The Other California: Fleener Chimneys, Spatter Cones and Fault Scarps

This post continues our exploration of one of California's hidden corners, Lava Beds National Monument, on the flank of California's biggest volcano, the Medicine Lake Highland.

So what, pray tell, is the feature below? A Sarlaac from a Star Wars movie? A Doomsday Machine from Star Trek? Brendan Fraser's pathway to the center of the earth? No, it's one of the abandoned volcanic vents from Lava Beds, the Fleener Chimneys. There are three holes like this, around fifty feet deep, that were the conduits for the Devil's Homestead Lava Flow around 12,000 years ago.

The eruptions were mildly explosive and messy, at least in the final stages. The Fleener Chimneys (below) are spatter cones, formed as molten bits of basalt were flung from the vents. Lava emerged and almost immediately floweed into lava tube systems, the openings of which can be seen in the near vicinity. After traveling some distance, the lava flows cooled enough to start forming a'a flows that give the Devil's Homestead its characteristic appearance.

Another fine example of a spatter cone, much younger and fresher looking, can be seen at Black Crater, just a short distance away from the Fleener Chimneys turnoff on the main park road. It erupted only about 1,250 years ago, but might as well have been yesterday given its youthful appearance.

Why have these eruptions occurred on the lower flanks of the Medicine Lake Highland? A stereotypical volcano has a crater at the summit that produces the explosions, the ash, and the lava flows. As has been pointed out previously, the Highland is a unique exception to the "rules" of volcano types. Shaped like a massive shield, it has numerous small magma chambers with a variety of magma compositions. Fluid basalts follow weaknesses in the crust, and fault zones make a fine conduit to reach the surface.

Medicine Lake Highland sits at the boundary between the Cascades and the Modoc Plateau, and fault zones project into the volcanic edifice. Gillems Bluff, seen in the picture below, is a fault scarp, with the sunken graben of Tule Lake on the right side of the photo. It just happens that the fault that formed Gillems Bluff projects right through the Fleener Chimneys.

These features are easy to access during a visit to the park. If you are tempted to toss a rock into the Fleener Chimneys to see how deep they are, please resist the urge. A clean up crew recently removed 35 tons of rock from just one of the holes.

The Other California: Geologists Who Live on Glass Mountains Shouldn't...(fill in moral)

Oh, come on, who of you hasn't done the manly-man or womanly-woman thing when confronted with a mountain of pumice boulders? That's one of the cool things a person can do when visiting one of California's unknown corners, Medicine Lake Highland, the biggest volcano in the state.

In the previous post about the volcano, we were exploring the varieties of non-viscous basaltic lavas, the so-called a'a and pahoehoe flows. Basalt is a silica-poor volcanic rock that is usually derived from melting below the crust in the earth's mantle. In places like Hawaii, basalt is essentially all one will find. Medicine Lake Highland sits on silica-rich continental crust, and hot magmas rising from the mantle mix and melt the continental crust, forming a variety of more silica enriched lavas, such as andesite or rhyolite. These are the lavas that lead to the distinctive nature of the Medicine Lake Volcano. Several unique kinds of volcanic cones dot the summit region of the volcano, including Glass Mountain, shown in the pictures below (and above, for that matter). Glass Mountain is also the youngest volcano on the Highland, having erupted only 950 years ago. Note how few trees have gained a roothold on the new rock.

The silica molecules that make up much of rhyolite magmas tend to stick to each other, forming polymers, and making the lava very viscous (sticky). Atoms can't move readily and the cooling lava tends to form glass (disorganized atoms) instead of a crystalline rock. Natural volcanic glass, as many of you know, is called obsidian. This is the material used by early cultures to make spearpoints, arrowheads and other tools (it makes a sharper edge than metal and is used in surgical scalpels today). Glass can form from other lavas like basalt or andesite as a result of sudden quenching, as in water, but such glass tends to be only on the surface of the rock.

