Posted by: Rob Viens | February 21, 2012

Fernando de Noronha II: Singing the Praises of Phonolites

February 21st found the Beagle’s sails slack for lack of wind

“We sailed at night, but have not made much way this morning. —latterly it has been a dead calm, the ships head standing the wrong way.” (Feb 21)

There is nothing more impressive than a 18th or 19th century sailing ship.  But take the “wind out of their sails” and there isn’t much they can do.  So, while Darwin drifts at sea, let’s explore the Geology of Fernando de Noronha…

Rocks of Fernando de Noronha – note the columnar shape of the rocks in the background (from 1000lonelyplaces.com)

rocks of Fernando de Noronha

Much like the Canaries and Cape Verde Islands (but not St. Paul’s Rocks), Fernando de Noronha is a hot spot island. These islands are formed were plumes of hot rock rise from the lower mantle of the Earth (not the core as some diagrams suggest – that would just be silly).  At some point during their ascension, these hot rocks start to partially melt and form magma. (Yes partially melt – there are actually very few places in the earth where it is hot enough to fully melt a rock.)

Map of mantle hot spots – note the Fernando de Noronha hot spot off the east cost of South America:

Map of mantle hot spots

An opportunity for a brief geology lesson – bear with me, it helps later on:

Geologists refer to most magma by how much silica (silicon dioxide) it contains. (All magma has at least some silica in it, typically close to half or more of its chemical makeup).  “Mafic” magmas have lower silica content (they are about 40-50% silica) and “felsic” magma has a high silica content (above about 70%).  We can call the stuff in the middle “intermediate”. To make a long story short, the Earth’s mantle has a very low silica content (compared to the crust), so we refer to its composition as “ultramafic”.  And, as it turns out, when you partially melt ultramafic rock, you get mafic magma. If this magma erupts at the surface, as it does at hot spot islands, it hardens into dark-colored, heavy, mafic volcanic rock. The most common type of mafic volcanic rock, the one many folks are familiar with, is basalt.

So the moral of the story is…partial melting at mantle hot spots typically creates mafic magma, and therefore, these islands are typically made up of dark-colored mafic rock, such as basalt.  Moral number two …when you find mafic volcanic rocks, there is a very good chance the magma they formed from originated in the mantle.

Now one more twist – Not all mafic rocks are created equally. In fact, although they all share the common bond of lower silica content and higher levels of iron and magnesium, there is a lot of variety in the other chemical elements they contain.  This is a great tool for modern geologists, because we can use the concentration of elements to try and uncover the specific origin of thes mafic rocks (for example, where in the mantle did they come from, or how much partial melting occurred).

With that background, let’s head back to our friend Darwin, “geologizing” on Fernando de Noronha.  Here are a few of his descriptions of the rocks, and a couple of his  hypotheses. I’ve also added some modern annotation. Again, Darwin seems to be spot on.

(1) On the age of volcanism:

“As far as I was enabled to observe, during the few hours we stayed at this place, the constitution of the island is volcanic, but probably not of a recent date.” (Voyage of the Beagle)

As mentioned earlier, Fernando de Noronha is a volcanic island – formed from the eruption of magma formed from the partially melted mantle. Interesting , volcanic activity on the island seems to have occurred between about 12 to 2 million years ago.  This suggest that the hot spot either (1) is done erupting, (2) erupts very infrequently, or (3) has shifted offshore and is erupting underwater. For various reasons I would favor the later, and there is a potential candidate for this activity – a seamount located to the east of the island – in a spot roughly consistent with where the hot spot should be.

(2) On the type of rock:

“The most remarkable feature is a hill 1000 feet high, of which the upper 400 feet consist of a precipitous, singularly-shaped pinnacle [Moro do Pico – see photo from yesterday], formed of columnar phonolite, containing numerous crystals of glassy feldspar, and a few needles of hornblende. … Near the base of this hill, I observed beds of white tuff, intersected by numerous dikes, some of amygdaloidal basalt and others of trachyte; and beds of slaty phonolite ” (Geologic Observations of Volcanic Islands)

“On the beach there were numerous fragments of compact basalt, of which rock, a distant facade of columns seemed to be formed.” (Geologic Observations of Volcanic Islands)

True phonolites are generally more intermediate in silica composition, but there are some types (such as phonolitic containing nephaline, a feldspar mineral) that are essentially basalt with more potassium (K) and sodium (Na). I suspect the later is what Darwin found. Trachytes have a higher silica content than phonolites but are also high in K and Na.  It is not uncommon to find them with phonolites, since they can come from the same magma source (geologists, ironically, would call them more “evolved”). Honestly, I doubt I could have ID’ed these rocks correctly in the field (I’d need more practice or a thin section of the rock and a petrologic microscope). This chemical difference is one of those little details that helps igneous geologists better understand the formation and source of the magma.  As Darwin notes (below) when comparing volcanic structures, these same types of rocks are found on St. Helena, whereas Ascension Island has lower K and Na (more “typical” basalt)

Phonolite in hand sample and under a petrologic microscope (this one is from the Comoro Islands) – the large rectangular crystals (called phenocrysts) are feldspar like those Darwin described (from the Phonolites Choir page – really, it’s a choir named after the rock type)

Phonolites

Amygdaloidal basalt, is basically a basalt (or in this case, probably something with a higher K and Na content) that had a bunch of air bubbles in it which are now filed in with other minerals.

Amygdaloidal basalt from the midwestern US (from James St. Johns web site at Ohio State University)

Amygdaloidal basalt

Lastly, those “columns” Darwin notes – those are columnar structures that form when a relatively thick (usually mafic) lava flow cools and cracks.  Worth pointing out here, but a story for another day…

(3) On differential erosion of a volcanic neck:

“On viewing one of these isolated masses, at first one is inclined to believe that it has been suddenly pushed up in a semi-fluid state. At St. Helena, however, I ascertained that some pinnacles, of a nearly similar figure and constitution, had been formed by the injection of melted rock into yielding strata, which thus had formed the moulds for these gigantic obelisks.” (Voyage of the Beagle)

“If this hill has had, as is probable, a similar origin, denudation has been here effected on an enormous scale.” (Geologic Observations of Volcanic Islands)

What Darwin is describing, and is likely correct about, is differential erosion. In this case the intruded magma cuts across older lava flows and solidifies in its volcanic conduit.  The older flows (which are softer) erode away, leaving the harder intrusion standing as a monolith. Classic examples of this in the US, include Devil’s Tower, WY and Shiprock, NM.  Based on this interpretation, Moro do Pico would represent one of the former eruptive centers of the island.  (Interestingly, Devil’s Tower in Wyoming is also composed of a columnar jointed phonolite. )

OK – my geo-geek kicked in today, I’ve been writing for a month now, and I still can’t get over how good Darwin was at geology.

One more week of sailing and Darwin would be on the continent – almost exactly two months from his Plymouth departure. (RJV)

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Responses

  1. […] primarily felsic while ocean rocks are mafic (see the “lecture” on rock composition in Fernando de Noronha II), they where also quite different in age.  The volcanic island rocks Darwin chipped away at in the […]

  2. […] difference we have to look back at our old friends mafic and felsic rocks (for a refresher, see Fernando de Noronha II). The bottom line is that the continents are primarily made of felsic rock, and the oceans are […]

  3. […] we also see columnar basalt), these rocks are most likely the result of hot spot volcanism (see Fernando de Noronha II). From what I can tell, the Abrolhos Islands were above the Trindade Hotspot about 40 million years […]


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