The recent and destructive eruption of La Palma in the Canary islands has likely made
a group of residents on another similar island chain a bit uneasy.
The Portuguese Azores are also completely volcanic in origin, except unlike La Palma
its eruptions are almost always highly explosive.
This fact is best displayed by several large circular depressions in the ground which are
known as calderas.
These calderas formed when after a large volcanic eruption, a massive swath of ground collapsed
downwards like a sinkhole.
In the Azores, the most remarkable caldera can be found on the western end of the island
of Sao Miguel.
There, you can find the impressive caldera of Sete Cidades which is 5 kilometer wide
and up to 500 meter deep.
Due to the numerous secondary volcanic vents and crater lakes within the caldera, it is
truly a sight to behold.
However, volcanic eruptions do not always occur within this caldera.
For example, two eruptions in both 1110 and 1713 formed a cinder cone and small lava delta
on the western shoreline of the island.
And, even more recent eruptions have occurred completely offshore of the island.
The reason why the Sete Cidades volcano exists is surprisingly still up for debate, but has
to do with one or two factors.
To the west of the island of Sao Miguel is the intersection of 3 tectonic plates.
Here, the African Plate, North American Plate, and Eurasian Plate are spreading apart via
a mid ocean ridge in what is known as a triple junction.
This interaction has created a series of large fault lines and ridges which go outwards from
these plate boundaries in a perpendicular direction.
One of these ridges goes through the island of Sao Miguel, where magma intrudes into the
crust through a fissure, eventually erupting via a path of least resistance.
Some scientists believe that the rate of volcanic eruptions in the Azores cannot be solely attributed
to the triple junction and think that an underlying hotspot is also involved.
However, it is unclear if a hotspot truly exists here.
The Sete Cidades volcano began forming around 250,000 years ago, when a volume of trachyte
composition magma intruded into the crust.
This material eventually erupted in an explosive manner, producing a chain of explosion craters
which trended in a southeast direction.
After several additional eruptions, one of these cones became dominant and started to
siphon magma which otherwise would have gone to more distant vents.
This began building a broad stratovolcano out of overlapping layers of lava and ash.
Sete Cidades flanks largely had surprisingly gentle slopes, owing to the fact that much
of its edge was constructed by highly fluid basalt lava flows.
By 36,000 years ago, the center of this scenic stratovolcano stood at around 1,300 meters
in height.
However, a large magma chamber had built up underneath the volcano, and this eventually
led to a highly explosive eruption.
This eruption came 35,700 years ago, causing a continuous plume of ash to shoot 30 kilometers
into the atmosphere.
Part of this eruption column then collapsed downwards, creating pyroclastic flows which
travelled up to 15 kilometers distant.
Due to the large amount of ejected material, totaling on the order of several cubic kilometers
of tephra, a large section of the underlying magma chamber had been drained.
This caused a significant quantity of overlying rock to collapse downwards, forming a several
kilometer wide caldera.
This caldera would grow to its modern size through two subsequent similar magnitude eruptions
28,750 and then 15,740 years ago.
Afterwards, Sete Cidades ceased erupting for several thousand years.
A new eruptive period commenced around 2050 BC, where unusually abundant volumes of pumice
ejected in an explosive eruption created a 160 meter tall and 1600 meter wide pumice
cone.
Pumice cones may look like cinder cones, but tend to have slightly more gentle slopes and
form through eruptions which are 1 to 2 orders of magnitude more explosive.
Over the next several thousand years, several sub-plinian and plinian eruptions occurred
within the caldera, forming more of these cones which would later be called caldeiras.
Some recent eruptions also occurred to the southeast, where magma interacting with the
area's abundant groundwater generated a series of explosion craters in the ground known as
maars.
While the last eruption on the island occurred in 1713, this volcano has produced 6 other
recent eruptions offshore of Sao Miguel.
Two of these formed temporary volcanic islands which soon washed away.
The last such eruption occurred in 1880.
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