The Forgotten Volcanoes in Slovakia; Vihorlat & Poľana

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.