Saturday, July 4, 2015

Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Underside of the Volcano

Quick, name the nation's oldest national park. I'll even give you a clue, it starts with 'y'. Did you guess Yellowstone? You are right. Did you guess Yosemite? You are also right, in a sense. Have you ever said "Yosemite" when you meant "Yellowstone"? I've done that plenty of times.
Castle and Beehive Geysers at Yellowstone National Park, Wyoming
Yellowstone technically is the nation's oldest national park, having been established in 1876 by an act of Congress, but Yosemite gains credit as the first "idea" of a permanently protected area when the valley was ceded to the state of California in 1864 by Abraham Lincoln.
Yosemite Falls from Swinging Bridge in Yosemite Valley

It's hard to think of two parks more different from each other. Yellowstone, with geysers, hot springs, bison, and moose. Yosemite, with huge vertical cliffs of granite and high alpine peaks. In Yosemite, the bears break into your car. In Yellowstone, the bears occasionally eat you.
Sunset in Yosemite from the Gateway View
The parks in one sense are very similar, and the similarity can be summed up in one word: "felsic". The word combines the mineral "feldspar", and the chemical "silica". These are the principle components of the plutonic rock granite, and the volcanic rock rhyolite. The rocks are identical in chemistry, but differ in origin: granite is silica-rich magma that cooled slowly deep in the crust, while rhyolite is a volcanic rock that often forms during extremely violent eruptions. The rocks of Yellowstone are mainly rhyolite and related rocks that formed in caldera eruptions, while those of Yosemite are principally granitic.
El Capitan in Yosemite Valley
We've reached the culmination of our journey across the most dangerous plate boundary in the world. As I've said before, it's not as dangerous today as it was in the Cretaceous period, but we can use these rocks to better understand subduction zones elsewhere in the world (it's a lot easier to study rocks at the surface than it is to drill miles into the crust, after all). We started the journey on the coast at Point Reyes National Seashore, worked our way along the Marin Peninsula, crossed into the Coast Ranges at Lick Observatory and Del Puerto Canyon, crossed the Great Valley, and worked our way up into the Sierra Nevada. And now we stand in Yosemite Valley, one of the most spectacular exposures of granitic rock on the planet.
Lassen Peak and Brokeoff Mountain, an active volcanic center in the southern Cascades Range

The subducting slab of oceanic crust in Cretaceous time was heated, and water that was released changed the melting point of the rocks at the base of the continental crust. The masses of molten rock worked their way up into the upper crust. Some of the rocks erupted at the surface as large Andean-style volcanoes, or as rhyolitic caldera eruptions. The chain of volcanic features is called a magmatic or volcanic arc. The surface may at times have even resembled Yellowstone, although present day Lassen Volcanic National Park or Mt. Shasta provides the best analog. But deep in the crust the rock was cooling slowly, developing into large crystals of quartz, feldspar and mica, then principle minerals of granite and related plutonic rocks.
El Capitan Granite, with gray-looking quartz crystals, white feldspar, and black biotite mica

Yosemite Valley provides a 3-D view of seven or eight different intrusions of granitic rock (for the purists, these include granite, granodiorite, tonalite, and diorite). Some of the rocks, principally the granite, stand out as bold cliffs like El Capitan and Cathedral Rocks. Other rocks were more easily eroded and formed recesses. The combination gives Yosemite the unique appearance that sets it apart from so many other glacially carved canyons.

When you visit Yosemite, take a moment to realize you are exploring the underside of massive volcanoes, quite literally Yellowstone or Lassen or Shasta from the inside out. It never ceases to amaze me that while the 3,000 foot cliffs are spectacular, they once were buried 4 or 5 miles deep in the crust, and that erosion has removed the missing rock (and filled our Great Valley in the process).
Half Dome from Washburn Point above Yosemite Valley

We've completed our exploration across the most dangerous boundary, and we are still alive! It's been several months with lots of interruptions, so you can expect a compilation of all the posts soon so you can get the whole story in one place. I hope you've enjoyed the journey!


Anonymous said...

This has been a fascinating series, which has been very educational to me, a lay reader. Thank you for presenting this in a way that is beautiful and understandable.

myfingersaresore said...

Amateur geologist here. This has been a great ride - thanks so much for putting it together!