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.
1 Pressure builds and magma explodes to the surface causing a volcanic eruption. 2  The lava from the eruption cools to form new crust. 3  After several eruptions, the rock builds up to form a volcano.4  The magma chamber is a collection of magma below the volcano.ey points. 1  Volcanic eruptions can happen at destructive and c onstructive boundaries. 2  Volcanoes form when magma rises through cracks or weaknesses in the Earth's crust. 3  Pressure builds and magma explodes to the surface causing a volcanic eruption. 4  The lava from the eruption cools to form new crust.