Is the foam on a mug full of root beer actually root beer or is it something else? Of course it is root beer, it just has a lot of airholes in it. Pumice has the same relationship to obsidian. It is also volcanic glass, only it has a lot of gas bubbles in it. As such the rock has very low density so that it can float for one, and out-of-shape middle-aged geologists can pretend to be really strong hefting it around for another. The two rocks are found in abundance at Glass Mountain.

The rhyolite lava is so viscous that it rarely flows at all, and instead builds steep rubble-covered cones called plug domes or silica domes. Glass Mountain is a prime example (on the right skyline in the photo above), but it includes some flows as well (below). In class, I describe the behaviour of the lava as being like toothpaste being squeezed straight up so that it just kind of flops over.

Glass Mountain is high on the east flank of Medicine Lake Highland, and nice views can be had of the Modoc Plateau in the far distance. Getting to the plug dome requires travel on some dirt roads with a few confusing junctions, but get a national forest map and follow the signs, and normal cars can get there with no problem (it is stop 4 on this sketch map). There is a rough access road that allows people to hike over the extremely rugged surface, but be forewarned: the glass is very sharp and unforgiving. Almost every field trip I take there results in a few bloody cuts. The pumice has been mined in the past, and the mine claims are current, so there are "no rock-collecting" signs here and there.

There are several plug domes in other parts of California, most notably in the Mammoth Lakes area and on the south shore of Salton Sea, but Medicine Lake Highland is one of the best places (in my opinion) to explore these fascinating landforms. Check it out!

Next: walking inside volcanoes!

Magnitude 6.5 Quake Offshore of Northern California

A magnitude 6.5 quake has struck offshore of northern California, about 25 miles offshore of Ferndale and Eureka, at 4:27 PST. It was apparently felt widely across the northern part of the state. Seven aftershocks in the range of 30-4.5 magnitude have followed the main shock so far. This area is quite close to the Triple Junction where three plates, the Gorda, Pacific and North American come together. The fault motions were primarily horizontal so a tsunami is not considered likely (and it would have hit onshore by now anyway). Andrew Alden at About.com:Geology provides a nice perspective of the quake, the biggest in the lower 48 states in a couple of years.

Update, 1/10: 23 aftershocks thus far, including three that exceeded magnitude 4. Some minor damage reports, in the form of fallen chimneys and broken windows. A nice history of seismicity in the region by the Berkeley Seismological Laboratory can be seen here.

The Other California: Waiter! There's A'a in my Pahoehoe!

Medicine Lake Highland is one of the finest places in California to compare and contrast volcanic landforms, lava flows, and pyroclastic debris. The volcano has erupted a complete range of different lavas in the last few thousand years, from silica-poor basalt to silica-rich rhyolite, resulting in the presence on the mountain of practically every kind of volcanic landform: a shield, cinder cones, plug domes, a caldera and tuff cones. And every kind of lava flow....

Today we are looking at basalt flows. Basalt can be highly fluid (non-viscous), forming smooth-surfaced flows that ancient Hawaiians compared to shark skin. When such lavas flow over edges, they form structures that look like coiled rope, as seen in the photo below. These are called pahoehoe lavas ("pa-hoy-hoy"). I have to admit I took the photo in Hawaii on the 1974 Maunu Ulu flow, not at Medicine Lake. As many times as I have visited Medicine Lake Highland, I have failed all those times to simply look down and take a picture of some pahoehoe lava...

As the lava cools, or if it has a different gas or silica content, the basalt lava can become more viscous. As it flows, it crusts over and the unstable surface breaks up repeatedly into an uneven rubble, even as the interior remains hot and molten. The Hawaiians called these a'a lavas ("ah-ah"). Some a'a is visible on the right side of the photo above, and fills the picture below. This shot is the Devils Homestead Flow in Lava Beds National Monument on the north flank of Medicine Lake Highland. It erupted around 12,000 years ago from an area in Lava Beds called the Fleener Chimneys.

