why do volcanoes blow their tops ?
Answers
Answer:
Earth's crust rides on a sea of molten rock called magma, which bubbles to the surface wherever two plates meet. Earthquakes work like pressure valves for this magma (known as lava when it reaches the surface). In effusive" volcanoes (such as the famous volcanoes of Hawaii, U.S.A.), the lava flows at a steady rate, often forming new mountains and islands. Volcanoes with a lot of gases dissolved in their magma and a high content of a chemical known as silica have"explosive eruptions in which they literally blow their tops. Potentially much deadlier than effusive eruptions, explosive volcanoes vaporize the landscape in every direction with hot gas and carpet the terrain with choking ash. The explosive eruption of Mount Vesuvius in A.D. 79 destroyed the ancient Roman town of Pompeii, burying under 13 to 20 feet (4 to 6 m) of hot ash. The geyser-riddled Yellowstone National Park in Wyoming, U.S.A, sits above a"supervolcano" that last blew its top 640,000 years ago.
Answer:
As if people living in the world's major earthquake zones don't have enough to worry about, a new analysis of two of the biggest quakes of the past century reveals a sharp spike in volcanic eruptions after the events, sometimes in volcanoes located hundreds of kilometers from the epicenters. The researchers are quick to point out that not all large earthquakes trigger eruptions, but the work does suggest that in areas where both earthquakes and volcanoes are common, such as in Indonesia, increased volcanic activity could be looming in the wake of big temblors.
The findings make sense. A large earthquake shaking Earth's most active seismic zones with the energy of millions of nuclear bombs is bound to stir up pots of magma deep beneath the surface. Indeed, Charles Darwin speculated about the possibility of volcano-triggered quakes as far back as 1835. And previous studies have linked the two phenomena based on quakes and subsequent eruptions in their proximity (ScienceNOW, 27 February 2007). But no one had conducted a long-term analysis or looked at the possibility that big temblors could set off far-away eruptions.
So volcanologist David Pyle of the University of Oxford in the U.K., along with colleagues Sebastian Watt and Tamsin Mather, pored over historical records of earthquakes and eruptions along the Chilean fault on the west coast of South America. In particular, they focused on the data covering the past 150 years, including the great 1906 and 1960 quakes in southern Chile--the latter event the biggest quake in recorded history. In an upcoming issue of Earth and Planetary Science Letters, the team reports that eruption rates among 15 volcanoes as far away as 1000 kilometers from the quake epicenters quadrupled within a year following those two events: four major eruptions versus the typical one. "We are confident that the observations are real," Pyle says. "There are no other such peaks in eruptions rates in southern Chile that are not associated with large earthquakes."
