• Home
  • About
  • Email List
  • Past Speakers
  • SFNHS Coordinator
  • Upcoming Speakers

SF Natural History Series

A lecture series exploring nature in the San Francisco Bay Area

Feeds:
Posts
Comments
« Next Lecture – Nov 20th: Sky Phenomena
Feb 19th – SF Carbon Cycles, Humans & the Climate »

Living in the Plate Boundary and Through the Ice Ages

October 24, 2014 by Adrian Cotter

(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!

Share this:

  • Click to share on Facebook (Opens in new window)
  • Click to share on Tumblr (Opens in new window)
  • Click to share on Pinterest (Opens in new window)
  • Click to share on Twitter (Opens in new window)
  • Click to email a link to a friend (Opens in new window)

Like this:

Like Loading...

Related

Posted in Lecture Notes | 4 Comments

4 Responses

  1. on October 25, 2014 at 5:36 am THINKWALKS » Notes from Tanya’s Talk

    […] and plate tectonics animator Tanya Atwater presented to our Natural History Series talks at the Randall Museum October 16, […]


  2. on October 25, 2014 at 7:00 am Miksha

    You were very privileged to have Tanya Atwater present to your group. (I am envious!) She was closely involved with the greatest revolution that has ever happened in Earth Sciences – the discovery of plate tectonics. Just 50 years ago, the origin of mountain ranges was fiercely debated and the idea of mobile continents was largely rejected. Ms Atwater was one of the scientists involved when plate tectonics unfolded.


  3. on October 25, 2014 at 11:35 pm ingridcc

    Thanks for that great recap of Tanya Atwater’s great presentation!
    Some of those details I had already forgotten, or maybe they went over my head at the time (ice ages coming on slowly but ending suddenly, for example – yikes!). Her lecture was so great – not just the incredibly interesting facts (and helpful animations/ illustrations), but also the way she conveyed her own awe and enjoyment in learning about them!


  4. on October 26, 2014 at 7:25 pm Adrian Cotter

    Miksha: We know, we were very excited to have her with us.
    Ingridcc: Glad you enjoyed!



Comments are closed.

  • SFNHS

    Our lecture series explores all aspects of nature in the Bay Area, and seeks to understand our impact both past and present on those natural systems, and their impact on us.
  • Find Us Elsewhere

    • Facebook
    • Google+
    • Twitter
  • Local Science & History

    • Ask a Scientist SF
    • California Center for Natural History
    • California Naturalist Program
    • Friends of Five Creeks
    • Nature in the City
    • Shaping SF
    • The Long Now Seminars
    • Think Walks
    • Urban Adventures for Kids
  • Our Hosts

    • Exploratorium Bay Observatory Gallery
    • Green Apple Books
    • Rotary Nature Center
    • SF Public Library
    • The Randall Museum
  • Science in Other Cities

    • Bay to Beach Life
    • NYC Wildlife
  • Speaker

    • Cartographer's Notebook
    • Keith Hansen
    • Marin Carbon Project
    • Nowtopians
  • SF Naturalist

    • RT @Sierra_Jobs: Are you interested in developing web applications, and shaping technical solutions for complex use cases at a mission-cent… 6 months ago
    • @Longreads @jkehe @WIRED This does just essentially seem like a reboot of intelligent design. Why is it easier to b… twitter.com/i/web/status/1… 11 months ago
    • My couple minutes of Corvid fame on @kalw's @CrosscurrentsFM this week -- shoutout to friends who clued me in to th… twitter.com/i/web/status/1… 1 year ago
  • Archives

    • July 2018
    • January 2017
    • June 2016
    • April 2016
    • March 2016
    • February 2016
    • November 2015
    • October 2015
    • August 2015
    • July 2015
    • June 2015
    • May 2015
    • April 2015
    • March 2015
    • February 2015
    • January 2015
    • November 2014
    • October 2014
    • September 2014
    • July 2014
    • June 2014
    • May 2014
    • April 2014
    • March 2014
    • February 2014
    • January 2014
    • October 2013
    • September 2013
    • August 2013
    • July 2013
    • June 2013
    • May 2013
    • April 2013
    • March 2013
    • February 2013
    • January 2013
    • December 2012
    • October 2012
    • August 2012
    • July 2012
    • May 2012
    • April 2012
    • February 2012
    • January 2012
    • November 2011
    • October 2011
    • August 2011
    • July 2011
    • June 2011
    • May 2011
    • April 2011
    • March 2011
    • February 2011
    • January 2011
    • December 2010
    • November 2010
    • October 2010
    • September 2010
    • August 2010
    • July 2010
    • June 2010
    • May 2010
    • April 2010
    • February 2010
    • January 2010
    • November 2009
    • October 2009
    • September 2009
    • August 2009
    • July 2009
    • June 2009
    • May 2009
    • March 2009

Blog at WordPress.com.

WPThemes.


  • Follow Following
    • SF Natural History Series
    • Join 35 other followers
    • Already have a WordPress.com account? Log in now.
    • SF Natural History Series
    • Customize
    • Follow Following
    • Sign up
    • Log in
    • Copy shortlink
    • Report this content
    • View post in Reader
    • Manage subscriptions
    • Collapse this bar
%d bloggers like this: