Artifacts from the history of geology

27 February 2017

I had the good fortune last week to join a gathering of the Explorers’ Club at the great David Rumsey Map Center, a famous collection housed since 2016 in the Green Library at Stanford University. Laid out for our enjoyment were precious original maps from the 18th century West Coast exploring voyages of Vancouver, La Pérouse, Cook and more; maps from the 19th century Western surveys of Fremont, King, Powell and more; and two monuments of geologic mapping: William Smith’s 1815 map of England and Wales, and an edition of William Maclure’s 1809 map of the United States. I couldn’t resist a phone pic of that one.

macluremap

But you can examine the map online as closely as if you were holding a hand lens up to it, thanks to davidrumsey.com.

It’s hard to overstate how much effort it took to make this map, crude as it is. Geology was in a crude state at the time — Maclure distinguished only four classes of rocks, using the Neptunist scheme of Werner — and the territory of America could only be surveyed laboriously by foot, horseback, boat or coach. But this is how it began.

The 1815 Smith map, widely known as “the map that changed the world,” recently had a well-celebrated bicentennial, and strata-smith.com is where to get deeply into it. It was a rare privilege to take it in with my own eyes.

Another item that impressed me was a heavy lump of dark stuff encased in a silver box, engraved with the owner’s name in 1791. It was a genuine working lodestone.

lodestone1791

A lodestone is a naturally magnetized piece of the mineral magnetite. Not every piece of magnetite actually acts like a magnet, although of course a magnet will attract it strongly. It has to have a certain amount of impurities that, in effect, lock down its magnetization in a persistent state. It also appears to need its magnetization to be “set” by the strong field of a lightning bolt.

Lodestones were the first compasses. Mariners first learned to navigate with them around 1200. Later it was learned that you could magnetize ordinary iron by stroking it with a lodestone, although this was not permanent. Iron compass needles had to touched up periodically, and that was what this lodestone was for. A ship’s captain might keep one as a backup, but they’ve always been rare and expensive.

Not until the 1700s were truly permanent compass needles made using steel, and not until we mastered electricity did we no longer need lodestones. I have this magnetite specimen, about a thumbtip’s size, that acts as a magnet. I hesitate to call it a lodestone because nowadays you could make one by just zapping it.

lodestone

I was touched by these artifacts from a time when science was closely tied to artisanship, when everything was done by hand. Today, geologizing still benefits from the human hands and senses in ways that, say, physics and chemistry have long left behind.

The Rumsey Map Center is open to the public.

Two bits of gabbro

20 February 2017

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

gabbrobitsgeomap

But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

gabbrobit-seneca

I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

gabbrobit-middleton1

This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

gabbrobit-middleton2

And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

gabbrobit-middleton3

I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building that there today.

The Idaho connection

13 February 2017

I’ve been getting into the weeds as I work on my book manuscript about Oakland’s geology (tentative title, Deeper Oakland). Where did Oakland’s rocks come from? Specifically, how did they get from where they formed to where they are? This problem is particularly vexing for the older rocks with Mesozoic ages. The western edge of North America has been built, unbuilt, shifted, rebuilt and disassembled for hundreds of millions of years.

Generally the pieces have been carried northward by the vagaries of plate tectonics. Rocks that were once Californian now sit as far north as Alaska, and likewise rocks that live here now come from as far south as Mexico.

The rocks in lower Shepherd Canyon (the Shephard Creek and Redwood Canyon Formations) belong to the huge set of sandstones and related rocks underlying the Central Valley — the Great Valley Group — but are separated from them. How they broke off and how they wandered to where they sit today are, as we say, poorly constrained. One clue may be within the sand itself.

Last year a paper in Geology laid out an intricate case that the sediment making up one part of the Great Valley Group was eroded from an ancient set of rocks in Idaho, the Lemhi subbasin of the Belt Supergroup. Papers dealing with the assembly of the North American Cordillera usually have gnarly figures, because the story is so complex, and this one, the product of an all-California team of geologists led by Stanford’s Trevor Dumitru, was no exception.

idaho-gvg-connection-figure

The whole scenario is based on microscopic grains of zircon, which lurk in many sandstones because the mineral is extremely durable. Luckily, we can determine the age of zircon grains because they’re superbly suited for the gold standard of dating techniques, the uranium-lead method. Think of them like pennies with dates on them.

So there’s a big body of rock in the Lemhi subbasin full of a unique combination of zircon ages, including a bunch around 1380 million years old. At one point during Cretaceous time, around 80 million years ago, a mountain-building episode pushed these rocks into a knot of high peaks, which eroded into sand that was carried by rivers in at least four directions. Dumitru and his coauthors duly gave these hypothetical ancient rivers names, because that’s one of the perks of doing historical geology.

Sandstones containing Lemhi zircons, with their telltale 1380 Ma peak, are found in Wyoming, Oregon, Washington and California. In the Bay Area, the paper identifies them in sandstone from Del Puerto Canyon, west of Patterson in the Central Valley. Apparently a huge pulse of Lemhi sand poured down the “Kione River” and filled the whole sedimentary basin for a while. (Basins keep sinking as they fill, because the sediment load depresses the crust.) That sandstone is mapped as the Kione Formation, a portion of the Great Valley Group that’s been considered mysterious because the sand clearly didn’t come from its usual source, the nearby (ancestral) Sierra Nevada and Klamath Range.

The point of all this is that the Oakland rocks I’m talking about date from this same period! If only we could get a zircon researcher to check them out, we might learn a little something. I mean, Dumitru dated rocks from Albany Hill, Stinson Beach, Bolinas Ridge and Sutro Baths (localities H, G, F and E in the inset map) among other places. He dated rocks from the Novato Quarry terrane of the Franciscan complex, the same unit our own Franciscan rocks belong to. He dated rocks from the Sierra de Salinas Schist, down near Monterey.

sierrasalinasschist

I love it; it has its own cool story. So why not visit Oakland too? I guess rocks on this side of the Hayward fault aren’t as important for the bigger story. But you never know until you find out, right?

By the way, I will be speaking at the upcoming East Bay Nerd Nite, Monday the 27th; watch its Facebook page or website for details. The topic is, “Are Rocks People?”