 7 × 12 km across. It isn’t the dramatic, deeply excavated type of caldera seen at places such as Crater Lake. Instead, it is a relatively shallow, broad depression. Medicine Lake occupies part of the caldera floor.
The volcano’s enormous volume is partly hidden because the edifice has been built into and over the pre-existing Modoc Plateau. Geophysical and drilling evidence indicates that the plateau surface beneath the volcano has been downwarped by roughly 0.5 km beneath its center. Consequently, the actual volcanic edifice may be considerably larger than what is apparent from surface topography.
Why there are so many lava tubes?
The northern Medicine Lake Highlands are particularly famous for their basaltic lava flows and lava tubes, especially around what is now Lava Beds National Monument.
The process is relatively straightforward:
- A basaltic eruption produces a fluid lava flow.
- The outer surface cools and solidifies.
- Lava continues flowing underneath the hardened crust.
- The eruption wanes and the molten interior drains away.
- A hollow tube remains.
Repeated eruptions produced extensive networks of these tubes. The Lava Beds area therefore provides an unusually good cross-section through the shallow volcanic plumbing system of Medicine Lake volcano. USGS notes that the area contains scores of lava-tube caves and exceptionally well-preserved young volcanic features. This is one reason the Medicine Lake Highlands are so interesting to a caver: the caves aren’t primarily erosional caves—they are volcanic caves formed directly by lava emplacement.
Glass Mountain tells a different story
On the eastern side of the volcanic complex is Glass Mountain, an enormous rhyolitic/obsidian flow complex. This is almost the opposite of the fluid basaltic lava responsible for the lava tubes.
Basalt:
- low silica
- relatively low viscosity
- travels long distances
- commonly forms lava tubes and broad flows
Rhyolite:
- high silica
- extremely viscous
- tends to accumulate near its vent
- can produce obsidian, pumice, domes, and explosive eruptions
Glass Mountain therefore demonstrates the extraordinary chemical diversity of the Medicine Lake magma system. The volcano can produce both extremely fluid basalt and extremely viscous rhyolitic magma.
The deeper magma system is more complicated than the surface suggests
One of the most interesting discoveries from drilling is that the volcano’s visible basaltic surface doesn’t represent the whole magma system. The subsurface contains considerably more rhyolite than surface mapping alone would suggest. This implies that basaltic magma rising from the mantle has interacted with and/or supplied heat to a more evolved crustal magma system. A simplified conceptual model is:
Mantle melting → basaltic magma rises → magma ponds in crust → fractional crystallization and crustal interaction → evolved and rhyolitic magma → eruptions of increasingly diverse compositions
This helps explain why a volcano whose surface is overwhelmingly basaltic can nevertheless produce rhyolitic eruptions.
It is still geologically young
Medicine Lake isn’t an extinct volcanic relic. The most recent eruption occurred approximately 950 years ago, and USGS considers Medicine Lake an active volcano with high threat potential. Its eruption history is episodic. There can be long quiet intervals, followed by clusters of eruptions. The USGS estimates at least 17 eruptions during the last ~12,000 years. That young age is also why the landscape is so spectacularly well preserved: many lava flows, cinder cones, spatter deposits, obsidian flows, and lava tubes haven’t had enough time to be destroyed by erosion.
The Modoc Plateau underneath it
The surrounding Modoc Plateau is important because Medicine Lake volcano didn’t develop on empty ground. It was constructed on a pre-existing volcanic plateau dominated by older basaltic volcanic rocks. Consequently, the Highlands are essentially a stack of volcanic landscapes:
older Modoc Plateau volcanism
↓
Medicine Lake volcanic edifice
↓
successive basaltic lava fields
↓
cinder cones and vents
↓
young rhyolite/obsidian eruptions
↓
modern erosion and glacial/weathering processes
This makes the region considerably more geologically complex than simply “a volcano.”
Glaciers and erosion have played a relatively minor role
The Medicine Lake Highlands reach almost 8,000 feet, so they have experienced substantial winter snow and some glacial activity, but they don’t have the deeply dissected landscape of the Sierra Nevada. That’s largely because the volcanic rocks are young and because enormous areas are covered by relatively recent lava flows. Consequently, you can still recognize the original volcanic landforms remarkably well:
- shield volcano slopes
- cinder cones
- lava-flow margins
- pressure ridges
- collapse pits
- lava tubes
- spatter ramparts
- obsidian flows
- pumice deposits
- caldera structures
From a geological perspective, it is almost a living laboratory of basaltic volcanism.
