What you are looking at is the gray and heavily eroded and scarred landscape of one of the
most active volcanoes in Indonesia; Mount Bromo.
This volcano is almost always venting volcanic gasses off of the stratocone which the volcanic
complex is named after, rising into the sky.
On a good day, you can even see two volcanoes at the same time; Bromo in the foreground
and Semeru in the background.
And, to make the Bromo volcano more interesting, it is emplaced in a massive caldera filled
with gray ash and tephra that formed a vast area of sand sized particles, thus giving
the caldera its name, the Sand Sea Caldera.
At its widest point, the Sand Sea Caldera is 10 kilometer wide, with its caldera walls
towering up to 610 meters or 2,000 feet above the caldera floor.
This caldera and the adjacent 9.2 kilometer wide Ngadisari Caldera likely formed in powerful
volcanic eruptions which were each probably more than a dozen times larger than the 1980
eruption of Mount Saint Helens, reaching a volcanic explosivity index of 6.
And, as seen by the high points on the caldera walls on edges of the two calderas, there
used to be a much taller volcano here.
There, the ancient and now destroyed Tengger volcano achieved a height of around 4,572
meters or 15,000 feet, meaning it was tall enough to on rare occasions even receive minor
snowfall.
The Bromo volcano is located within Indonesia on the eastern section of the island of Java
where it is 35 kilometers east of the city of Malang.
Like other volcanoes on the island of Java, which has a higher population than the entirety
of modern Russia, Bromo exists due to an offshore tectonic plate collision.
There, the Indo Australian Plate is subducting underneath the Eurasian Plate.
The melted material from this collision then migrates upwards, often intruding into the
crust and occasionally erupting onto the surface, forming a chain of volcanoes.
The Bromo volcano began erupting approximately 800,000 years ago when a volume of basaltic
andesite lava erupted onto the surface.
Ash emissions were moderate during this period, and numerous vulcanian explosions alongside
flows of aa lava advanced in several directions.
Over time, this built a stratocone which continued to get larger and larger until it reached
a height of 2,000 meters.
At this point, eruptive activity shifted to being more explosive, creating heavier ash
emissions and beginning to also erupt some basalt and andesite.
After 648,000 years of erupting, the Tengger volcano reached an impressive height of nearly
4,600 meters.
However, by this time a large magma chamber had grown significantly and became primed
for a large eruption.
Such a large explosive eruption eventually occurred, scorching 100s of square kilometers
in pyroclastic flows and causing the eastern two thirds of the volcano to lose 2800 meters
in height, forming the Ngadisari caldera.
This caldera was then heavily eroded by water, causing it to be breached to the east and
carving the terrain at the caldera floor.
The remainder of this once tall volcano then collapsed in another powerful eruption in
around 36,299 BCE which likely ejected 20 cubic kilometers of tephra.
Within the newly formed caldera, numerous sand sized particles defined as being between
1/16th of a millimeter and 2 millimeters in diameter mostly stayed within the caldera
walls where they eroded into large groupings of material.
Since then, smaller but somewhat frequent eruptions have built 5 medium sized stratocones
which from oldest to youngest are Segarawedi Kidul, Mount Kursi, Mount Batok, Segarawedi
Lor, and Mount Bromo.
It is quite likely that the 230 meter or 750 foot tall Mount Bromo cone only began forming
around 6,500 years ago.
Since 1800, Bromo has been the only vent to be historically active, producing frequent
eruptions which have occurred at a rate of about once every 5 years on average in the
last 200 years.
These eruptions are often phreatic or strombolian in nature, with some eruptions having a vulcanian
eruption style.
Recent ruptions as a whole from Bromo are generally moderate in size, rarely creating
an ash plume which reaches more than 5 kilometers above its crater rim.
As of the writing of this video on April 13th 2023, Bromo is at a yellow alert level with
the last confirmed eruption to have occurred from this volcano occurring in late 2020.
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