First 438 words of the document:
Case Study of a Volcano: Mount St Helens
Date: Started 20th March // Main Eruption 18th May 1980 at 08:30am (Pacific Standard
Where: Mount St Helens is a peak in the Rocky Mountains (fold mountain range) `Cascade
Range' of North America.
Cause: The mountain range was formed approximately 70 million years ago at a convergent
plate boundary. A minor plate, Juan de Fuca, which is oceanic crust (which is heavier than
continental crust) moving eastwards colliding with the continental crust of the North
American Plate (which is lighter than oceanic crust).
The oceanic crust is forced downwards into the mantle. The increase in temperature
destroys this crust, turning it into magma. Pressure increases in the mantle and the magma rises
to the earth's surface. This leads to volcanic eruptions.
Type of Volcano: Composite Volcano A term used by geologists for a steep sided volcano,
often symmetrical cones constructed of alternating layers of lava flows and ash. Composite
volcanoes tend to erupt explosively posing considerable danger to nearby life and property.
20th March 15:47 1980 4.2 Earthquake on the Richter Scale (First sign of an imminent
27th March Mount St Helens spews ash and steam.
Early May Northern side of the mountain began to bulge by 1.5m a day.
18th May 08:30 Small ash and steam eruptions were rising from the crater of the volcano.
18th May 08:32 Earthquake magnitude 5 caused the bulging northern slope to move
forwards and downwards. This became a great landslide of ash, glacier ice, snow and rock to
fill Spirit Lake. The material now mixed with lake water formed a mudflow down Toutle
Valley. This eventually blocked the channel on the navigable Columbia River 60Km away.
18th May 08:33 the landslide exposed magma in the volcanoes vent. This blast called a `nuee
ardente' sent out hot gases, steam, dust and rock fragments at speeds of over 100kph. It
felled tress up to 24km away. Every form of plant and animal life was destroyed within this
Rest of the morning Eruptive column rose more than 20km high. 120Km away in Yakima ash
deposits were 1cm thick. Volcanic debris fell from the column onto the remains of the northern
slope called Pyroclasts. The swirling ash particles created lightning which in turn started
Early 19th May The eruption stopped. Ash cloud had now spread to the central United
States. Some of the ash encircled the globe within 2 weeks of the eruption.
Other pages in this set
Here's a taster:
57 people died (most were tourists that had come as sight seers and some scientists).
More than 200 houses and cabins destroyed, leaving many people homeless.
In the long term many jobs have been created through tourism (see below).
Over 185 miles of highway and roads plus 15 miles of railway destroyed.
27 bridges destroyed.
4 billion board feet of timber damaged or destroyed.…read more