I’d like to say that on this day in history (my birthday) Darwin did something particularly cool. But alas, on March 9th he spent the day wracked with pain. So, I thought I’d use the day to comment on his geological observations in Bahia.
I continue to be amazed and impressed when I read Darwin’s geologic notes. Its been 180 years, yet I can still easily understand the geologic story they tell – whether it be mantle rocks, beach terraces, lava flows, or continental crust. I think this is a tribute the accuracy and detail that he puts into the descriptions. Granted the language of basic field geology has not changed (a granite is still a granite), but it is more than that. From his descriptions of the rocks and their relationship to one another, I can usually decipher the geologic big picture. And although, the concept of plate tectonics was well over 100 years away, I can insert his descriptions into the modern theory and have it all make sense.
This is one of the beautiful things about science (when it is done right). As an former professor once said, “good data are immortal” – Darwin’s observations are just as good today as they were 180 years ago. Interpretations may change as we gather more data, but observations simply accumulate – adding to the overall evidence. To put it another way, the facts remain, but the interpretations evolve. So Darwin’s description of the unusual serpentines 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 hot spots. So how do Darwin’s notes from Bahia speak to the modern geologist?
Within a few weeks of landing in Bahia, Darwin’s notes clearly reflect that he is standing on a continental craton, made up of what are almost surely very old rocks. The term craton in geology refers to the “tectonically stable part of a continent”. This is in contrast to the “active margins” of a continent where the forces of plate tectonics result in volcanic eruptions, earthquakes and the massive deformation of rock that forms mountain ranges. In North America, the west coast is an active margin – think the Cascade Mountains in Washington or the San Andreas Fault system in California, as well as the Rockies further inland. Although they are not being deformed in the same way today, the East Coast of North America has been active recently enough (in the past few hundred million years) that the rocks there are also quite deformed – think the Appalachians or Taconic mountains. But if you look at the rocks in the middle of the continent you find very old igneous and metamorphic rocks (granites and gneisses in particular) covered in massive flat layers of sedimentary rocks (deposited when higher sea level covered the continent). These rocks have been stable for hundreds of millions of years – sometimes 2 or 3 billion years. This is the craton of North America. (To complete the lesson: the craton can actually be broken into two parts – places where bare naked igneous and metamorphic rocks are exposed are called the continental shield, while areas that are covered with massive flat layers of sedimentary rock are called the continental platform. The shield and platform together make up the craton.)
Cratons of the world (the red areas are continental shields, and the orange are continental platforms):
- “There is some fine-grained syenitic granite” – Granitic rocks (granites and rocks with similar composition) are the foundation of the continents. Syenitic granite (or syenite) refers to very felsic (silica-rich) rocks that do not contain a much (if any) of the mineral quartz. A large percentage of the rock tends to be made of the felsic mineral orthoclase. (The name derives from the Egyptian city of Syene (now Aswan)).
Syenite (from Andy Tindle’s Open University page):
- “The prevailing rock is gneiss, often passing, by the disappearance of the quartz and mica, and by the feldspar losing its red colour, into a brilliantly grey primitive greenstone.” – Gneiss, granite and greenstone are an assemblage cratonic rocks often associated with what are called “greenstone belts”. These regions are often some of the oldest sections of continental crust. To put it as simply as possible, greenstone belts represent some of the earliest “mini” continents made up of small islands and ocean crust that have been squeezed together and metamorphosed (the islands becoming gneiss and the ocean crust becoming greenstone). They represent a time when the Earth was much hotter and plate tectonics operated at a much faster pace.
- “The gneiss is traversed by numerous dikes composed of black, finely crystallized, hornblendic rock, containing a little glassy feldspar and sometimes mica, and varying in thickness from mere threads to ten feet. … Hence the gneiss has certainly been softened, its composition modified, and its folia arranged, subsequently to the breaking up of the dikes, these latter also having been at the same time bent and softened.” – I’m not too sure what to make of the dikes Darwin describes here, though it is quite common in highly metamorphisized rocks to see intrusions such as these. Dikes are linear (or tabular in 3-D) features formed by the injection of magma that cut across preexisting rock layers. Since gneiss forms under high pressure and temperature, it is not unusal to find molten rock nearby as well – hence the dikes. Darwin’s description nicely illustrates the relative timing of the geologic events: (1) The continental rocks form (the material that would become gneiss), (2) it is intruded by dikes, and (3) the whole thing is caught up under high pressure and metamorphosized.
- “The folia of the gneiss within a few miles round Bahia generally strike irregularly, and are often curvilinear, dipping in all directions at various angles: but where best defined, they extended most frequently in a N.E. by N. (or east 50° N.) and S.W. by S. line, corresponding nearly with the coast-line northwards of the bay.” – Darwin used his clinometer to measure the orientation of the “layers” in the metamorphic rock . “Folia” form perpendicular to the direction that pressure is applied to the rocks. In this case, a SW-NE orientation of the foliation in the rock, suggests that they were “squeezed” from NW-SE (perpendicular to the coastline). This alignment may also explain the orientation of features along the coastline, as folia form weaker planes in the rock along which erosion can more readily occur. If this orientation is correct, I would strongly suspect it explains the SW-NE orientation of the Bay of All Saints and the peninsula on which Salvador is located.
Up until now, the rocks Darwin observed were affiliated with the ocean crust of the Earth (and the mantle below it). In Brazil, Darwin got his first taste of continental rocks. Not only were these rocks quite different in composition (they are 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 Atlantic where probably not more than 10 million years (give or take a few million years). The rocks of the Brazilian Craton could easily have been anywhere from 1 to 3 billion years old. This was the skin of the Earth’s “middle ages” – a time before life had eyes and legs.
Little did Darwin know that these cratonic rocks were exactly the same cratonic rocks found in west Africa – split apart by the opening of the Atlantic Ocean. But that is another story for a later post…(RJV)