Posted by: Rob Viens | February 15, 2012

St. Paul’s Rocks I: Serpentines and Bird Poop

On the evening of February 15th, the Beagle reached St. Paul’s Rocks (also called the St. Peter and St. Paul Archipelago). I am fascinated by two things in particular about this “stop” – (1) geologically these rocks are very unusual – not your typical ocean islands, and (2) Darwin got almost everything (geology and biology) about them correct in his short visit to the islands.  In fact, recent research I have found online more or less elaborates on the same things Darwin thought to write down in his diary, Voyage of the Beagle and Geologic Observations. Very cool.

The crew actually explored these island on the 16th, but since they are so darn interesting I though I’d spend a couple days digging into their geology and biology.

Our story starts on the 16th:

” When within 3 miles, two boats were lowered, one with Mr Stokes for surveying the island, the other with Mr Wickham & myself for geologizing & shooting.” (Feb 15/16) – More on Wickham and Stokes at a later date…

If I had to summarize, four main things happened that day – (1) Fitzroy and the Beagle surveyed the island and the depths of the water, (2) Darwin examined the rocks, (3) Darwin observed the flora and fauna, and (4) the crew collected food by beating the daylights out of the local bird and shark populations.

YouTube video of the islands from 2006:

(1) Geography and surveying:
Geographically speaking the archipelago consists of 15 islets totaling about 4 acres of land. Their highest elevation is a mere 18 meters above sea level. They are a mere 100 km off the equator and the nearest continental land is almost 1000 km away. Finding them is sort of like “speaking a ship” in the middle of the sea – it is amazing that they can actually find these islands at all.

Map of location of St. Paul & St. Peter Archipelago:

Fitzroy did some detailed surveying and determined (correctly) that these islands were the tips of a “submerged mountain”. His fancy new sounding lines couldn’t reach the ocean floor and Darwin summarizes:

“Bottom could not be found within a mile of the Island, & if the depth of the Atlantic is as great as it is usually supposed, what an enormous pyramid this must be.” (Feb 15/16)

Right on Charlie – these “mountains” rise over 4000 meters off the ocean floor! In my neck of the woods that is almost as tall as Mt. Rainier.  And that is not the strange part…

(2) Geology:
It is not unusual for ocean islands to rise 1000’s of meters off the ocean floor. For example, the big island of Hawaii surpassed the height of Mt. Everest, rising over 10,000 meters from the bottom of the ocean to its summit.  But, like almost all of the islands found in the middle of the world’s oceans, Hawaii is a volcano – a million years worth of lava flows piled one on top of each other until they reach the sky. St. Paul’s rocks, as Darwin correctly surmised, are something else entirely.

“It is a remarkable fact, that all the many small islands, lying far from any continent, in the Pacific, Indian, and Atlantic Oceans, with the exception of the Seychelles and this little point of rock, are, I believe, composed either of coral or of erupted matter.” (Voyage of the Beagle)

This is an amazing observation in the 1830’s.  Today, it is easy to see a geologic map of the world and make this connection.  But the only way to do it in Darwin’s time was through a long voyage and careful observation.  Luckily, this was his specialty.

Geologically speaking these islands are extremely unusual (I’m a life-long geologist and still don’t fully understand them). Darwin notes:

“It is composed of rocks, unlike any which I have met with, and which I cannot characterize by any name, and must therefore describe. The simplest, and one of the most abundant kinds, is a very compact, heavy, greenish-black rock, having an angular, irregular fracture, with some points just hard enough to scratch glass, and infusible.” (Geologic observations of Volcanic Islands), and

“Its mineralogical constitution is not simple; in some parts the rock is of a cherty, in others of a felspathic nature, including thin veins of serpentine.” (Voyage of the Beagle)

“Serpentinized mantle peridotites” from the Alps (a collisional plate boundary) similar to St Paul’s Rocks (from the Virtual Tourist web site):

What Darwin is describing here – particularly the green rocks and serpentine veins – are a piece of rock from below the ocean crust – literally a part of the Earth’s mantle. This type of rock doesn’t see the light of day much.  Occasionally, in a massive continental collision or along a subduction plate boundary they are scraped off the ocean crust and pushed to the surface along massive faults.  However, there has not been a plate collision or subduction zone in these parts since before the Atlantic formed (a very long time ago).  So that raises the question – how the heck did they get here?

