Walk, swim, eat, sleep, repeat; walk, swim . . . Yes, it
seems that we are developing quite a rhythm in this excursion. Today we admired
the volcanic landscape of Bartolome Islet, which is separated from the much
larger Island of Santiago by the Seymour channel. What a difference with
respect to flat Genovesa! Bartolome, and the adjacent coast of Santiago, are
formed by the juxtaposition of several Surtseyan tuff rings and subaerial
cinder cones. Surtseyan eruptions take place when magma erupts at a depth of a
few meters below the surface of the ocean; because the water pressure is low,
the contact between the hot basaltic magma (1,000 to 1,200 degrees C) flashes
the water into steam and causes powerful explosion jets. The explosion jets
were observed for the first time in Caphelinos (Azores) in the 1950’s, and
later as the new island of Surtsey (Iceland) was born in 1965 (it was then that
trained volcanologists saw and described the eruption, and hence the name
Surtseyan was memorialized in the scientific literature). As described, the
explosion jets resembled black pine trees that grew instantaneously in all
directions, only to turn white once the steam condensed into tiny droplets. The
force of the explosions comminutes the magma, so the layers of tuff formed are
vey fine-grained, and sometimes bear evidence of powerful lateral forces in the
form of thin laminations, ripple marks, or cross bedding. As the new volcano
rises from the sea the waves immediately start cutting it down, so for a while
the new island struggles for survival. If it is to last it must grow fast
enough to cut the connection between magma and sea water, and either coat
itself with lava or at least grow into a cinder cone armored with larger
volcanic bombs. Sometimes, long after the eruption has ceased, the waves manage
to cut down a flank of the tuff ring, exposing the crystallized plug of the
vent to form a pinnacle (which happens to be the case at Bartolome).
The fact that several Surtseyan tuff rings and Strombolian
cinder cones are clustered in Bartolome and the adjacent portion of Santiago
suggests to me that this was a point of active basaltic magma intrusion through
the oceanic crust. However, I don’t think a magma chamber had the chance to
form, so the whole ended being just a field of small monogenetic volcanoes. The
bulk of Santiago, however, eventually was formed by a shield volcano, not
unlike the one in Santa Cruz, and so provided me with a good example to
describe the difference between monogenetic and polygenetic volcanoes.
All this I explained in fits and starts to my fellow
travelers as we climbed the 350 m high tuff ring closest to the shore. On the
back side we found a filed of spatter cones, formed by small central vent
eruptions, which again made a textbook example of central vent versus fissure
eruptions of the type we had seen in Genovesa.
From the top of the tuff ring we had a fabulous view of
Bartolome, the Seymour channel, and Santiago. We could see that a very young,
large lava flow had erupted in the not so distant past (1897 as it happened to
be) from the flank of the Santiago shield, and had flooded the partially eroded
or relatively new cinder cones and tuff rings, leaving them as “inselbergs” sticking out of a sea of
black rock (I am borrowing the term inselberg
from glaciology, where it is used to refer to mountain tops that stick out of
an ice sheet).
To the delight of our friend Catherine, who is the avid bird
watcher of our group, we sighted the Galapagos Hawk, albeit soaring high in the
sky.
Our snorkeling this time was off the dinghy, following the
coast. I sighted a shark, so close I could have almost touched it, a different
type of isolated coral colony that looked like a stack of cereal boxes, and any
number of colorful fish. Our guide saw and took a movie of a shark sleeping on
the sandy bottom that was fascinating (sharks do not have a swim bladder to
control their buoyancy, so they must swim continuously, or rest on the bottom
while asleep).
After another delicious lunch the sip crossed Seymour
channel, and our second walk of the day was a geologic walk through the 1897
lava flow. This lava flow exhibits the most beautiful gallery of ropy pahohoe
(we all went nuts taking photographs of cool ropy structures), with the odd
patch of aa lava in between (did you know that in Germany little kids that need
to go to the bathroom are asked by their moms “Do you need to go peepee or
aa?”). Gives a completely new meaning to the term aa lava!
Quite surprisingly the surface of the lava flow is
practically devoid of life. I have always claimed that life, in the form of odd
seeds and small reptiles, is quick to colonize new areas. I stand corrected, at
least in the case of a rocky, black surface in a dry climate. Not much has
gotten a foothold here in over a 100 years.
Our final dive was delightful, with sightings of two
turtles, isolated coral clusters, and my first sighting of small white mollusks
attached to the rocks, and some sort of yellow submarine lichen (?) that gave
the jumble of rocks the aspect of a gold field.
After diving I had a delightful conversation with Catherine
and her boyfriend Michael. Catherine, our bird watcher, was a professor of
ornithology at State University of New York (SUNY), until she got tired of the
pressure to do research and get grants, and now she works as researcher for the
Swiss Research Institute. Michael is the other geologist on board. He is a
petroleum geologist, specialized in field development and well completion. He
was there for the heyday of the British exploration and development of their
portion of the North Sea, and now seems to spend a lot of time consulting for
companies doing development in the Norwegian oil fields. His guess is that the
petroleum industry will not recover from the current slump (too much oil
available already, plus the bad boy image of the oil industry), and that
geologists would be well advised to specialize in something else. I like this
couple very much.
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