The Spanish Canary Islands have numerous active volcanoes, with 7 of the 8 islands having
volcanoes which have or are suspected to have erupted at some point in the last 11,650 years,
with only La Gomera not having an active volcano. This includes Cumbre Vieja on
the island of La Palma which erupted in 2021, Mount Teide on Tenerife which erupted in 1909,
and Lanzarote which produced a lava flow between 1730 and 1736 which covered a little more than 26%
of the island, totalling 225 square kilometers. Yet, the Canary Islands are not the only location
in Spain to contain active volcanoes. This is best shown by the Calatrava volcanic field which
produced its last eruption around 5,000 years ago and has a grouping of explosion craters
termed maars, or the Olot volcanic field east northeast of Barcelona which contains groups of
cinder cones and last erupted 8,000 years ago. Yet, there is also another point in Spain which
contains geologically recent volcanism, still having reported active hydrothermal vents which
emit volcanic gasses from the sea floor. I am not referring to some hidden submarine
volcano but rather a grouping of 20 islands which are in a location frequented by scuba
divers. What I am referring to are the Columbretes Islands which from satellite
you can easily seen a circular ring shaped tuff ring that includes an ancient explosion
crater which is 460 meters or 1509 feet wide. The Columbretes Islands are located off the
eastern coast of Spain in the Mediterranean Sea where the largest island, Illa Grossa,
is approximatelt 101 kilometers east northeast of the city of Valencia. These 20 islands are overall
quite small with the largest island having an area of around 0.15 square kilometers or 37 acres.
Altogether, the Columbretes Islands formed in a brief geologic timespan between 1.1 million and
200,000 years ago, with four main eruptive centers forming. Each eruptive center formed what was once
a larger island that has since mostly eroded away along with one or more satellite eruptive vents.
Because of this, the Columbretes Islands could be thought of as a form of extinct volcanic field.
Yet, you might ask how can this volcanic field be extinct if it still has a reported
active hydrothermal vent? The answer is that, surprisingly, molten rock is somewhat shallow
underneath the region, perhaps as little as 5 kilometers depth. Yet, this is not pooled
up as a magma chamber as it rather represents a fairly large rift zone where the crust happens to
be unusually thin, meaning that the shallow heat source is in fact a section of the Earth's mantle.
This feature is known as the Valencia Trough and is part of an ancient rift zone that formed around
24 million years ago when continental Europe looked somewhat different. The rift zone that
would eventually form and be active for perhaps as many as 10 million years expanded while it was
active to the north-northwest and south-southeast. It is now believed that a section of a tectonic
plate that underlied the region millions of years ago dramatically suddenly began to descend at a
steeper angle, causing it to be pushed further and further to the east in a process known as
slab rollback. This caused molten rock in the mantle to rush upwards to fill the empty space,
sometimes intruding into the crust above it and erupting onto the surface in what
could be described as a back arc setting. After no rifting occurred for 13 million years,
the Valencia Trough seemingly reactivated albeit temporarily around 1.1 million years ago. This
caused low silica basanite magma to intrude into the crust before erupting explosively onto the sea
floor. Eruptions were highly explosive, producing significant amounts of ash as the lava at a depth
of between 50 and 100 meters interacted with the overlying ocean water, which flashed to steam and
created powerful explosions. Several ash plateaus were created, and sometimes lava erupted on top
of these, allowing the islands to remain after the eruption ended. This even created a 1500 foot wide
explosion crater, which is one of two craters in the tuff ring island of Illa Grossa. After 400,000
years of intermittent activity, the volcanic field became dormant for 300,000 years. During
this time, magma likely remained at depth, and leached material form the surrounding crust,
becoming more silica rich. Thus, when it erupted again between 400,000 and 200,000 years ago,
it produced a more explosive and viscous type of lava known as phonolite. Luckily,
the rifting at the Valencia Trough in my opinion is now inactive and thus the
Columbretes Islands should never erupt again. Thanks for watching. If you would like to
request a specific topic, please leave a comment below! Additionally, I would like
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