A Destructive Plate boundary - Mt St Helens

According to geological evidence found by the United States Geological Survey, there had been

two major eruptions of Mount St. Helens in the recent past: around 1900 B.C., and

about A.D.1500.

On March 27, 1980, a few clouds of smoke

formed above the summit, and slight tremors were felt. On the 28th, larger and

darker clouds, consisting of gas and ashes, emerged and climbed as high as 20,000

feet. In April a slight lull ensued, but the volcanologists remained pessimistic.

Then, in early May the Northern flank of the mountain bulged, and the summit

rose by 500 feet. On the 18th May at 8:32 a Nuee Ardente erupted from Mt St Helens leaving the mountain 1300 feet shorter than it had been when its growth began. The Nuee Ardente was unstoppable, moving at over 200mph. Eventually viscous lava, accompanied by the burning daddy

clouds of ash and gas, welled out of the volcano's new crater, and from lesser

vents and cracks in its flanks.

61 deaths were reported during the course of the eruption.

A Constructive Plate Boundary

Galapagos Island

The volcano on Isabel, the largest of the Galapagos Islands, is composed of five smaller volcanoes known as the Five Hills.

The Galapagos Islands have been built entirely by volcanism, and about 60 previous eruptions have been recorded over the last 200 years, so it is not surprising that another eruption has occurred. But the surprise is where the eruption happened. Although many Galapagos eruptions have been in and around the large calderas on top of the shield volcanoes, the activity that started Jan 25 was from a vent far down the flank of Fernandina, near the sea. The last time this occurred at Fernandina, in 1968, there was a huge collapse of the floor of the caldera - it fell more than 350 meters. The new eruption has not caused another collapse, so far.

The average slope angle is 25 degrees, and the eruptive fissures are concentric. It is not totally understood why the slopes are so steep, but one theory is lava flows mantling together pyroclastic cones, which were formed at an earlier stage of activity

the Galapagos are formed from basalt, the most basic of all types of lava. Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes, and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland. These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls. Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Ferdina to the east, the volcanoes are lower and more eroded.