One of the things I enjoy doing is combining my appreciation of the earth with diving. Most of the time the two things are in close juxtaposition as dive sites are often close to shore. And any time a rock exposure presents itself adjacent to the shoreline there is an opportunity to do a bit of geology before jumping into the water for a dive. Most often it’s best to be in a dive skiff which is able to get up close and personal with the rock exposure. That was the case in a recent dive trip I took with Nautilus.
Recently, I was onboard the Gallant Lady, a 116′ dive yacht. The exciting ‘Orcas and Mobulas’ trip traverses the southern Sea of Cortez in the vicinity of La Paz, visiting Cerralvo Island to the east of La Paz, Espiritu Santo Island which is close and to the north of La Paz and San Francisco and San Jose Islands further north. The format for this excursion is for the boat to cruise around between the islands and the mainland looking for whales, orcas, and schools of mobula rays. And of course making visits to each of the sea lion colonies in this portion of Baja California, which some claim to be the highlight of any trip to these parts because sea lions enjoy the presence of divers and will goof around and pose for the enjoyment of their guests.
This portion of Baja California is very arid with little rainfall, meaning that rock exposures are plentiful and commonly in the form of sheer cliffs rising out of the sea. Most of the area consists of intrusive basalts (meaning they did not erupt on the surface) that tend to be dark gray or brownish and massive (meaning they lack detail). Basalts in general are oceanic in origin because they originate in the mantle (below the continents). We can tell this because they lack silica (such as quartz which is silica dioxide) which only occurs within continental landmasses.
There were also some extrusive (meaning they erupted) ash and lava flows. This was clearly seen on the northern portion of Espirito Santo Island when we visited a sea lion colony called La Reina. Here the lowest unit (that we could see at sea level) was a light-colored ash that was covered (unconformably) by a thinnish dark brown lava flow followed by a reddish-brown lava flow, then a soil zone (seen as a thinnish light red and white layer) which was in turn covered by a massive grayish lava flow.
The only sedimentary rocks were seen on the mainland coast to the east of La Paz (across from Cerralvo Island) where recent sandstones have been uplifted from the sea due to the tectonic movement of Baja California to the northwest, angling obliquely away from the Mexico mainland. These sandstones show clear layering indicating how they were deposited. The layers sometimes change dip and truncate one another which reveal changes in flow direction. Units like this can be deposited by streams and rivers (called ‘fluvial deposits’) but these sandstones were deposited offshore when sediments carried by these rivers flowed into the sea and were dispersed.
These are just some of the examples of geologic formations found on our recent adventure. But, they are good examples of formations that surround the places we dive. I would encourage you to look beyond the mammals, fish and corals and observe the structures around us when we head out diving. There is a lot more there.
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