Driving to the Center of the Earth in Del Puerto Canyon...Piercing the Ocean Crust

Rugged terrain in the upper Del Puerto Canyon just beyond the Tesla-Ortigalita fault (on the right near the people)

As in the last post, I'm exaggerating a little bit. We're not going to the center of the Earth, we are instead using California's unique geology to explore the mantle, the layer that extends from 20 miles to about 1,800 miles depth, about half way to the center. These rocks have been on a long journey to reach the Earth's surface, and they are not often seen by casual explorers.

Volcanic rocks in Del Puerto Canyon. These are either pillow basalts or highly jointed rocks.

In our first post, we had driven through the five mile (8 km) thick sequence of sedimentary rocks laid down on the floor of a relatively shallow ocean (the Great Valley Group). We reached a major fault near the base of the sedimentary rocks and the canyon changed in a major way. The smooth gentle slopes of grass gave way to rocky slopes covered with brush, scrub oak and the occasional cypress tree. The rocks had changed. We had reached the ancient oceanic crust, known here as the Coast Range Ophiolite.

An ophiolite sequence is a unique series of rocks that are usually understood to represent a cross-section of oceanic crust. The top of an ophiolite is composed of pillow lavas, lumpy chunks of basalt that form as molten rock encounters cold ocean water (see some forming in this short video). Those might be some pillow basalts in the picture above, but I've never been able to get close enough to confirm it. It could also be highly jointed volcanic rocks.

Beneath the pillow basalts, one might expect to find sheet dikes, fractures that have been filled with volcanic rock that had been on the way upwards to the ocean floor. The sheet dikes are not obvious in Del Puerto, although they can be picked out in a couple of places (we didn't stop in the right places on this trip).

There is a prominent dike in the canyon, but it is not actually part of the ophiolite. The rugged ridge is composed almost entirely of quartz. It probably formed millions of years after the others as hot hydrothermal fluids flowed through cracks and fractures. It's been investigated for gold mineralization, but I don't think anyone has found any ores worth mining (not that they wouldn't try; it's still under claim).

Quartz vein and gabbro outcrops in Del Puerto Canyon

Beneath the sheet dikes, one would expect to find the plutons that once fed the eruptions of basalt and other lavas on the seafloor. The magma that remained cooled slowly over thousands of years, forming a coarse-grained rock composed of crystals of amphibole, pyroxene, plagioclase feldspar, and maybe some olivine. The plutons are usually composed of a dark rock called gabbro, but some of the rocks are lighter-colored, a variety called diorite.

Highly jointed gabbro and diorite in Del Puerto, rocks of the lowest part of the oceanic crust

The presence of diorite and some silica-rich volcanic rocks in the upper parts of the sequence throws  a wrench in the normal interpretation of ophiolite, especially those that occur in California. Ophiolites that form at oceanic ridges (divergent boundaries) are usually poor in silica (composed almost entirely of basalt and gabbro). The ophiolite in the Coast Ranges of California may have formed in a more complex tectonic setting, in and near an island arc (a chain of volcanic islands like the Aleutian Islands today) associated with an oceanic trench.

Diorite in Del Puerto Canyon

In any case, we've penetrated the oceanic crust, a thickness of around three or four miles (6-7 km). These rocks don't often see the light of day, because when you think about it, what does it take to bring the ocean floor and crust to the slopes of a mountain range on land? Geologists have been trying for years to drill a hole through the oceanic crust, unsuccessfully so far, but a new effort has begun this year. Del Puerto Canyon is a place where we can literally walk from the base of the oceanic crust to the underlying mantle.

And that's what we'll do in the next post!

Gabbro near the quartz vein in Del Puerto Canyon

Fire Down Below II - a Geological History of the Colorado Plateau

Continuing a long-running adventure, we pick up the tale of the geology of my cherished corner of the United States, the Colorado Plateau....we've come through 2 billion years of the story, and are now only 20 million years from the end!

Volcanic activity had not been a prominent part of the geology of the Colorado Plateau for hundreds of millions of years, throughout Paleozoic and Mesozoic time, except for the volcanic ash that drifted in from elsewhere and ultimately provided much of the color of the Morrison and Chinle formations. That all changed in Cenozoic time. As outlined in yesterday's post, there was a vast outpouring of lava and tephra across much of the western United States around 30 to 20 million years ago, which resulted in the formation of numerous volcanic necks, calderas and the subject of today's post, laccoliths.

There are some real oddities in the landscapes that make up the Colorado Plateau, islands of rock with alpine snowfields and deep green forests that seem more like the Rocky Mountains than part of a desert. These strange out-of-place mountains include the La Sal Mountains (top photo), the Abajo Mountains, Navajo Mountain, the Henry Mountains (center photo), the Sleeping Ute Mountains (bottom photo), and a number of others. The highest, the La Sals, top out at over 12,000 feet. They are a beautiful addition to the landscape; imagine Delicate Arch in Arches National Park (top photo) without the dramatic backdrop of the La Sal Mountains in distance.

How can such mountains develop out of an otherwise flat landscape?

They aren't volcanoes, not in the sense that we normally think of them, but they are volcanic in origin. The mountains are made of a semi-coarse-grained igneous rock that has a number of names, but diorite will do for the moment. 'Coarse-grained' usually means the rock results from slow cooling of the magma miles underground, but the rocks forming the core of these mountains are sort of a hybrid, showing a fine enough texture that they were probably a few thousand feet below the surface, rather than several miles beneath the surface. G.K. Gilbert was studying exposures of these rocks in the Henry Mountains in the 1870's and he realized that although there were complexities, the rocks tended to squeeze between sedimentary layers, and to make room, they pushed the overlying layers upward into a domelike structure, much like a blister pushes the skin upwards. He called these igneous 'blisters' laccolites, and eventually they came to be known as laccoliths.

A simple laccolith, courtesy of http://geology.utah.gov/teacher/tc/tc0108.htm

Navajo Mountain appears to be an ideal example of a simple dome-style laccolith. It certainly looks the part. The more complex mountain ranges include multiple laccoliths at different levels, sometimes branching out in several directions. They often probably constituted the plumbing systems of volcanoes that developed on a surface many thousands of feet above, but have since been eroded away.