A quick disclaimer.
I am being paid to make this video by the European Union and NS Kožare as part of a
series meant to share several wonderful geologic features within the nation of Slovakia.
What you are looking at is a specimen of Slovakian opal which contains multiple shades of brown.
This is no ordinary common opal, as its name of wood opal hints at its true origin.
In other words, this is a piece of wood that was buried more than 10 million years ago
and over a significant period of time was replaced with opal.
The original reason the source wood became buried was not insects, disease, or old age,
but rather a volcanic eruption.
As, approximately 14 million years ago, a powerful volcanic eruption occurred from the
Poľana volcano.
This sent pyroclastic flows in all directions around the mountain, which moved at speeds
exceeding 150 kilometers per hour.
Along the way, they flattened dozens of square kilometers of thick forest, causing trees
to burn and fall, which were then subsequently buried by a layer of thick ash.
The aforementioned eruption sequence occurred during the opening stages of Poľana's caldera
forming eruption, which caused this once glacier topped 3500 meter high stratovolcano to collapse
to a height of only 1500 meters.
Today, you can see the remnants of this 6 kilometer wide caldera which is breached to
the west, alongside the 190 square kilometer outline this volcano once held.
And, once Poľana eventually became extinct, it would only be 500,000 years before a separate
group of 5 stratovolcanoes near the border of Ukraine would erupt, forming what are known
as the Vihorlat Hills.
As, while Poľana erupted between 15 and 13 million years ago, the 5 volcanoes in the
Vihorlat Hills erupted between 12.5 and 9.5 million years ago.
This is the story of these two volcanic complexes, which erupted at approximately the same time
as several other stratovolcanoes and cinder cones during a time period when Slovakia was
far more geologically active than it is today.
Approximately 15 million years ago, a vast inland sea known as the Pannonian Sea existed,
which was approximately half the size of the modern Black Sea and covered swaths of Austria,
Slovakia, Hungary, Slovenia, Croatia, Serbia, Romania, Bosnia & Herzegovina, and Ukraine.
For a time while this sea existed, the same mantle source fed what would become two separate
volcanoes 50 kilometers apart, likely being connected by a magma resevoir in the lowermost
crust.
The southwestern volcano would eventually become the Stiavnica supervolcano, while the
northeastern volcano would become Poľana.
While Stiavnica was beginning to climb above the seafloor, Poľana was also erupting, but
did not produce any submarine eruptions as it started 50 meters above the level of the
adjacent sea and 25 kilometers inland from its edge.
This volcano produced numerous moderately sized vulcanian eruptions over hundreds of
thousands of years, depositing interwoven layers of ash alongside thick layers of blocky
andesite lava.
These andesite lava lobes were up to 50 meters thick, and over some eruptions that lasted
more than a year, travelled up to 11 kilometers distant.
By 14 million years ago, this gray topped stratovolcano had grown to 3500 meters or
11,483 feet above sea level.
Before the Stiavnica supervolcano produced its cataclysmic eruption, Poľana produced
an ultraplinian eruption that devastated the landscape in a 25 kilometer radius, causing
this volcano to collapse into a caldera.
During the final 1 million years of its lifespan, a few smaller eruptions produced andesite,
dacite, and rhyolite lava domes within its caldera.
Beginning 12.5 million years ago and 200 kilometers to the east, a new set of volcanoes began
to grow.
Unlike Poľana, these volcanoes all initially erupted through the shallow seafloor along
an ancient beach, creating large deposits of solidified volcanic ash known as tuff.
These layers of tuff reached up to 50 meters thick, forming the initial shallow platform
which the volcanoes would later grow from.
Fast forward 200,000 years, and five gray topped stratovolcanoes each measuring 10 to
15 kilometers wide had been constructed, largely from a combination of strombolian and less
common vulcanian eruptions.
If you were to walk around any of these volcanic remnants in the Vihorlat Hills today, you
would find numerous pillars and boulders of layered gray andesitic lava flows.
While most of these horizontal fractures formed long after the lava cooled, they can help
show us which direction the lava was flowing, which as an example moved from left to right
in this image.
The edges of these lava flows contained piles of andesitic rubble, as the advancing lava
flows had sides exceeding 45 degree slopes.
While the Vihorlat volcano would go on to become the tallest volcanic cone of the group,
reaching 2500 meters in height, it and the three closest volcanic cones produced fairly
low gas content in their eruptions.
The 5th volcano which primarily exists in Ukraine was different, producing far more
powerful explosive eruptions.
This volcano, referred to as Vihorlat-Popricny would eventually climb to a height of 2000
meters, being covered by numerous domes of both dacite and rhyolite lava.
Around 10 million years ago, this volcano ceased erupting, but not before it partially
collapsed into a 3.5 kilometer wide caldera.
Today, all of these aforementioned volcanoes are long extinct and will never erupt again,
but provide for some beautiful scenery and interesting rock formations.