Geology

Technically speaking Isla Socorro is one of the Revillagigedo Islands. However, it is the largest of the four islands that make up this island chain, the others consisting of San Benedicto, Roca Partida, and Clarion (Figure 1). But since ‘Revillagigedo’ is almost unpronounceable in English, most people merely refer to the island chain as the Socorro Islands.

The islands are a popular dive location due to their location as an isolated seamount in the open ocean, thus resulting in it being used by migratory and transient ocean-dwelling creatures. The principal animals passing through this chain are giant mantas, hammerhead sharks, whale sharks, and humpback whales. Resident large animals include silky sharks, Galapagos sharks, whitetip reef sharks, silvertip sharks, and bottlenose dolphins. The primary dive attraction is the giant mantas that cycle through to visit cleaning stations populated by abundant schools of jacks and other fish species. Supposedly the mantas are accustomed to divers and swim overhead so that diver bubbles tickle their undersides, which mimics the action of cleaner fish. All of this provides some pretty exciting dive adventures.

Some of the more inquisitive among us might ask why these remote islands are out in the middle of the deep ocean in the first place. Well, I’m glad you asked that question. Here, in installment 2 of my Dive Site Geology Series, I will explain how and why these islands are here. You can see the first Dive Site Geology article here.

The Socorro/Revillagigedo Hotspot

In a nutshell, the Socorro Islands are a hotspot within a relict mid-ocean spreading ridge. Now, let’s deconstruct that statement. Hotspots are a common feature in the deep ocean and even on land occasionally (Figure 2). They result from a plume of magma rising through the mantle, piercing the crust, and erupting on the earth’s surface (Wikipedia, 2024a). The most common example that Americans are familiar with is the Hawaiian Island chain (Figure 2). Chains such as the Hawaiian Islands do not look stationary because pieces of the earth’s crust are continually moving. This is known as plate tectonics. So as a particular plate moves past a hotspot, a chain of volcanoes will trace the direction and speed of movement of that crustal piece. In our particular case, notice how Clarion Island and Roca Partida are off to the west of San Benedicto and Socorro Islands. This traces the movement of this section of the earth’s crust.

As expected, Clarion Island and Roca Partida do not have recorded eruptions and are in the process of eroding and slowly sinking into the sea from the weight of rock on top of a relatively thin oceanic crust. This is a common occurrence that is well documented in the abundant volcanic island atolls of the southwest Pacific. Roca Partida in particular doesn’t have a very long life ahead of it since it currently only consists of two spires above the surface of the ocean (Figure 3).

The same cannot be said of San Benedicto and Socorro Islands. Mount Evermann on Socorro Island last erupted in 1993 with prior eruptions in 1951, 1905, 1896, and 1848. Vulcanologists think they can trace eruptions back at least to 3090 BC (+/- 500 years) (Wikipedia, 2024b). San Benedicto has two large craters, the southern of which is currently active. Its most recent eruption was in 1952 and was so large that it wiped out all life on the island. It has since been repopulated naturally by species from either the mainland or other islands, that is except for the San Benedicto rock wren which became extinct (there are other species of rock wren on other islands of this chain) (Wikipedia, 2024c).

The East Pacific Rise

The East Pacific Rise (EPR) is a divergent plate tectonic boundary, which means that new ocean crust is being created along its entire length and the crustal plates on either side are moving away (Wikipedia, 2024d) (Figure 4). In the Mexican portion of the EPR, the plate to the west is the Pacific Plate and to the east is the Cocos Plate. The Cocos Plate in turn is being subducted beneath the North American Plate in a feature known as the Middle America Trench (this is known as a ‘convergent’ plate tectonic boundary) due to the fact that the North American Plate is moving westward and will eventually completely consume the Cocos Plate.

Currently the EPR is located some distance to the east of the Socorro Islands (Figure 1). This is because 3.5 million years ago the EPR shifted to its current location, having previously been located directly underneath the Socorro Islands. Today this relict feature is known simply as Mathematicians Ridge. Lateral translation (or jumping) of spreading centers sometimes does occur due to reasons that are not well understood but are likely related to the fact that the oceanic crust is thin and weak in the vicinity of these spreading centers and if the mantle plumes feeding them shift slightly then the spreading center will also shift.

Geology (and dive experience) of the Socorro Islands

Let’s take a tour of what sights a diver might see when visiting these islands on a dive trip. Since the boats all come from Cabo San Lucas, the first island you reach will be San Benedicto. It is common to visit a popular site on the south end of the island called El Canyon (Figure 5). From the sheltered bay there you get a great view of the active volcano called Volcan Barcena (Figure 6). This barren landscape reminds one of being on the moon or some other similarly desolate place. The dives here are on volcanic rubble bottoms that have lots of fish but the primary attraction are the schools of sharks that make VERY close passes to divers (Figure 7) because we are associated with possible food handouts (not because we ARE food).

The Boiler

You might then be treated to a dive site on the other side of the island called The Boiler. Here is where you get your first chance to see giant mantas performing acrobatics above you. The pictures from here often have steep volcanic cliffs as a compositional element, which make for some very dramatic images (Figure 8). I’ve seen some company advertising literature framed on these volcanic walls, and for good reason (Figure 9)

Roco Partida

Next is Roca Partida. The time I went we spent two days diving on the same ‘rock’. That gives you an impression of what we thought of this isolated seamount in the middle of the Pacific. The underwater environment is very similar to The Boiler in that you swim around the seamount waiting for the mantas to show up. While you’re waiting there is lots of fish and sharks to photograph. Here you use the same compositional technique as you did previously by including rock outcrops on the seamount whenever possible (Figure 10). An additional world-famous ‘must-see’ picture from here is of resting white-tip reef sharks (Figure 11) which have the unique ability to lie on the bottom while sleeping, unlike most sharks that cannot breathe unless they are moving.

Socorro

Eventually, you make it to the southernmost island of Socorro, the largest island in the chain. Here the geology is very interesting because you see layered igneous stratigraphy wherever you look (Figure 12). There are two special things you might experience here. First is a night snorkel behind the boat with a whole herd of silky sharks just beneath the surface (they hunt flying fish, so the trick here is not to act like one so they don’t take a bite out of you). The second is a Zodiac ride to the rugged coastline where you go on a tour of the dramatic landscape up close and personal, which as you can imagine is like a kid in a candy store for a geologist (Figure 13).

This all adds up to an experience not to be missed in this lifetime.