Plate Tectonics: A Revolution in Geology
Almost everybody who’s looked at a map of the Atlantic Ocean has noticed that South America and Africa fit together. Alfred Wegener noticed too and proposed in 1915 that they indeed were together long ago. He was laughed out of the room. The reason was that he couldn’t say how and why they had split apart and that’s a big part of science: you can propose any idea you want, but you need to come up with a mechanism to get you there.
Then, in 1960s, a revolution happened in geology, and we learned that Wegener was right. Plate tectonics was discovered.
I was an undergraduate at Caltech just after the first groundbreaking papers were published, some by Tanya Atwater, currently a professor at UCSB. It was an exciting time to witness the complete transformation of a science. One of my professors brought back a suite of rocks from Cyprus that represented a cross-section of the oceanic crust. There are only a few places in the world where we find these so-called ophiolites. Three of the best known are in Cyprus, Oman, and right here in our own backyard at Point Sal. What had happened was that the “how” was discovered after extensive exploration of our oceans was undertaken.
Paleontologists and others had sided with Wegener in the early 20th century as their observations of similar rock formations and fossils in Africa and South America seemed to show they were indeed joined long ago. Later, seismometers began showing that earthquakes were concentrated along the edges of continents where active volcanoes were also observed — such as the so-called “ring of fire” around the Pacific Ocean.
The last piece of the puzzle was put into place when ships performed a magnetic survey of the oceans and found that the volcanic crust underlying the ocean basins was “striped” with alternating bands of magnetic polarity. It was known that Earth’s magnetic field reverses periodically (the last time was about 700,000 years ago), so the stripes were an indication that the oceanic crust varied in age systematically.
Following the stripes back to the youngest age led the scientists to a new landform, a mid-ocean ridge that was a continuous line of active volcanoes. So oceanic crust was produced at the ridge and moved like a conveyor belt outward, forming the magnetic stripes. Where the ocean crust ran into a continent, it either moved with the rigid continental plate, or was pushed under the continent, as the volcanic ocean crust was denser than the thick continental crust.
Where the ocean crust dove under the continents, such as the west coast of South America, it created a deep offshore trench. As the plate sank deeper into the mantle under South America, it began to melt which then rose into the continental crust and popped out as the Andean volcanoes.
Those are the simplest scenarios; many more complications were discovered, including those rare instances where the ocean crust is pushed up on land. The uplift of the Himalayas and Tibet were explained by the fact that the Indian Plate has rammed into the Asian Plate. Two continents impacting led to India being pushed under Asia and lifting the Himalayas and Tibet. It also forced large faults to form that allow the eastern part of Tibet and southeast Asia to “escape” from the crunch by slipping eastward much like squeezed watermelon seeds.
Learning About Transform Faults
Another geological feature that was explained by plate tectonics was the presence of what was called transform faults. These were found to be faults that connected two segments of mid-ocean ridge. An example we know very well is the San Andreas Fault; it connects a volcanic ridge that runs north from offshore northern California to a section of the ridge that happens to lie in the Gulf of California or Sea of Cortez. Thus, all of California west of the San Andreas Fault, including Baja California, is moving northwest at about two inches/year. Geologists had long sought to explain the large amount of offset on the San Andreas Fault: most known faults have only a few miles of offset over their lives, but the San Andreas appeared to have 150-300 miles.
It turns out the tectonics of the western U.S. is even more complicated than just having a huge transform fault on land. As described very eloquently in John McPhee’s book Assembling California, the state has been built up literally by volcanic arcs, like Japan, being transported by the Pacific Ocean Plate and ramming into the North American Plate. Thus, the Sierra Nevada, which are an old, eroded remnant of an Andes-like chain of volcanoes sitting over a plunging oceanic plate, are now isolated way inland by the newer parts of California having floated in from the west. Further north, we all know that Oregon and Washington have some active volcanoes and that’s because the Pacific Plate is diving under the continent and melting like the Andes.
The Hawaiian Island chain shows how the Pacific Plate has moved over the last several million years. If we date the various islands from Ni’ihau and Kauai (about 5 million years old), Oahu (3.4 million years), Maui (1.3 million years) to the Big Island, where a new volcano, Loihi, is just beginning to build a new island off its south coast, we can see a progression of the Pacific Plate as it slides over a fixed hot spot, where molten rock is rising from the deep Earth. We can even project back further in time by noting that the Emperor Seamount chain starts northwest of the Hawaiian chain and then turns more northerly, implying the Pacific Plate changed its motion in the distant past.
By carefully piecing together the same rock formations and their ages across various continents, geologists have worked out how the continents have moved around over the past billion years or so. There are some great animations of those gyrations out there (www.youtube.com/watch?v=QhldiOaFqpE, www.you
tube.com/watch?v=q-ng6YpxHxU). It turns out about 300 million years ago, a supercontinent named Pangaea formed from all the continental plates. Then about 150 million years ago, it split into Gondwanaland and Laurasia, opening the Atlantic Ocean. Those two super-continents then split up over the next 100 million years into the present continents: North America split from Eurasia about 60 million years ago and gradually, the continents drifted into their present locations.
The continents continue their drift and there is at least one place that a continent is currently being ripped apart. The Great Rift Valley in East Africa is a place where what’s called a triple junction has developed. That’s a place where a hot spot has risen, causing the continental plate to be torn apart, eventually to form a new ocean. It’s called a triple junction because there are three arms. In East Africa, the arms run through Kenya, Ethiopia, and Djibouti; up the Red Sea; and through the Gulf of Aden.
The activity of the East African Rift has been in the news lately as Mount Nyiragongo, a volcano on the rift, has erupted, damaging the town of Goma at its base. The corner where South America once connected to Africa at Nigeria is the location of another triple junction. In this case one arm failed and remains today as the Benue Trough along which the Niger River flows.
Strike-Slip and Thrust Faults
All this jostling of continental and oceanic tectonic plates is the cause of most of the earthquakes we feel. As an oceanic plate dives beneath a continent, it sticks sometimes and when it finally slips, we can have a very large earthquake. Japan suffers from those as well as other areas in southeast Asia. Transform faults like the San Andreas also stick and then slip occasionally, usually along a long stretch. These faults, called strike-slip, slide sideways; and if you look across the San Andreas fault, you’d see the other side moving to the right, so it would be called a right-lateral, strike-slip fault. The last big earthquake along the San Andreas was the 1906 San Francisco earthquake. The last one in the southern section of the San Andreas was in 1872. As the stress builds up on the stuck fault, the probability of a large earthquake increases.
Along the South Coast and to the east toward L.A., we have a somewhat different situation: there is a big bend in the San Andreas Fault north of us and due to the right-lateral motion, the crust gets “piled up” in the northern part of L.A. and in the Santa Barbara area. We see that pile up as the Transverse Ranges, which run east-west as opposed to every other mountain range in California. The pile-up causes faults to develop called Thrust Faults as they’re pushing the crust up into the mountains. Our offshore islands are part of the Transverse Ranges and that means there are Thrust Faults out in the channel as well. We had a small earthquake in the channel on May 26 on one of those faults.
Because of the importance of monitoring the motions of tectonic plates and stress build-up on stuck faults, scientists have been using fixed GPS stations that record their movement continuously. In the U.S., the National Science Foundation has funded a consortium of scientists and engineers, who have established a network of GPS stations in the west called the Plate Boundary Observatory. There are several stations in the Santa Barbara area, including one in the middle of the Coal Oil Point Reserve in Isla Vista. Anyone can download the recorded data and watch as coastal California moves northwest relative to the rest of the nation. I’ll discuss the GPS set of satellites in a later column.