The Islands of Sao Tome, Principe, and Annobon, West Africa
Text and photographs by Dr. Dan N. Barfod
Isotope Geosciences Unit, Scottish Universities Environmental Research Center
San Tome
Sao Tome is a volcanic island on the Gulf of Guinea, West Africa and is part of
the Cameroon line volcanic chain. The image shown above is of Morro Peixe, one of the many
recent basaltic cinder cones that crop out on the north and south ends of the island.
Argon isotopic data for similar cones on the island indicate ages of less
than 100,000 years. Despite these signs of recent volcanism, Sao Tome has had no
historic eruptions.
The eruptions on Sao Tome produced mainly basalt and some trachyte. Pictured above are the columnar-jointed basalt flows from the central part of Sao Tome.
Annobon
Annobon is the last island in the Cameroon line volcanic chain. It is a small island
with a central volcanic crater (occupied by a lake). This image shows one of the many
sets of 4.8 million year old vertical basalt dikes that intrude a yellowish basaltic tuff.
The tuff is widespread and blankets large parts of the island. More recent volcanic
activity (less than 100,000 years ago) includes eruption of basalt flows on the north
and south ends of the island and the eruption of a basaltic tuff from the crater lake.
Annobon is a small island, measuring only 3 by 7 kilometers, with a maximum
elevation of 654 meters above sea level. However, this is only the tip of a very large,
mostly submarine volcano that rises ~4,200 meters off the seafloor. This image shows Annobon from the north. The prominence just to the right of the summit is Pico del Fuego,
a trachyte plug that intrudes the margin of the crater lake. In front of that is a
massif, just above the coastline, of 4.8 million year old basalts. As with the other
islands of the Cameroon line chain, Annonbon has seen no historic eruptions.
Principe
Principe is a deeply eroded volcanic island that shows no signs of the more recent volcanic activity that marks the other islands of the Cameroon line. Shown here are three
phonolite towers, similar to ones found on Sao Tome. Phonolite is a volcanic rock that is highly resistant to weathering and erosion, much more so than basalt. These towers form
first by the phonolite intruding into a basaltic volcano, and then by erosion of the surrounding basalt. The towers can be several hundred meters high.
When volcanic rock cools, it shrinks slightly and fractures to form polygonal columns. This
image shows a block of phonolite (found at the foot of the central tower in the previous
image of Principe) with well-developed columnar jointing. The individual columns are nearly
a meter across, much larger than the typical column seen in basalt lava flows. The person
in the photo is Dr. C. J. Ballentine, ETH Zurich.
VolcanoWorld wishes to thank Dr. Dan N. Barfod for generously sharing the
information and photographs.
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