Saturday, October 1, 2016

California's Cascade Volcanoes: Medicine Lake Highland & Lava Beds National Monument

Medicine Lake Highland is a massive shield-like volcano in Northern California, east of Mount Shasta in a region that combines features of the Cascades with those of the Modoc Plateau and Basin and Range provinces.  The volcano has a volume of 130 cubic miles, making it California's largest volcano.  And most people have never even heard of it!  The volcano rises nearly 4,000 feet above the Modoc Plateau, from a starting elevation of roughly 4,000 feet to nearly 8,000 feet at its summit.  The Medicine Lake Highlands span about 30 miles north-south, and 20 miles east-west. 
Looking southwest, across Lava Beds National Monument, toward the massive shield edifice of Medicine Lake Highland.
On my recent geology field studies trip, offered through Modesto Junior College (taking extra classes purely for the joy of learning seems to be a sort of hobby of mine), I had the privilege of traveling to Lava Beds National Monument to see the products of the Medicine Lake volcano - specifically, lava flows and lava tubes. 

Basalt lava flows in the foreground, with the lava source, the Medicine Lake shield volcano, in the background.
After our stay at Lava Beds, we drove up and over the Highlands, traversing its length to experience Mammoth Crater, the source of 70% of the lava flows in Lava Beds National Monument, and the caldera at the summit which contains the beautiful Medicine Lake itself.
Mammoth Crater, 350 feet deep and a quarter-mile wide, is a large vent on the flank of Medicine Lake Highland.

Lava Beds National Monument contains miles and miles of cave systems, lava tubes that formed as channels of lava cooled from the outside.  As the top, sides, and bottom of a lava flow cooled, it hardened into a crust, allowing hot, fluid lava inside to continue to flow and drain out, leaving behind a hollow tube, or cave. 
Opening of a lava tube, or cave.
Some features inside lava tubes are similar to those found in more familiar limestone caves, but instead of being formed as dripping water leaves behind mineral deposits, formations like "lavacicles" were formed as lava dripped from the ceiling as a tube cooled.  Access to lava tube caves is gained when a portion of the roof collapses, providing an opening to the chamber below. 
Entering Skull Cave
Lava Beds National Monument is home to 14 species of bats.  Many are permanent residents, choosing from a variety of habitats in the monument, from miles of caves, to cliff faces and even trees; but some species migrate here from Mexico or as far south as South America.  The bats at Lava Beds are insectivores, catching flying insects using echolocation.  I was thrilled to stumble upon a few bats in the darkness of one cave; I don't know what species they were, but according to the National Park Service, the most common type seen in the lava tubes is Townsend's Big-eared Bat (Corynorhinus townsendii). 

The best thing to do if you come across bats is observe them quietly and respectfully for a moment if you'd like, then leave them in peace.  Don't shine your flashlight directly at them, and be very quiet.  Do not try to touch them; don't make too much noise or disturb them in any other way!

Bats are fascinating animals, and a valuable part of the ecosystem; I'm afraid they are too often underestimated.  We should be thankful for the insect control service they provide, eating up to their body weight in insects each night!  Humans have absolutely NO reason to fear bats or cause them any harm.

A good portion of our time at Lava Beds was spent exploring these fascinating caves, wearing protective clothing (basalt is sharp!) as well as helmets and headlamps for all those dark passages and tight squeezes! 
A very happy naturalist, delving into the realms of geology and caving.
An interesting fact about the Medicine Lake shield volcano (or, more accurately, "shield-like" volcano) is that it exhibits bimodal volcanism.  This means that it has more than one magma chamber, with more than one magma type; in addition to erupting basalt lava, from magma chambers of basaltic magma, it also erupts rhyolite lava from separate magma chambers.  Evidence of this bimodal volcanism can be seen at Glass Mountain, a fairly young (900-year-old) rhyolitic plug dome on the southwest flank of the Medicine Lake Highlands.  And as you may already know, rhyolite lava, when it cools extremely quickly, forms beautiful smooth, black volcanic glass: obsidian.  When that same lava mixes with gases, it forms the feather-light, frothy pumice.  Both can be found side-by-side at Glass Mountain.
Obsidian at Glass Mountain
At the summit of Medicine Lake Highland is a large 4-mile by 6-mile caldera, formed after the collapse of the volcano's summit 100,000 years ago.  The Highlands were glaciated at one point after this, and through their slow grinding processes, the glaciers created enough glacial clay to essentially seal up the caldera.  This layer of impervious clay prevented water from seeping into the porous basalt as it does in the rest of the Highlands and Lava Beds areas, and allowed the formation of a lovely subalpine lake.  Medicine Lake, as it is called, is a shallow lake of extremely cold water, and is a welcome sight to wildlife and travelers alike after traversing miles of lava flows.
Medicine Lake
 


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