Local Geology: Estero Bluff

Hey there every peoples!

Sorry for being a little off topic lately (and will be again very very soon) but I promise I will keep things relevant. So here’s some geology. Last week my Geology class went on its first field trip. Mr. Grover took us to a piece of land up in north county, north of the town of Cayucos. It’s a place called Estero Bluff, and while it has a very nice beach, it sports some interesting geology (I may be a little spotty; I tried to remember as much as I can).

Mr. Grover ain’t lying when he says we have such wonderful geology in this area. He demonstrated this by bringing us to Estero Bluff. He brought us here because Estero Bluff demonstrates many geological processes. For example, notice the terrain:

Estero Bluff

It’s a flat open terrace. But sitting on the terrace is this hill:

A hill sticking out like a sore thumb on the Continetal Terrace

So how did this happen? It’s a combination of things. The first is a change of sea level. Sea levels have changed so much throughout the earth’s history. It rises and drops with the growth and waning of glaciers and other factors. But a drop in sea level doesn’t explain the terrace alone (as we’ll later see, uplift also had a hand in it). Erosion is always at work, whether we see it or not. And what went on here at the bluff was a process called differential erosion. Everything erodes at different rates for various reasons and in the case of Estero Bluff, it’s the hardness of the rock. The hill here is made up of layers of chert:

Layers of chert that make up the hill

When you have a blob of chert in a site that’s mainly made of soft soil, the soil will erode away while the chert stays relatively intact since it’s much harder. And that’s how that’s how this hill came to stick out of a flat terrace.

Now, the other reason for the terrace is uplift. Did you notice the chert layers were at a slant? That is due to the process of geological uplift. This is where the earth pushes up (that’s often how mountains are formed). And Estero Bluff is unique because it sits on the edge of two continental plates. Currently the Pacific Plate is being subducted (pushed under) the North American Plate. And this subduction has caused the layers of the earth here to scrunch up and tilt on their sides. It’s wholly possible that the chert layers could be vertical in a few million years.

This uplift is the reason why Estero Bluff is unique. It represents an Ophiolite, which Wikipedia describes as “a section of the Earth’s oceanic crust and the underlying upper mantle that has been uplifted or emplaced to be exposed within continental crustal rock.” The chert represents old ocean sediments. It rests near the top of the Ophiolite because it was among the top layers when the crust began to scrunch up. Usually below sea floor layers we get basalt lava with what are known as “pillow” structures:

Pillow lava basalts (not the best example though)

Pillow lava is formed under water, but they will get their own post eventually. Estero Bluff illustrates the top of the Ophiolite in this pocket beach, where we can see the layers of basalt, chert, and younger conglomerates:

Mr. Grover expalining the different layers

Finally, Estero Bluff has one last goody for us:

A bluse schist enjoying the California sun

It’s called schist. It’s a form of metamorphic rock formed from basalt. It is formed at high pressure but low temperatures. Generally it cooks at temperatures of 200-500 degrees Celsius at depths of 15 to 30 kilometers. So how did it get here? Let’s review: Estero Bluff exhibits layers of rock uplifted from the crust and upper mantle and schist is formed from basalt. I think what happened is some of those pillow lavas got sucked down into the subduction zone and formed schist. Then through uplifting they were brought to the surface, where they came to rest on the beach and get polished by the ocean.

Whoa man, that was a mouthful! How’d I do? Because I’m better with paleontology than geology. Anyway, I might feel another rant coming on, but after that, back to the good stuff.

Till next time!

5 thoughts on “Local Geology: Estero Bluff

  1. Pingback: Local Geology: Estero Bluff a Central Coast Paleontologist

  2. If I recall correctly, rocks have to buried more than 20 kilometers below the surface before they can metamorphose into blueschists, so that should give you some idea of how much they’ve been uplifted.

    Great stuff. We don’t get a lot of really clear, new ophiolites on the east coast; there hasn’t been any subduction here for almost 300 million years.

    • Yeah, it does. The figure i got said 15 to 30 kilometers. That’s a lot of uplifting!

      That’s a long time without subduction. I wonder if that’s why we have so many volcanic rocks around here.

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