(notes by Joel) Geologist and plate tectonics animator Tanya Atwater spoke to us October 16, 2014. She gave us her presentation called Living in the Plate Boundary and Through the Ice Ages.
The presentation was largely based on diagrammatic animation videos which these notes cannot describe. But you can see them yourself on Atwater’s website (or you can Google Atwater animations). I apologize that I wasn’t as able to take notes as usual since my eyes were up on the video screen a lot.
The surface of the earth is made of plates floating on (and diving down thousands of miles into) the earth’s molten mantle, which is always flowing.
When you look at the animation of the continents coming into their present formation, India seems to move faster than the rest. This is partly due to the size. Smaller means it can move faster. Of course the speeds are all pretty slow, taking many millions of years. (Speeds are similar to how quickly a fingernail grows, according to Julian’s talk back in July.)
The Himalayas are uplift (crumpling and wrinkling of the plate that pushes the surface up). This is due to the direct-hit crunching of the Indian plate against the plate to its north.
The Pacific Plate is the largest plate, taking up almost 1/3 of the surface of the earth. It’s being dragged past the North American Plate to the NW, scraping against it and at the same time leaving bits behind along the edge.
The area west of North America has been for a long time (until recently) made up of three ocean plates. Ocean plates are formed by spreading mid ocean rifts where mantle magma wells up in the gap. (Continental plates are formed by uplift, by volcanos, by wind and glacial deposits, etc.) The two plates on the east side of the rift zone have flowed under the Americas now (a process called subduction). There’s a tiny bit left of one (Juan de Fuca Plate) in Oregon and North California Coastal waters and a little (of the Farallones Plate) near Central America.
As the rift spreads, the Pacific Plate gets bigger on its east side faster than it moves northwest. That means it grows and seems to come closer while in fact it is moving away. The direction of movement is not directly away, but scraping NW along our coast, pulling our coast out and stretching the North American continent. That’s why the high areas that used to be Utah and Nevada had room to collapse and fall over becoming the basin and range provinces with lots of gaps between high ridges. It’s also why the Gulf of California has opened up.
The San Andreas
Tanya’s graduate thesis was to try and figure out the San Andreas Fault, which turns out to be a unique fault over the whole earth.
If a slipping fault is a perfect line in the direction of slip, it has no gaps or places that push against each other (forming wrinkles, i.e. hills). But in reality, all places have some kinks, so there are hills and gaps formed.
Pinnacles Park in California has a very different kind of volcanic rock. Rocks the same have been found hundreds of kilometers away so we know that one side of the slipping fault moved that far since the eruption.
Bodega Head has granite that has moved up from Southern California, 500 km, as do a few other spots on the Northern California coast.
About 25% of the motion of the fault’s energy is spread into the Tahoe region in smaller stresses. In 10 million years, there will be an ocean alongside Las Vegas, because the Pacific Plate will have dragged what is now California away, probably.
Subduction
Before the San Andreas, we had a subduction zone as the ocean rift pushed plates under us. The mid ocean ridge where the spreading areas were is now mostly under us. A subduction pretty much has to be along a straight line. When the subduction hits the melting point (not from friction but from the internal heat of the earth) it bubbles up through the plate above it. That is why there are volcanos in the Cascade Mountain Range. It’s the Juan De Fuca Plate coming back up. Some is trapped under and that becomes granite. Some makes it to the surface and is lava. That lava flowed to the coast and was crumbled and tossed into a mixer of rocks, seawater, etc, some of which was dragged back down by the subduction to enter the whole cycle again and again. Meanwhile, some of the rubble of this cycle is left on the edge of the North American Plate. This is the accretionary wedge.
Other geographical features
The Great Valley (a.k.a Central Valley, San Joaquin Valley, etc.) is the collected debris of the volcanos and cycles of subduction at the edge of the continent. The land west of it (the coast range) is the uplifted part of that debris (cause by bends in the slipping fault pushing mountains up) mixed with melange of things dragged along from elsewhere. The melange is blue schist and chert and all sorts of stuff.
The Transverse Ranges (Tehachapies) that curl toward the coast are granite like the Sierra Range, but the farther south you go the more deeply eroded it is, so the farther under the formation you are looking. The granite in places like Joshua Tree Park are not smoothed into canyons like the Sierra’s because there were no glaciers to scrapes and smooth them. They formed rounded boulders instead.
The L.A. basin is deep and filled with mud.
Between Santa Barbara and San Diego there was a break and a gap opened. The land that was there got pulled and rotated. It is now the block of material that the Channel Islands and Santa Barbara sit on. It tumbled and was rotated to its current position by the two plates grinding past one another within a gap where the coast was pulled out allowing for rotation. Geologists can tell this is so, even though it is unlikely, because the rocks formed with their magnetism lining up with the poles. Now it points east in all those areas, so they know by paleomagnetism studies that the whole section pulled apart and rotated.
Ice Age
The present sea level is about as it has been since the last ice age ended 6,000 years ago. It’s melt from the ice age glaciers. The maximum of the ice age was about 17,000 years ago after tens of thousands of years of ice age. We are now in an interglacial period. The previous one was about 100,000 years ago, and they happened in the past about every 100,000 years. Ice ages always come on slowly and end quickly.
As the glaciers melt quickly, the sea level rises a lot. (For the most recent, between 300 and 400 feet all around the world at once.) Each place where a sea level remained a while, the wave action cuts a terrace on the edge of the continent. By the time the sea level goes gradually down and then quickly back up the next time, the continent has risen some distance, so the next wave terrace is below the old ones. This eventually forms a stair-step coast line. The older steps are more eroded but they are highly visible even to the untrained eye.
If there is a lot of sand in the waves, it takes up the wave energy, but if not, the energy cuts new terraces.
There was so much more she said!
SF Natural History Series 