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.
Volcanic eruptions are caused when magma from the magma chamber of the Earth rises up the pipe and out of the vent.Magma is continuously supplied from the mantle of the Earth. Shield volcanoes usually have a gentle eruption, as it contains basic lava.nother type of eruption in what is known as a boiler volcano results from water that gets trapped inside porous rock, and is then heated by the mantle, until is turns into steam and the steam pressure causes the volcano to explode. These explosions can be extremely large.