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Geological Tour of Loch Leven Trail

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Emily Kentta

on 24 October 2013

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Transcript of Geological Tour of Loch Leven Trail

A Geological Tour of
Loch Leven Trail
The Loch Leven trail is in the Tahoe National Forest. To get to the trailhead from Sacramento, drive east on I-80. Take Exit 166 towards Big Bend and turn Left on Hampshire Rocks Road. Go 0.4 miles and the parking lot will be on the left.
A Magmatic History
The granite mountains of the Tahoe National Forest and of Loch Leven have a fascinating geologic history. Everything you see around you used to be molten rock deep underground called magma, and the magma chamber of Loch Leven tell a special story.

Let's take a look at a magma chamber.
We often think of magma in reference to a volcanic eruption, but that is not always the case.
Magma Chamber
Often, instead of erupting, the magma in a magma chamber will just cool in place forming a pluton.
Tectonic forces push the pluton upwards.
Finally, the upper surface erodes way exposing the pluton. The whole Sierra Nevada Mountain Range is exposed plutons!
So what is it that makes the Loch Leven pluton special...?

Let's go on a Tour!
Google Earth Satellite Image of Loch Leven
As soon as you exit your car, before or after that trip to the bathroom after the long drive, you will notice distinct dark patches in the otherwise mostly light colored rock. You will see these almost everywhere you look. They are called xenoliths.
What the heck is that?
Xenoliths are actually pieces of a different kind of rock that surrounded the magma chamber. They chipped off and fell into the magma. Notice that most of them are rounded or oddly shaped or look like they are coming apart. This is because they were in the magma long enough for the heat of them magma to begin to dissolve them.
Flowing Magma
You can also see that the xenoliths are elongated and oriented parallel in some areas. This tells us that the magma was flowing with the xenoliths in it, and oriented them the same way, before it completely cooled.
Shapes and Sizes
The xenoliths will come in many shapes and sizes. We even found some heart-shapes ones. As you continue along the trail towards the bridge they will get larger and have more distinct edges and corners. This means they were in the magma for less time and that you are getting near to the edge of the magma chamber.
Glacial History
The exposure of this pluton certainly wasn't the last interesting thing to happen to it. Hundreds of thousands of years ago a mile high sheet of glacial ice sat atop the Sierra Nevadas. Here you can see how the slow grinding movement of the glacier polished these rocks. You can see this phenomenon, called glacial polish, about one hundred feet from the beginning of the trail.
The Boulders
Almost anywhere you look you will also see boulders in what seem to be rather odd places. They also look a bit different than the rock that they are resting on. These boulders are called erratics and were dropped there by the receding glacier as it melted.
Exfoliation, not just skincare
When the pluton is deep underground it is under an immense amount of pressure. This is not the case at the surface of the earth. As the rock slowly releases it's internal pressure it forms cracks that look like peeling skin. This process is called exfoliation.
More Glacial Deposits
A third to half way along there is a heavily forested part of the trail. The bits of broken up rock and soil that the trees are growing in is called glacial till. The glacier deposited much of this smaller material as it was melting and receding.
Chatter Marks
These deep indentations are also signs of Loch Leven's glacial past. The surface of the rock has been chipped by rock fragments that were stuck to the bottom of the glacier and carried along with it.
A Rock Layer Cake
Not really, but we can tell by these reddish lines that this magma cooled in layers with different compositions. Conditions at the surface of the earth are causing changes in the minerals in this rock. This is called weathering and in this case the iron in this rock is oxidizing (rusting). So we know that the red layers are high in iron.
Veins of Quartz
These long white lines are visible throughout the trail, but they become more prominent about half way through. They are called quartz veins and the white rock is almost pure quartz .
Veins of Quartz
As the magma cools and becomes a pluton it becomes denser and takes up less space. This process causes the rock to fracture. These fractures make it easy for water to pass through the rock and as it does it deposits the quartz which fills up the fracture.
Graphic Granite
A bit before you come to the bridge you can see this odd, worm-like textured rock. It is called graphic granite, named for it's resemblance to ancient cuneiform-style writing. It is the last gasp of cooling magma in the chamber and in this final cooling phase whatever minerals are still in the molten phase must form crystals as best they can in whatever space is available.
The End
Congratulations! You have made it to the bridge, which is the edge the the pluton and the end of our Geological Tour of the Loch Leven Trail. Sit back, have a snack and enjoy the scenery or continue on to the Loch Leven Lakes. Happy Trails!
This Presentation was Researched and Created by
Wyatt "The Desert Yeti" Brewer
Brendan "Full-Sized Man for Scale" Clark
Chris "Big Bad Voodoo Daddy" Day and his trusty field assistant
Emily "Kitchen Sink" Kentta
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