So I spent a few days in Lake Tahoe. Well, I wasn’t in Lake Tahoe for a few days, more like a couple minutes. And I was in all the way to my ankles. But I was in the area for a few days. Was it ever gorgeous.

I dare you to click on that and not fall instantly in love.

Or how about that one?

Or that one?

It seems like everywhere you look there’s a breathtaking view. And knowing a little about the processes that made those views makes it all the more spectacular. But is the geology of the Sierra Nevada ever complicated. So many things going on in the same place (but not at the same time).

First you have to know that the eastern side of the Sierra Nevada was once the west coast of North America. But then the Atlantic Ocean started opening up, and shoved North America to the west, where it smashed into the Pacific plate, which started subducting and scraping off bits and smashing them up against the North American plate. Those rocks are now the eastern side of the Sierra Nevada and North America has grown a bit and is at about the line of the Sierras.

And then it happened again, adding more smooshed and folded and deformed rocks and growing North America even further west. And these rocks form the center and western side of the Sierras.

And then…again! And those rocks form the Sacramento Valley all the way out to the current coast.

And while all this is going on, there are volcanoes. Because you can’t have subduction without volcanoes. That’s just how it works. So all this magma of varying compositions is working its way up from way down deep in the earth. Some of the more mafic (basaltic) magma would blurp up into the more felsic (granitic) magma. And then they’d solidify as plutonic rocks and eventually get exposed by erosion. And you’d get inclusions like this:

When you know what these are, you suddenly notice they’re everywhere. Like 3 of every 4 hunks of granite you see have these inclusions. Technically these are gabbro, which is chemically identical to basalt, but has larger crystals because it cooled slowly. (The same is true of granite and rhyolite: granite cooled slowly and has large crystals; rhyolite cooled quickly and has small crystals).

Oh and because of all this magmatic heat and water from the subducted plate moving around you get quartz veins.

Sometimes you see rocks with both! And of course, where you have quartz veins, you have gold. So there was a lot of gold pulled out of the Sierras back in the 19th century, and it left a huge mark on the appearance of the area. A lot of the damage has since been covered over by tree growth, but at one point, mining operations dumped so much sediment into the Sacramento River, that the river flooded and destroyed tons of agricultural land in the Sacramento Valley.

Oh did I mention the glaciers? As if all of this subduction and volcanism weren’t enough, there were massive glaciers in the Sierras during the ice ages.

It’s not hard to see that there were glaciers when you see something like this huge erratic on the shore of Donner Lake. No force short of ginormous could have moved this here. And glaciers qualify as ginormous.

I’d like to think that these linear erosions are evidence of glacial activity, too. But I’m not certain. Glaciers are famous for leaving boulders with striations or scratches, because it’s not like they’re pure ice. They’ve got all kinds of gravel and rocks riding along with them. Maybe these striations created another surface for erosion, which deepened them and allowed more room for freeze-thaw cycles to expand them further. I found some pretty dramatic examples of this same process, if that’s what it is.

Oh and there’s one more element to the geology of Lake Tahoe. It is the western end of the Basin and Range province. I’ll talk more about that in my next post, but for now just know that it involves lots and lots of faults that create valleys (that’s the Basin part) and mountains (that’s the Range). Lake Tahoe actually lies in one of those basins, and its eastern side is one of those ranges (the Carson Range).

So the whole story goes something like this:

Repeated rounds of subduction caused mountain building and volcanism, while glaciation was carving and eroding the growing mountains. Meanwhile, faulting from the Basin and Range province split those mountains in two, creating a valley which filled with water after a volcanic eruption closed its main outlet, leaving the Truckee River as its sole, pitiful outlet. Ice dams from melting glaciers sometimes blocked this outlet, causing the water level to rise greatly until the ice dams broke, unleashing huge torrents of water which further sculpted the area. Glaciers also created features like Emerald Bay:

Ok enough talk. Let’s get back to the pretty pictures.

In the above picture of Mt. Tallac, you can see some of the small glaciers that remain. Glaciers flow from the heads of valleys called cirques, which are usually steep on the uphill side and open on the downhill side. And they pick up lots of debris as they scourpad their way down the mountain, leaving it in great piles called moraines. You can see both cirques and moraines in this picture.

That last one is actually Spooner Lake, the one I mentioned in a previous post. As promised, here are some better pictures of the fire fighting helicopter that was scooping water out of the lake for a nearby fire.

Donner Pass is very close to Lake Tahoe, so I took a quick trip out there. I’m sure you all know the story of the Donner Party: group of pioneers try to get to California by wagon train, take a bad shortcut that adds weeks to their trip, get stuck by early snows in the Sierras, half of them die, the other half eat the dead ones, the end. Or something like that.

The base of this memorial shows high the snow was that year. Near that memorial is this boulder.

It’s interesting for a few reasons:

  1. There’s a plaque on it! It lists the names of every member of the Donner Party, and whether they died or survived.
  2. It actually formed the wall and fireplace of the cabin that the Murphy family lived in.
  3. It’s huge! It’s clearly a glacial erratic.
  4. It seems really complicated. Now, there’s a lot of dark lichen growing on it, so it’s tough to decipher for my inexperienced eyes, but it seems like it’s a granite intrusion into something else. And the granite intrusion has its own dark inclusions.

But not everything in the area is granite. Like I said there’s lots of other stuff going on in these mountains. Like serpentinite.

At least I think that’s what this is. It’s weathered into a rather unattractive color.

This might be an example of exfoliation, where the rock is weathering by peeling like an onion. Or maybe it’s just been plucked at by a glacier. Either way, the ever present inclusions are there.

I believe that’s an andesite boulder along the shore of Donner Lake. So more evidence of volcanism.

I think this is an Atlantis fritillary, based on my cursory glance at a field guide. But I’m no expert!

I have no idea what this little guy is!

These were the biggest dandelions I’ve ever seen. As big as my fist. And I don’t have dainty hands.

This golden-mantled ground squirrel was posing for me.

As was this deer near Spooner Lake.

And its parents(?) nearby.

I think that’s a good sampling of what I saw at Lake Tahoe. See you next time!