by Joel Pomerantz
October 25th, 2014
Living in the Plate Boundary and Through the Ice Ages
Geologist and plate tectonics animator Tanya Atwater presented a talk with the title above to our Natural History Series talks at the Randall Museum October 16, 2014.
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 wasn’t as able to take notes as thoroughly 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 we looked 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 Lozos’s talk on How Quakes Are Measured 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 continental 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.
[Background: Continents ride on higher plates than ocean plates. These are formed by uplift, by volcanos, by wind and glacial deposits, etc. The North American Plate is like this.]
The area west of North America has been for a long time (until recently) made up of three separate ocean plates. Ocean plates are formed by spreading mid ocean rifts where mantle magma wells up in the gap. 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. In some places it pulls and other places it just recedes away from the North American Plate, depending on the local twists of the boundary. That is why the Gulf of California has opened up and why there was room for Santa Barbara and the Channel Islands to twist out of line.
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 that pull apart or places that push against each other (forming wrinkles, i.e. hills). But in reality, all places have some kinks and twists, so there are hills and gaps formed.
Pinnacles Park in California has a very identifiable 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 from the Mojave area 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 stress faults than the main ones on the coast. In 10 million years, those will probably break away and there will be an ocean alongside Las Vegas, because the Pacific Plate will have dragged what is now California away, probably. Tanya loves diagramming ‘the future’ because it’s entirely (and fun to) conjecture.
Before the San Andreas, we had a subduction zone as the ocean rift pushed the two (now almost gone) plates under us. The mid ocean ridge where the spreading areas were is now mostly under our plate.
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. And those have lots of steam because the plate drags ocean water under along with the rocks. Some rising magma is trapped as magma, hardening before emerging and that becomes granite, cooled slowly so it has big crystals. Some of the melted crust makes it to the surface and is lava. That lava flowed to the coast and was crumbled and tossed into a mixer of rocks, alluvial soils, 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) is the collected debris of the volcanoes 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 that melange of things dragged along from elsewhere. The melange is blue schist and chert from the seafloor (radiolarian skeletons) 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 away 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 bowl-shaped canyons because there were no glaciers to scrape and smooth them. They formed rounded boulders instead by their natural fracturing patterns.
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 clockwise (as viewed from above) 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 space. Geologists can tell this is so, even though it is unlikely, because the rocks formed with their magnetism lining up with the poles. Now the magnetism points east in all those areas, so they know by paleomagnetism studies that the whole section pulled apart and rotated.
The present sea level is about as it has been since the last ice age ended and stabilized 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 that built up gradually and ended suddenly (as they do). 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.
As the glaciers melt quickly, the sea level rises a lot. For the most recent, it rose between 300 and 400 feet all around the world at once. Each shore location where sea level remained a while, the wave action cut 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! That’s all I wrote about in my notes.