If you’ve ever read about volcanoes, you may have heard the term “Plinian eruption”. Named after the famous historian Pliny the younger of ancient Rome, he originally described it as looking like a certain type of pine tree: with a tall straight trunk and a circle of branches at the top…a more modern description might be “mushroom shaped” cloud, towering tens of thousands of feet into the air.
It is the column of ash and fire produced during an explosive volcanic eruption, and he was describing what he saw in 79 AD when Mt. Vesuvius erupted, killing thousands of people in Pompeii and Herculaneum , Italy. In a series of explosions and surges of pyroclastic flows, the clouds of ash and superheated material towered tens of thousands of feet into the air then collapsed, racing down the side of the mountain and burying villages as it sped to the sea.
In fact, Mt. Vesuvius is really only a small cone, growing inside the much larger caldera of what is currently known as Mt. Somma. The first evidence of volcanic activity in this area dates to 400,000 years ago, and is the result of the motion of the African tectonic plate being forced under the Eurasian tectonic plate. 25,000 years ago, the top of Mount Somma collapsed after what is known as the Codolan Ultra-Plinian event, forming a caldera, and scientists believe that Vesuvius started to grow inside the caldera almost immediately.
Ultra Plinian? On the Volcanic Explosivity Index, Plinian eruptions have a VEI of 4,5 or 6. Ultra Plinian events have a VEI of 6,7 or 8. (The VEI is similar to the Richter scale for earthquakes. An increase of one number means the event is 10 times more explosive than the preceding number…so, 1=100; 2=1,000; 3=10,000; 4=100,000; 5=1,000,000 more explosive, and so on.)
8 is the highest number on the scale, and is described as “mega-colossal”. Scientists say there may have been 40 eruptions of VEI magnitude 6 or higher in the last 132 million years (Source Wikipedia search Volcanic Explosivity index). Mt. Vesuvius, during the eruption that destroyed Pompeii, earned a VEI 5. (Mt St Helens in 1980 barely made a 5, but was a very different sort of eruption, with a lateral blast instead of a vertical one and very little magma). How big was the Mt Somma eruption? Scientists are still looking for the answer because deposits of ash and volcanic debris are still being found hundreds of kilometers away.
Not all Vesuvius eruptions are explosive…sometimes magma rises and flows through the inside of the volcano (called the conduit), escaping from fissures in its sides to deposit in sheets or waves on the outside of the cone. Over time the magma cools and becomes different types of rock.
Most rocks around Vesuvius are andesite, which contains about 50% silica. It also contains magnesium and iron. Also present is the mineral Olivine, one of the most abundant minerals in the earth’s mantle. As the magma cools, crystals of Olivine are formed.
If the crystals are nicely formed and don’t include bits and pieces of other rocks, it can be called Peridot…you may even have a pair of Peridot earrings or a Peridot necklace, and now you know how it came to be.
In this slide, we have a thin section of Phonolite from Mt. Vesuvius. It has feldspar crystals, some aluminum, potassium, calcium deposits. I haven’t figured out what the dark crystals are yet, I thought it was some sort of Obsidian, but there’s not enough silica… Oh, and the beautiful Olivine crystals, shining their perfect green color in small deposits throughout the slide.