Friday, July 10, 2020

Geology of the National Parks Through Pictures - Lassen Volcanic National Park

My next post about the Geology of the National Parks Through Pictures was from a trip to meet some friends at Crater Lake. Since that was a rather long drive for us with the trailer we stopped over at Lassen Volcanic NP in California along the way.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website

California State Geological symbols can also be found HERE.


Lassen Volcanic National Park
During the summer of 2019, we decided to take the trailer on our longest trip yet, to Crater Lake. Along the way we camped at Lassen Volcanic National Park. The main campground was full by the time we made reservations so we ended up at the out of the way Butte Lake campground.

Since our campground was so out of the way, we needed to wait till the next day when we entered the main part of the park to grab our entrance sign shot.

Lassen Peak is the remnant of a once much larger volcano, Mount Tehama, that erupted leaving behind a 2 mile wide caldera located between the surrounding mountains of Lassen Volcanic National Park.

The peaks of the surrounding mountains can be seen here. These outer mountains represent the edge of Mount Tehama (also known as Brokeoff Volcano) that measured ~11 miles across and rose to ~11,500 feet, 1,000 feet higher than modern day Lassen Peak.

Lassen Peak is the southernmost volcanoes in the Cascade Range, a series of volcanoes that stretch up into Canada along the Pacific Northwest coast. Image courtesy of the USGS

These volcanoes formed from a process called subduction. The crust of the Earth is broken up into very large pieces called plates. These plates move around, with some sliding past each other, some pulling apart from each other, and others going towards each other. The edge of these plates are where earthquakes frequently occur, due to the rubbing of the plates against each other. When a plate composed mostly of oceanic crust goes toward a plate composed mostly of continental crust, the denser oceanic crust gets forced downwards into the Earth. This area is known as a subduction zone. As the denser oceanic plate moves downwards into the Earth it starts to heat up and eventually melt. That liquid rock rises up through the crust forming a string of volcanoes called a volcanic arc. The Cascade Range is such an arc with Lassen Peak representing the southernmost extent of the subduction zone. Image courtesy of the USGS.

There are many different types of volcanoes. Here is a view of Lassen Peak in the distance, rising up to 10,457 feet in elevation. It is so high that during our trip at the end of June it is still entirely covered in snow. Lassen Peak is what is known as a "Plug Dome", this forms when the lava is too thick to flow great distances. Lava rich in silica (quartz), has a higher viscosity (thickness) and doesn't flow as far as thinner, basaltic magma, which is hotter and has less silica content.

The original Mount Tehama was known as a "composite volcano", which means that it was composed of alternating layers of ash, lava flows, rock fragments, and cinders that slowly piled up over time from smaller eruptions. Most of the volcanoes in the Cascades are composite volcanoes. Mount Tehama was active with frequent eruptions from about 600,000 to 400,000 years ago until the magma shifted and the volcano, losing its magma, started to collapse and erode away.

Once we reached the highest part of the road way at Lassen Peak, the snow really was still piled up. Here is a view of the snowfields along Lassen Peak at ~8,500 ft.

Along the main road there were a lot of glacial and volcanic features. Here is Emerald Lake, which is known as a "tarn". A tarn is a glacial lake that was formed when a glacier erodes a divot out of the side of a mountain. When a glacier sits on the side of the mountain, snow slowly accumulates on the top portion of it. As more snow is added the entire glacier starts to get pulled down the mountain by gravity. This movement down the mountain causes the bottom surface of the glacier to erode the side of the mountain out, like an ice cream scoop. Eventually a bowl shaped depression is formed, called a cirque, where sometimes those bowl shaped depressions are able to hold water and a lake is formed, called a tarn.

Within the central part of the caldera, towards the bottom of the valley between the mountains, is a large hydrothermal area. The hydrothermal area contains many features that are heated up from the rich source of magma that still resides below the surface. This particular area is known as the Sulphur Works, which does have that glorious rotten egg smell, but there are a few other hydrothermal areas within the park. What is seen here is a mudpot which is produced when ground water flows down into the surface and interacts with the magma and volcanic gasses deep down. That heated water then rises back through the crust and heats up the water within the streams and groundwater coming down from the surface. This heated water produces steam that we see within other features and heats up puddles of water and mud such as this mudpot.

Hydrothermal features also have a tendency to have very vivid colors from the heavy minerals that get brought to the surface by the groundwater from the magma. Here is another portion of the Sulphur Works, looking out into the lower parts of the caldera.

Out towards the eastern edge of Lassen Volcanic National Park is Butte Lake, which borders the Fantastic Lava Beds. The Fantastic Lava Beds and Cinder Cone, which is seen in the background, were formed from an eruption in ~1650 CE. A Cinder Cone is another type of volcano which is produced from the eruption of black chunks of rocks, typically referred to as cinders or the rock type scoria. In this image you can even see Lassen Peak sticking out behind Cinder Cone slightly to the left of the cone.

Here is one of the edges of the Fantastic Lava Beds. The Fantastic Lava beds are formed from a basaltic lava flow, meaning that the lava had a low viscosity and a dark color, containing a high amount of iron in it, low amount of gasses, and low amount of silica. The silica (quartz) gives lava a thicker, viscous quality, like the lava that forms the plug dome at Lassen Peak.

You can actually hike to the top of the Cinder Cone as well, which was initially protected by Theodore Roosevelt as Cinder Cone National Monument in 1907 before being rolled into Lassen Volcanic National Park. While hiking up the volcano you can really feel the loose pile of cinders that create the volcano. A cinder cone is formed as a volcano sputs and spurts lava into the air. The air then quickly cools the lava forming holey, light pieces of rock called scoria. The smaller chunks of rock are spit out further while the larger chunks stay near the vent. Over time the chunks are built up around the vent forming the cone.

Here is a view from the top of the Cinder Cone looking out onto the Fantastic Lava Beds and Butte Lake in the distance.

A view into the center of the cinder cone. At the base of the crater there is nothing of interest that I had hoped for. Somebody had apparently built up a chimney structure with rocks at the base of the crater, but otherwise it is just a flat smooth surface.

Here is an overview of the lava beds and the Cinder Cone to give you an overview of the area. 


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