The youngest basalt lava in Lava Beds National Monument, the Callahan flow, originated at Cinder Butte just south of the monument only about 1,100 years ago. A small quarry (below)reveals a cross section of an a'a flow, with the rubbly base, a solid interior, and the rubbly surface (a sketch map showing the location of Cinder Butte and the Callahan flow can be found in yesterday's post; it's a narrow dirt road, but accessible to cars with reasonable clearance). The barren surface of Cinder Butte is a wonderful example of a young cinder cone, formed as small fragments of lava (cinders) were explosively ejected from the eruption site.

A'a and pahoehoe flows can show transitional forms. In the photo below, from Black Crater in Lava Beds, the pahoehoe surface was beginning to break up as it congealed, forming slabby pahoehoe. This particular flow is probably only a few decades older than the Callahan Flow.

Medicine Lake Highland also has silica-rich rhyolite lavas. We will explore these in the next post.

In the interest of full disclosure, I stole today's title quip from an absolutely fantastic DVD called Lava Flows and Lava Tubes which is one of the best movies I have ever seen illustrating the dynamics of lava flows. It intersperses active Hawaiian flows with volcanic locales across the western United States, including Lava Beds. If you are curious about lava flows or are a teacher, it is a great resource.

2nd Blogoversary! Hanging in There

This week I join NOVA Geoblog, Lost Geologist, Looking for Detachment, and Dinochick (did I miss anyone?) in the ranks of geoblogs reaching their second anniversary! This year I managed to finish my long series on the geology of the Colorado Plateau, and I got off to a good start on a new series on the unheralded geological wonders of the state of California.

I've had a satisfying level of traffic during the year (via Statcounter), with a bit over 115,000 hits since last December. I really love reading the statistics. I had some interesting spikes, once in February when I got linked to a CNN transcript of Bobby Jindal's anti-volcano monitoring speech, in April when I got linked to the heavily trafficked Yosemite Blog (run by one of my earliest students; thanks Loyd!) because of the Ahwiyah Rockfall in Yosemite, and in August when I commented on some giant waves hitting the beaches at Balboa in Southern California. The October spike came when I commented on the Keeler Earthquake Swarm. I guess a lot of people were curious!

As I said at my first anniversary, this continues to be a lot of fun, not just the blogging, but to see the exploration of the world throughout the geoblogosphere. Many thanks are due to Chris at Highly Allochthonous for his blog feed, and also to the producers of the Geoblogosphere News. Both sites have become indispensible sources of news in the earth sciences. I fervently thank everyone who has paid a visit to my site, and appreciate more than you can know the comments and conversations that have taken place here....And, if you are tempted, give it a shot: blogging is fun!

The Other California: If a Tree Falls in a Forest, Will, uh...If Lava Knocks Over Tree in the Forest...?

Lava has a well-deserved reputation for destroying everything in its path, not to mention melting human beings and chasing movie heroes. What really happens when lava flows into a forest? When I traveled in Hawaii, I found out that trees actually do persist, temporarily, as the lava flows through. The trees cool the lava enough to freeze the liquid, and then the tree burns, leaving behind a gap in the shape of the tree: a tree mold.

Hawaii has a great many tree molds, but California is not totally bereft of examples. One good place to see them is on the Medicine Lake Highland, California's biggest volcano. About 12,330 years ago, a minor basaltic eruption took place just north of the caldera rim. It flowed through a Western White Pine forest for a few hundred yards, and ended. In most places the basalt was no more than 10-20 feet deep. The doomed trees burned, but the molds remained (the charcoal from the burned trees provided the material for radiocarbon dating. The molds are 10-15 feet deep, and several are big enough to climb into (see the photo above).

Snow reportedly remains in the molds for much of the summer. Bark patterns and branches are also preserved; in the photo below, a person can reach into both holes and clasp their hands about two feet down in the rock.
The molds can be seen on Forest Road 49 about 10 miles from park headquarters at Lava Beds National Monument, and 6 miles from Medicine Lake (see map below).