The big geological picture
I’d summarize the Medicine Lake Highlands this way:
They are a huge, relatively young shield volcano constructed on the Modoc Plateau where Cascade volcanism intersects an actively extending and faulted crustal environment.
The result is an unusually broad and chemically diverse volcanic system. Basalt dominates the surface and created the enormous lava fields and lava-tube networks; tectonic extension and faulting provided pathways for magma; a large summit caldera developed at the center; and evolved magma produced rhyolite, pumice, and obsidian such as the Glass Mountain complex.
That combination is what makes the Medicine Lake Highlands particularly fascinating: you can see the entire volcanic system from mantle-derived basaltic lava tubes to highly evolved rhyolitic obsidian flows within one enormous volcanic edifice.
The USGS’s geologic map of Medicine Lake volcano is especially useful if you’re interested in going deeper, because it shows the individual lava-flow units, vents, caldera, cinder cones, and rhyolitic centers. (USGS)
If your interest is specifically cave geology, I can also break down the Medicine Lake Highlands into the major lava-flow units and explain which kinds of eruptions produced the different lava-tube cave systems around Lava Beds, Glass Mountain, and the Medicine Lake area.
The cave geology of the Medicine Lake Highlands is dominated by volcanic caves—especially lava tubes. What makes the area exceptional is that the caves preserve the internal plumbing and cooling history of individual basaltic lava flows.
1. The basic cave-forming process
Most of the caves formed when fluid basaltic lava flowed across the surface.
A typical sequence is:
eruption → lava flow → crust formation → continued internal flow → drainage → tube
Basalt coming out of a Medicine Lake vent can initially be several hundred degrees Celsius and extremely fluid. The surface rapidly loses heat and develops a solid crust while molten lava continues moving underneath.
Eventually the eruption or lava supply diminishes. The molten interior drains away, leaving a hollow passage.
The result is a lava tube.
Unlike limestone caves, these caves aren’t carved out by groundwater dissolving rock. The cave is essentially the fossilized conduit through which the lava flowed.
Why Medicine Lake produces so many tubes
The critical ingredient is low-viscosity basalt.
Medicine Lake has repeatedly erupted large volumes of basaltic lava. Because basalt is relatively fluid, individual flows can travel many miles from their vents.
The Highlands also have relatively gentle slopes. That combination allows lava to remain flowing for considerable distances.
A simplified cross-section looks like this:
lava-flow surface
______________________
/ \
/ solidified crust \
/__________________________\
| |
| LAVA TUBE | ← formerly molten
|__________________________|
older lava
The tube initially isn’t necessarily a large cave. It may be only a few feet high. Continued lava flow can enlarge it, and later collapse can create entrances.
The ceiling is a geological record
One of the most interesting things about Medicine Lake lava tubes is that the walls and ceilings record the history of the flow.
You can encounter:
- flow layers
- lava ledges
- benches
- drip formations
- lava stalactites
- wall linings
- gutters
- shelves
- inflation structures
- secondary mineral deposits
These aren’t analogous to limestone stalactites. They’re mostly products of molten lava interacting with an already partially solidified tube.
For example, if lava level falls while a tube is still active, it can leave a horizontal lava bench on the wall.
Multiple episodes of flow can produce several benches at different elevations.
Lava tubes can have multiple levels
This is particularly important when looking at the Medicine Lake cave systems.
A lava flow doesn’t necessarily form one tube and stop.
As the flow evolves, new pathways can develop:
Surface
────────────────────────────
old tube
┌───────────────┐
│ │
└───────────────┘
↓ collapse
┌───────┐
│ │
└───────┘
│
│
┌───────────────┐
│ younger tube │
└───────────────┘
A later pulse of lava can establish a new tube beneath or alongside an older one.
Consequently, some cave complexes aren’t single tubes—they’re remnants of a three-dimensional lava-flow plumbing system.
Roof collapse creates many of the entrances
A lava tube can remain completely buried after the lava drains.
