The nation of Norway contains many beautiful geologic features including glacially carved
fjords and steep rock cliffs, of which many of the relevant rocks they contain are more
than 1 billion years in age.
Yet, despite the wide range of features in the country, something is seemingly missing
on the mainland; volcanoes.
However, the country does indeed contain two active volcanoes, which are both associated
with underlying mantle hotspots much like the Hawaiian Island chain and have erupted
at some point in the last 10,000 years.
Instead of being located on the mainland, they are far from it on two different and
distant islands.
The first and more famous volcano is the long and thin island volcano Beerenberg in Jan
Mayen, which although glacier covered has been surprisingly active, having last erupted
in 1985.
There is also a second lesser known volcano which strangely enough is part of Norway despite
being located in the southern hemisphere.
There, 2,570 kilometers south-southwest of the city of Cape Town is another island territory
of Norway, Bouvet island.
Despite being a long distance away from Norway and several other countries claiming the island
first, it was eventually decided to be a dependency of Norway.
Despite more than 90% of this island which has an area of 49 square kilometers being
covered by thick glaciers which obscure any chance at seeing significant outcrops of bedrock,
do not let this fool you as the island has produced a geologically recent volcanic eruption.
In 25 BCE, this volcano produced an eruption which sent lava flows that descended to the
eastern edge of the island.
Eventually interacting with the ocean water, it created a series of littoral explosions
due to the water quickly flashing to steam, which along with creating small volumes of
ash also created a small lava delta.
This delta, now at the site of Cape Meteor contains a black sand beach created from finely
pulverized volcanic rock.
Geologically speaking, Bouvet Island is located in close proximity to a mid ocean ridge where
the Antarctic and African plates are spreading apart.
It would thus seem a bit strange that an island is located here until you learn that there
is an underlying low intensity mantle hotspot which in recent geologic history perhaps the
last 20 million years popped up, thus sending large volumes of molten rock into the crust
which occasionally erupted onto the surface.
In this case, the divergent plate boundary and related stretched areas represented a
path of least resistance for this to occur.
The initial eruptions of what would become Bouvet island involved lava erupting effusively
onto the seafloor at a depth of approximately 1 kilometer.
After 500,000 years and perhaps more of infrequent eruptions, this volcano reached the ocean
surface, triggering an explosive phase of surtseyan style eruptions which left behind
a uniform layer of ash across much of the island.
Afterwards, basaltic eruptions continued, and due to the low viscosity these lavas had,
they travelled quite quickly for lavas, building a gentle slope.
Thus, in a short period of time, this volcano had become a recognizable shield volcano.
However, eventually this island reached a size and height which allowed for glaciers
to easily grow, changing the nature of its eruptions.
For the next 450,000 years, subglacial eruptions occurred which were a bit more explosive but
whose products deposited large volumes of a rock type known as hyaloclastite.
During this period, the nature of the volcano's magma changed as it slowly became more and
more silica rich until a section of it began erupting highly explosive trachyte then rhyolite
magma.
This caused a large amount of pressure to build which culminated in a highly explosive
eruption at some point most likely in the last 50,000 years.
As a sustained ash column incinerated the ice above it, large volumes of ash were ejected
and pyroclastic flows raced across the island and entered the Southern Ocean.
Then, due to the large amount of erupted material, a large swath of ground collapsed downwards
like a sinkhole to fill the empty space in the underlying magma chamber, forming a massive
caldera.
This caldera was soonafter the site of a lava dome forming eruption via viscous high silica
lava.
Then, a pause in eruptive activity occurred, and the rest of the volcano including the
caldera was re-covered in glacial ice.
Today, this caldera today measures 3.9 kilometers or 2.4 miles wide.
A future eruption from this volcano could disrupt regional air traffic if an explosive
component occurs during such an eruption.
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