Source: Johnston, D.A., and Donnelly–Nolan, J.M. eds., Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California: US Geological Survey Circular 838, p 171–176

The Other California: Exploring California's Biggest Volcano

So we begin our exploration of Medicine Lake Highland, California's biggest volcano. As noted in a previous post, it is essentially a basaltic shield*, about 30 miles long in a north-south direction, and 22 miles stretching east-west ("stretching" is a particularly apt term in this instance). Starting from the plains of the Modoc Plateau at about 4,000 feet, the volcano rises to a high point of 7,913 feet above sea level.



The summit region on the volcano is a basin just over seven miles long and 4 miles wide, containing Medicine Lake itself (about 0.6 by 1.2 miles, and up to 143 feet deep). The basin formed when eruptions emptied the magma chambers beneath the volcano, causing the summit area to collapse inwards, forming a caldera. Crater Lake (over the border in Oregon) is a similar-sized feature, but it formed more recently, and far more catastrophically (a single devastating explosion rather than a series of eruptions over time). The caldera at MLH is topographically indistinct, having been modified by subsequent eruptions, and shaped by glaciation.

The volcano has been producing lavas for less than a million years. They are generated in the mantle as east-west stretching of the crust has caused numerous normal faults to form, providing a conduit for the basaltic lavas to reach the surface. A certain amount of mixing and other chemical interactions occur in the crust as lavas rise, causing some magma chambers to change composition. Many of the younger flows on the volcano are rhyolite (in the form of obsidian and pumice) and gray andesite. Comparatively speaking, Medicine Lake Highland is one of the most active volcanoes in the western United States. It has erupted hundreds of times during its growth, with at least nine eruptions in the last 5,200 years, with the most recent only 950 years ago.

Medicine Lake Highland sits astride the ill-defined boundary between two of California's geologic provinces, the Cascades and the Modoc Plateau. Although it is often lumped with and compared to the volcanoes of the Cascades, the north and eastern sides of the volcano clearly lie on the flat terrain of the Modoc Plateau, especially at Tule Lake.

The Cascades Province extends from northern California to British Columbia, and includes some of the most famous volcanoes in the world, including Mt. St. Helens, Mt. Rainier, Mt. Hood, and Mt. Shasta, and Lassen Peak (the last two being California's best-known volcanoes). The volcanoes derive their magmas from the melting of rock in the Cascadia Subduction Zone, where the oceanic crust and upper mantle of the Pacific Plate Juan de Fuca and Gorda plates are being driven beneath the North American Continent. The magmas are primarily andesitic in nature, forming a type of lava that is capable of explosive, devastating eruptions. The explosion of St. Helens in 1980 and the formation of the Crater Lake caldera around 7,000 years ago provide vivid examples.

The Modoc Plateau is less familiar. Sparsely populated and relatively flat, the region is mostly "drive-through" country, unless one is looking for an interesting geologic story. The oldest rocks of the Modoc Plateau are the basalt flows of the Cedarville series, which are best exposed in the Warner Mountains on the eastern edge of the province near Nevada. The rocks are several tens of millions of years old (Oligocene to Miocene). Subsequent faulting disrupted the crust, resulting in the formation of fault grabens and lake basins, including those containing Klamath and Tule Lakes (see the diagram above).

I will be writing about some of the unique sights on the highlands in coming posts, but if you can't wait and want some great online resources, check out the following:

A USGS geologic road trip through the region

A 2007 Volcano Hazards Assessment of Medicine Lake Highland and a very nice poster based on the same research

Information from the Cascades Volcano Observatory on the Medicine Lake Highland

Latest research on age dating of the youngest flows on Medicine Lake Highland

*UPDATE: A definitional thing here: the latest research, sourced below, describes Medicine Lake Highland as a "shield-shaped" edifice that can be better described as a "composite volcano" or "central volcano". This follows from new revelations about the interior of the volcano, which is more silica-rich than expected, and not primarily basaltic. I'm going to continue to call it a shield, based on the geomorphology, with full recognition that it is quite unique, and certainly not all that much like classic shields such as those in Hawaii. Thanks to Forrest for the heads up!

Latest research: Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc volcano in the southern Cascades (abstract)