Serpentinized mantle peridotites

From what I can tell, these rocks were brought to the surface along a transform plate boundary that connects two sections of the Mid Atlantic Ridge (a divergent plate boundary). The San Andreas Fault is part of a similar boundary in California. It is a little different, since it is on land, but similar in that it connects the Juan de Fuca Spreading Ridge (off the coast of the Pacific Northwest) with a spreading ridge in the Gulf of California. It is not uncommon for the rocks on either side of an oceanic transform boundary to have different elevations (especially when divergence at the nearby ridges is slow). However, 4000 m is extreme, and I now of nowhere else a ridge like this forms islands quite like these.

I really can’t say enough about the fact that Darwin recognized the uniqueness of these rocks It was an extraordinary revelation for his time, and reminds me that he was a really good geologist.

Map of the Elevation of the Ocean floor around the islands (from Motoki et al, 2007 via Wikipedia):

Map of Ocean floor around the islands

That brings us to the other geologic feature of the islets – semi-fossilized bird poop – guano. Darwin notes:

 “The rocks of St. Paul appear from a distance of a brilliantly white colour. (Voyage of the Beagle)

In Geologic Observations of Volcanic Islands he describes details:

“a layer of a glossy polished substance, with a pearly lustre and of a grayish white colour; it follows all the inequalities of the surface, to which it is firmly attached. When examined with a lens, it is found to consist of numerous exceedingly thin layers, their aggregate thickness being about the tenth of an inch.”

Guano buildup at near the research station (posted to Google Earth by silmarferreira)

Guano deposits

Darwin hypotheses (correctly) that these deposits originate from “dung of a vast multitude of seafowl”. More than just a curiosity, guano was actually considered an economic deposit due to their high phosphate content. (If you remember, gardening fertilizers is defined by its N-P-K – the P stands for phosphorous.) It was also a source of nitrates for gunpowder. Though the first real commercial mining operations of guano didn’t start until the 1840’s. The “guano boom” (interesting thought) was over in about 50 years as many of the prime islands were completely mined of their deposits, and other alternatives became more readily available.  St Paul’s rocks were small enough that (to my knowledge) they were never mined.

Tomorrow – we’ll delve into the guano producers (the birds) and the ecosystem they support. (RJV)

Advertisements

Responses

  1. 🙂

  2. Rob,

    Thanks for the time and energy that you are putting into this blog. It has become my daytime soap opera or my electronic NPR.

    Darwin has always been my favorite scientist, but I am even more enamored with him because of this blog. My gosh, Darwin is a kid but his knowledge base is remarkable! And he is a true field scientist. Gerry Maki was an instructor at BC for 30 years and during his last five years, I would join him on field trips with his environmental science and botany students. Gerry, like Darwin, knew so much! He knew soils, rocks, plants, weather patterns, old-growth forests, etc. He was a true generalist, unlike many scientists today who are very specialized. In most cases, a molecular biologist wouldn’t care about the organism and its interaction with the environment. Just show him or her the A’s, T’s, C’s and G’s of DNA…

    • Thanks Jim!

      Just imagine what Darwin would have done with all data and knowledge of today. He was such a good synthesizer of information, I can only imagine what he would have seen in all those A T C G’s!

  3. […] at St. Paul’s Rocks fit nicely into our understanding of plate tectonic boundaries today (see St. Paul’s Rocks I: Serpentines and Bird Poop) and his observations of the volcanic nature of ocean islands support what we now know about mantle […]

  4. […] was when the ship reached the “unexplored” shores of St. Paul’s Rocks (see St. Paul’s Rocks I and St. Paul’s Rocks II). Darwin got to be in the first boat to go ashore and explore.  […]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: