Benny and his friend Griffin at Ocean Beach in San Francisco.

Wednesday, October 20, 2010

Hiking California: Angel Island

As with most parents, it's obvious that I Need to Get Out More. Just meeting up with our friend Doug at a tapas bar Friday night took two weeks and numerous emails to arrange, plus a very good friend offering to take Benny for a few hours.

The tapas bar was, of course, worth all the trouble. I found myself surrounded by teeny plates of meatballs, tomatoes on bread, corn and mushrooms, and a pile of chard and it was all amazing. San Francisco will not stop trying to make me a foodie. (Ten years from now I'll be debating sauces with waiters. Shudder.)

This post, however, is not about food (sorry) but about the geology hiking group I joined. I'm not one of those people who happily dives into new things, especially if said things are full of total strangers. But San Francisco's geology is really interesting and yes, I Need to Get Out More.

Now I've taken those online personality tests and they keep giving me labels like "Lively Center of Attention", but that doesn't mean I can march up to a group of 20 total strangers and start jamming about rocks -- even metamorphosed Franciscan rocks. So I was a little nervous, which either makes me very quiet or neurotically chatty. Fortunately for the group I chose the former and ended up near the back of the pack, scribbling in my notebook and trying not to look winded on the steeper slopes.

Angel Island rocks are kind of neat. They're Franciscan rocks, like most of the rest of the Bay Area. The Franciscan complex is a group of basalts, granites and serpentine along with cherts, sandstones and shales. The whole bunch is cracked up with faults and looks pretty much like a mess.

Angel Island's Franciscan rocks are different because they're metamorphosed. That means they've been exposed to enough heat and pressure to change their chemical makeup. So they look different, and in some sandstones the dark spots that are usually round are elongated.

The reason Angel Island's rocks were metamorphosed is because they ended up in what's called a subduction zone. In a subduction zone, one plate of the earth's crust is shoved under another plate. After Angel Island's rocks were pushed down and cooked up, another crack in the earth arrived. Called a thrust fault, it shoved the metamorphosed rocks over a big sandstone block called the Alcatraz Terrane. So at Angel Island, you can see the old, weird, metamorphosed rocks on top of the boring sandstone.

And there you have it. So off I went on a week ago Sunday, carrying my lunch and my notebook in a backpack. It was, of course, Fleet Week, which meant the Blue Angels were scheduled to loop-de-loop over the Bay that afternoon.

I took the ferry to the island and met the group. We trudged over to the first stop, which promised metamorphosed sandstone with flattened pebbles in it. Here it is. Yeah, it looked that exciting in person, too.

Our leader passed around little lenses, but we couldn't find any of the fabled flattened pebbles. Oh well. We hiked down to the beach, looking for pillow basalts, but the tide was too high and we found ourselves squabbling about whether a few wet, black, suspiciously round boulders were in fact pillow basalts or just wet sandstone.

Now pillow basalt is one of my favorite rocks. These basalts are formed at a crack in the seafloor when new seafloor is spurting out. Once the molten rock hits the icy seawater, it forms blobby, shiny shells like the chocolate shells on Junior Mints. Blobs pile on blobs until the whole thing looks like a bunch of black pillows. Then the molten rock inside slowly cools. So when you're looking at a pile of pillow basalts, you're looking at rocks from the ocean floor, frozen as they formed. Love that. There are places where you can see pillow basalts far away from any water and it's mind-bending to consider how they got there.

Since the rocks weren't that riveting, I shot a picture of the Golden Gate Bridge in fog:

But the real show happened on the ferry ride back to San Francisco. The Blue Angels started cork-screwing all over the Bay. They even buzzed the ferry with an ear-splitting boom.

Here is one of my favorite pictures ever with Alcatraz, a Blue Angel and a masted ship:

And here are some more Blue Angels:

All in all, I consider my hike a success. This Sunday I'm meeting the group to hike Ring Mountain in Tiburon. I'm told we'll see much more dramatic examples of metamorphic rocks associated with subduction zones. But sadly, no acrobatic jet planes are planned.

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