Last year I gave a presentation at a boys’ school in Virginia – although to call Woodberry Forest a school is a bit like calling the Pacific a puddle. It was a magnificent school – gorgeous buildings situated in rolling green parkland. I got to stay in a beautifully appointed guest house. The boys were polite, interested, fun. But best of all I came away feeling I had made some friends – particularly two of the teachers, who just happen to be married to each other.
Earlier this year those two teachers and I got together again in California and as we were driving along I-280 Michael gave a fascinating description of the San Andreas fault, visible to the left of the highway. It was then that I decided I was going to ask him to write something about the land that lies hidden far beneath the Pacific – the ocean bed. Here is what he had to say. I found it fascinating, and I hope you enjoy it too. It certainly adds an extra dimension – of time and geology – to my journey.
When my friend Roz first asked if I would be willing to write an essay about the geologic story that (quite literally) underlies her journey across the Pacific, my first reaction was probably very similar to that of many people who are reading this: She’s crossing the Pacific OCEAN! After departing the California coast, Roz won’t see another rock until she makes landfall in Hawaii. That doesn’t exactly make for an engrossing geologic narrative. But Roz was persistent (what a surprise!) and, the more I thought about it, the more I realized that geology does provide an interesting and unique perspective on her Pacific voyage. What follows is a rough geological “sea log” of the first leg of this remarkable adventure.
As Roz passed beneath the Golden Gate Bridge, her attention was quite understandably on the treacherous currents that swirled around her and the enormity of the challenge ahead. I doubt she spent much time looking at the rocks of the Marin Headlands to the north, and I’m certain she didn’t realize that those very rocks had made the same journey she was attempting-only in the opposite direction!
*Picture 1.* View of the Golden Gate Bridge and San Francisco from the Marin Headlands. Rocks in the foreground are radiolarian cherts of the Franciscan Formation. Photo – Michael Follo.
The Marin Headlands are what geologists refer to as an *exotic terrane*. They are comprised of rocks-in this case bedded cherts of the Franciscan Formation- containing the remains of microscopic marine organisms known as radiolaria. Some terranes are thought to have originated as far away as the southern hemisphere and/or western Pacific. Movement of the Earth’s tectonic plates transported these terranes thousands of kilometers. When the plate carrying the Franciscan cherts slid beneath western North America in a process known as subduction, oceanic rocks on the descending plate were scraped off and accreted to the North American continent. Intense folding of rocks in the Marin Headlands is evidence of the force of this collision and uplift.
Thankfully for Roz, her voyage differs from that of the Marin headlands and other exotic terranes in speed as well as direction. The average rate of plate motion is approximately 5 centimeters per year-about the rate at which a fingernail grows. On even her worst days, fighting headwinds and currents, Roz is well over a hundred million times faster!
Not long after paddling out of San Francisco Bay, Roz crossed a major tectonic feature, the San Andreas Fault – a *transform* plate boundary along which the North American and Pacific plates slide past one another. The movement along this boundary is primarily responsible for the numerous earthquakes that residents of California are all too familiar with. There are two other types of plate boundaries-*dive
*Picture 2.* Google Earth image showing approximate trace of San Andreas Fault (red line) where it crosses from the San Francisco peninsula to Point Reyes.
Roz’s Pacific voyage is very different from that of her earlier row across the Atlantic. The Atlantic Ocean is bisected by a divergent plate boundary, the Mid-Atlantic Ridge, separating the Eurasian and North American plates. They were once part of a single supercontinent, known as Pangea, which began to rift apart approximately 200 million years ago. As Europe and North America moved away from this mid-ocean ridge, the Atlantic Ocean grew progressively wider-as it continues to do today. The Earth is not getting bigger, so the amount of new crust created at divergent boundaries must be balanced elsewhere by subduction at convergent boundaries. In the Pacific, this occurs along the so-called “Ring of Fire” that circumscribes virtually the entire ocean basin.
Along this convergent plate boundary the Marin Headlands and other exotic terranes were accreted to western North America. However, complex plate interactions over the last 30 million years have transformed this formerly continuous subduction zone into the San Andreas Fault system. After crossing the San Andreas, Roz will not pass another plate boundary until she reaches the western Pacific near the end of her voyage. There, the Pacific plate collides with the Philippine and Indo-Australian plates along a complex boundary.
I’m certain that Roz will be thrilled to see the Hawaiian Islands – and they provide a break from 3700 kilometers of geologic monotony. For the past several months, Roz has been rowing some 5-6 kilometers above a featureless abyssal plain marked only by occasional volcanic seamounts, all below sea level. The Hawaiian Islands, on the other hand, rise from a depth of 6000 meters to an elevation of over 4000 meters above sea level. By this measure, Mauna Loa on the Big Island of Hawai’i is the tallest mountain on Earth, surpassing even Mount Everest (8850 meters).
The Hawaiian Islands are a chain of volcanic islands extending from Kaua’i in the northwest to the Big Island of Hawai’i in the southeast and require a different explanation. The most important clues to the origin of Hawaiian volcanoes come from their age distribution and composition. The oldest volcanic rocks are found on Kaua’i. The islands get progressively younger to the southeast, culminating in the currently active Kilauea volcano on the Big Island.
The Hawaiian Islands are thought to be the product of a more or less random “hot spot,” a thermal plume of mantle-derived magma that has burned its way up through the overlying plate. The Pacific plate is moving (to the northwest) over a stationary hot spot. As the plate continues to move over this, a series of volcanoes have built up and then gone extinct as each island was carried away from it.
The Pacific Plate has been moving over the hot spot at an average rate of approximately 10 centimeters per year. The prominent bend in the chain reflects a change in the direction of Pacific Plate motion some 40 million years ago. Prior to that time, the Pacific Plate was moving almost due north.
The Pacific plate is steadily carrying the Hawaiian Islands northwest at a rate of some 10 centimeters per year, the first leg of her journey is actually getting longer by the day. Sorry, Roz!
*About the author:*
Michael Follo is a science teacher at Woodberry Forest School in Virginia. He has a Ph.D. in geology from Harvard University, and has taught at the college and secondary school level for the past 23 years. He has conducted geologic fieldwork in Europe, North America, and Hawai’i-but, unlike Roz, nowhere in between.
Position as at 2100 19th August HST, 0700 20th August UTC: 22 26.238’N, 151 00.378’W.
It is a beautiful night tonight. As I rowed along after sunset, waiting for the moon to rise, the deck of my boat lit up and looked up at the sky to see a shooting star. I’d never seen one so bright – it really was like a camera flash going off in the darkness of the night.
Thanks for all the great messages. Special hellos to: Brian – have a great (and safe) time mountain-climbing in Pakistan. Hope to see you in London in November. Andy, Emer, Ailis, Saoirse and Caoimhe – so nice to know you are keeping an eye on the blog. Hope you’re settling into your not-so-new home. Lots of love to all. George – thanks for the quote. I used to live in the same village as Sir Peter Blake’s family – I met his widow and son. Lovely people – wish I’d met the man himself, but I was too late. John, Erin – great questions. Will respond in future blogs.
And finally. from George van der Meeuwen in New Zealand: There is a legend that says: On the occasion of a great forest fire, all the animals sought to escape. Only one little hummingbird was gathering a few drops of water from the river and flying high to drop them on the fire. They asked the hummingbird what use it was to do so little? The hummingbird answered – ‘If everyone were to do just a little!’