Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix. Volcanic rocks often have a vesicular texture caused by voids left by volatiles trapped in the molten lava.
A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usually andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava that piles up over a vent).ere are some of the most common types of eruptions: In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano.
The most violent type of volcanic eruption is a Plinian eruption.  This is also one of the largest types of eruptions. This was the  type of eruption of the famed Mount Vesu â¦ vius in 79 AD.0% of the world's active volcanoes are found occur along destructive boundaries. Aci â¦ d-lava and composite volcanoes also cause the most violent eruptions as the lava is viscous and the opening of the volcano is sometimes blocked by hardened lava, causing a huge massive explosion during the eruption.