Learning About the Central Coast Through Geological Oceanography

By Catherine Drake, Invertebrate Zoology Lab

Other than a few awesome, albeit too short, trips to the Monterey Bay Aquarium, I hadn’t spent much time in the Central Coast.  So when I moved up here for graduate school at MLML, I didn’t know much about the area; that is, until my MS 141 class.  Geological Oceanography—taught by Dr. Ivano Aiello—involves learning about the formation of minerals and rocks, as well as geological mechanisms such as plate tectonics.  We’ve taken field trips almost every week to various locations along the Central Coast and inland as well.  One of my favorite field trips was our overnight trip to Point Reyes, where we stayed in an old lifeboat station while we observed different types of rock formations.

The lifeboat station was built in 1927 at Chimeny Rock in Point Reyes.

We examined multiple sedimentary rocks both along our journey to the station and also once we had arrived.  One of the depositions we inspected was an outcrop of radiolarian cherts.  These deposits sit underneath about half of the Marin Headlands, are resistant to weathering, and can be up to 200 million years old.  They are comprised of radiolarians, which are protozoans that form siliceous (made of silica) skeletons.  As these organisms decompose, a radiolarian ooze is formed in the deep ocean; over time, deposition occurs along the seafloor, forming the well-bedded radiolarian cherts.

Radiolarian cherts are formed from years of deposition of radiolarian siliceous skeletons on the seafloor.

Igneous rocks were also on our list of stops, as we went to a formation of pillow basalts.  They are formed underwater as lava comes in contact with seawater and cools rapidly.  Basalts are generally aphanitic rocks, meaning that they cool down too quickly for any minerals to form as the magma cools.  As they are created, pillow basalts form ellipsoidal shapes and depict the direction of the lava flow.

Behind the class are pillow basalts, which are igneous rocks formed underwater as lava comes in contact with seawater and rapidly cools down.

It was so surreal to touch igneous and sedimentary structures that formed hundreds of millions of years ago.  Examining these rocks helped me better understand the geological mechanisms involved in their formation.  Not only did these sedimentary depositions and igneous rocks help me become more acquainted with the Central Coast, but they also demonstrated the fact that oceans are integral components to the geologic history of our planet.

The Great Glauchonite Hunt

Jeremiah Brower

by Jeremiah Brower, Geological Oceanography Lab

Over the last couple of years I’ve learned that certain grants have been difficult to get because some people don’t consider Marine Geologists (or Geologic Oceanographers…the terms are interchangeable, one just seems to roll off the tongue better) as “actual” geologists because they don’t consider us to be “field scientists.” While there is a certain amount of truth in that, due to advances in software technology (much of our work involves computers and mapping programs), we still need to go outside to collect the data in the first place!  Even the oceanographers who focus on the creation of habitat maps need to spend years surveying in the field before they can sit down in front of a computer and decide how best to play with the numbers.

A microscope view of glauchonite - that's fish poop you're looking at!

Case in point – early one Saturday I was out on the beach, hunting not for shells like so many tourists, but little green rocks.   Another student from Moss Landing needed to find iron-rich rocks to see how iron affects the growth rates of various species of kelp.  I offered to help him out and so I grabbed my rock hammer, hand lense, field notebook and we meet up at a beach just south of Santa Cruz. It was early in the day but we still needed to weave our way through a plethora of tourists to find a good exposed cliff-face. We were on a hunt for Glauchonite, a green, rounded rock that is formed in shallow marine sediments by the compaction of iron ore and…..well…….fish poop! (Or ‘fecal pellets’ as they are very scientifically called.) Small samples of Glauchonite can be found all over the Purisima formation I wrote about earlier, and they are rich in iron, so would provide a good test for the kelp experiment.

Traveling down the beach and climbing up cliffs, I guided my kelp-loving friend through thousands of years of history until we found a promising exposure of rock. Hacking away at it may have drawn some questionable looks from the beach-patrol, but humans are agents of erosion anyway, so we were just doing our part!  We found some good samples of the green rock and took a moment to enjoy the day before heading out. I think it was a smart move to pick a profession that would let me spend most of my life on a beach, and Saturday’s “rock hunt” was a perfect example of why I love the science. Marine Geology IS Geology……now if we could just get that grant we could buy a new cappuccino machine!

Sand man signing out.

Against the Grain

Jeremiah Brower

by Jeremiah Brower, Geological Oceanography Lab

One of the reasons I love being a Geological Oceanographer is that you can walk along the beaches of Santa Cruz and travel through thousands of years of history in a few short miles. Much of the coastline here is actually made up of shallow marine plankton deposits that have been crushed over the years into a fine silt and sandstone layer that is greater then 150 m thick.

Basically, the coastline of Santa Cruz was once underwater and has been slowly rising through a tectonic process called “uplift” for the better part of the last two hundred thousand years.  Because of uplift, these previously submerged sediment deposits of the continental shelf are now exposed above the ocean, and natural erosion has cut them back to the cliff walls we see today.

Geologists identify this cliff layer we find on our coastline as the ‘Purisima formation’ and date it to the late Pliocene era (1.6-5.3 million years ago, which is young as rocks go, if you can believe it!).  The Purisima formation is structured with thousands of shell fragments from clams and various other plant and animal life forms that used to live on the sea floor. By examining the different layers of the formation you can see when the ocean was more full of life, or when the sea floor was a sort of dessert (just another cool aspect of geologic oceanography).

A certified time detective and rock hunter in action.

No matter where you go in the field, you tend to find that the land has been affected by the ocean. The Santa Lucia range of central California is another great example: it was created by the compaction of millions of marine carbonate shells left behind by various planktonic life forms.

So I suggest that the next time you’re hiking in the mountains or hanging out at the beach with a cold mojito, consider the sand and rocks around you. Most of this planet is covered in water, and that water has affected most of the land we stand on – so don’t be surprised if you’re hiking in Montana and find a shark’s tooth at your feet!

Sand man over and out.