On the island of Santiago in the nation of Cape Verde is a strange group of rocks.
On this island are a series of large boulders, some of which are more than 6 meters or 20
feet wide that are weirdly emplaced on top of a prominent plateau on the northern tip
of the island.
These boulders are found at more than 200 meters or 656 feet above sea level and are
out of place as the plateau they can be found on is a local high point.
However, analysis revealed that these mysterious boulders formed during submarine volcanic
activity.
Yet, the island has not been uplifted sufficiently enough to explain their origin, and no nearby
submarine vents are known.
So, how did a part of the Fogo volcano end up on a high plateau on Santiago?
The answer is one of the largest geologically recent volcanic landslides on the planet that
began45 miles away, which carried these boulders and emplaced them there via a towering megatsunami,
which had a maximum runup height of 270 meters or 886 feet.
Around the planet, large scale landslides like the one I just described are surprisingly
common at volcanic islands.
At both shield volcanoes and stratovolcanoes you can find many remnants of these slides
via large collapse scars, which leave behind cliffs and a sort of figurative bite out of
the landscape.
In a manner of speaking, these events seem to be part of the typical life cycle of some
volcanic islands.
For example, the island of Oahu used to be much larger, but experienced a catastrophic
landslide or group of landslides in the distant past, transporting blocks of material up to
143 miles to the northeast.
This is why the northern section of the island has a series of steep cliffs; it represents
a landslide scarp.
As a result, many island based volcanoes have an incredibly low albeit ever present threat
of generating a massive landslide and tsunami, such as at the Hilina Slump of the Kilauea
volcano.
One such massive landslide occurred 73,300 years ago at the Fogo volcano.
For context, the islands in Cape Verde exist due west of both Mauritania and Senegal in
Africa within the Atlantic Ocean.
The Cape Verde islands which the Fogo volcano is a part of exists due to an underlying hotspot
which in the case of a linked paper is referred to as a thermal anomaly in the mantle.
The Fogo volcano has been the most recently active volcano in this chain with highly frequent
eruptions since 1500, including one eruption that may have even been 261 years in length.
This volcano today stands at more than 9,281 feet above sea level, but used to be even
taller.
The today has an 8 kilometer wide collapse scar which marks the site of where the aforementioned
major landslide occurred.
Although the true height of this volcano pre-collapse is unknown, topography suggests that it may
have reached a height of around 3,580 meters or 11,745 feet in height.
While what made this volcano catastrophically collapse is unknown, recent papers around
the world offer a number of possible explanations.
These include cryptodome growth, large phreatic eruptions, and a volcanic cone being pushed
horizontally by a taller cone.
However, my personal favorite explanation for large scale volcanic collapses are the
fact that dike intrusions seem to run parallel to the direction of collapse.
This suggests that ground deformation due to the influx of magma is strongest in a single
axis, which could then trigger a flank collapse.
Regardless of the cause, when the landslide began, what likely started as a moderate size
event within a few dozen seconds propagated to include the entire eastern side of the
volcano.
As between 130 and 160 cubic kilometers of material slid at a speed of several hundred
miles per hour, it may have even triggered a volcanic eruption.
Hitting the water and travelling up to 25 kilometers along the seafloor, the slide displaced
large volumes of water, generating a towering megatsunami.
This tsunami also pushed upwards and carved away at the seafloor, pushing up large boulders
that would be carried by the generated tsunami.
Upon reaching the northern tip of the island, it had a maximum runup of 270 meters, towering
dozens of meters over much of the northern plateau.
Thanks for watching!
If you would like to request a specific topic, please leave a comment below.
Additionally, I would like to thank my new patron Alex for supporting this channel.