Eventually the roof becomes unstable.
Collapse produces:
- skylights
- pits
- entrances
- collapse chains
- trenches
A line of collapses can be especially significant.
If you see something like:
pit → closed section → pit → closed section → pit
there is a good chance you’re looking at the surface expression of a buried lava tube.
At Lava Beds, many cave entrances originated this way.
Lava falls are important
Some Medicine Lake caves contain lava falls.
Imagine lava flowing through a tube and encountering a step or steep drop. Molten lava cascades downward and then continues flowing along the lower level.
When the flow drains, the frozen cascade remains.
This produces spectacular features such as:
- lava cascades
- flowstone-like lava coatings
- plunge pools
- vertical shafts
- terraced floors
Again, these are frozen flow features, not water-eroded features.
Inflation caves are a little different
Not every volcanic cave in the Highlands is a classic tube.
Some develop through lava-flow inflation.
A lava flow can continue receiving molten material after its exterior has solidified. Internal pressure causes the flow to thicken and inflate.
Think of it somewhat like inflating a partially frozen blanket.
The interior can remain molten while the surface rises.
Later, the interior drains or freezes unevenly, producing cavities.
These inflation structures can produce complicated relationships between:
lava sheets → cavities → tubes → collapse pits
and they can be difficult to interpret without understanding the surrounding lava flow.
The age relationship is crucial
One of the best ways to understand Medicine Lake caves is to think in terms of successive lava flows.
The Highlands have experienced repeated eruptions over hundreds of thousands of years.
An older lava tube may be:
- partly buried by younger lava
- intersected by a younger tube
- truncated by a later flow
- filled with sediment
- collapsed
- exposed again by erosion
Therefore, two caves only a few hundred yards apart can actually belong to different eruptive episodes.
The cave itself is only one piece of the geological puzzle. The surrounding lava-flow relationships tell you when and how it formed.
Why the caves are so well preserved
This is where the Medicine Lake environment is particularly favorable.
The volcanic rocks are relatively young, and the region experiences relatively limited chemical weathering compared with humid cave environments.
So delicate volcanic structures can survive for thousands—or in some cases much longer—than they would in a heavily weathered environment.
A lava tube can therefore preserve its original morphology surprisingly well.
That’s why you can still see things like:
original lava floor → flow marks → lava shelves → roof structures → collapse features
essentially frozen in place.
But water still matters
Water doesn’t usually create the primary caves, but it can substantially modify them afterward.
Snowmelt and groundwater can enter through cracks and collapse entrances.
This can produce:
- sediment deposits
- ice
- mineral coatings
- breakdown
- erosion of loose material
- biological deposits
In some caves, water can migrate through fractures and alter the basalt.
But the distinction is important:
Water is generally a secondary modifier of Medicine Lake’s caves; lava created the original void.
The cave is really a cross-section through a lava flow
This is perhaps the most useful way to think about the geology.
When you enter a Medicine Lake lava tube, you’re effectively walking through a fossilized magma conduit.
The cave allows you to see things that would normally be buried beneath a lava flow:
surface
↓
vesicular upper crust
↓
massive lava
↓
former molten core / tube
↓
lower lava crust
↓
older lava flows
The cave therefore gives you a rare three-dimensional view into how a basaltic lava flow actually transported magma.
The really interesting part: cave systems rather than individual caves
It is better viewed as a large volcanic cave landscape produced by numerous generations of lava flows.
The important questions become:
- Which volcanic vent produced the flow?
- Which direction did the lava travel?
- What was the lava’s viscosity and eruption rate?
- Where did the tube form within the flow?
- How did the tube branch?
- Did later lava flows bury or intersect it?
- Where did the roof collapse?
- What happened to the cave after the eruption ended?
Answering those questions lets you reconstruct the eruptive history from the caves themselves.
One particularly useful distinction
There are essentially two scales to study:
Cave-scale geology
The walls, ceiling, floor, benches, lava falls, linings, breakdown, and mineral deposits.
Flow-field geology
The relationship between hundreds of caves, lava-flow margins, vents, collapse chains, inflation structures, and successive eruptive units.
The second scale is where Medicine Lake becomes especially fascinating. The cave systems can actually be used to reconstruct how individual lava flows moved across the landscape.