Lessons from Mount St. Helens Eruption

"Mount St. Helens volcano in Washington State, north-west USA,

would be one of the most studied volcanoes on earth. On 18 May 1980, at 8:32 am, a surge of magma deep underground triggered an avalanche on the mountainside. Like a cork popped from a bottle of soda, the pressure inside the mountain unleashed the deadliest and costliest volcanic disaster in the history of the US: 57 lives were lost.

One remarkable effect of the Mount St. Helens eruptions is that geologists are now more accepting of catastrophic geologic processes. 

---Previously they were wedded to the idea that geological features

formed slowly over millions of years. But their ideas changed after they saw that thick beds of ash, deposited in less than an hour, displayed fine laminations. That proved that long periods of time are not essential for fine layers to form.
---Radioactive dating of rock that formed since the 1980 eruption gave ‘ages’ of hundreds of thousands, even millions of years, showing that the fundamental assumptions behind radioactive-dating were wrong.

During Noah’s Flood, 4,500 years ago, water not only rained from the heavens but also burst from underground from “the fountains of the great deep” (In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst forth, and the windows of the heavens were opened. Genesis 7:11). These fractures and upheavals in the earth’s crust caused great volcanic activity. It was many hundreds of years after the Flood ended before the earth settled down.

During Noah’s Flood some volcanic eruptions covered enormous areas, such as the Columbia River Basalt Group in north-western

USA. Here, as many as 300 individual lava flows engulfed some

63,000 sq. miles of the countryside to a depth of more than 1.1 miles.

The lava gushing from the earth was so hot and runny that it flowed across the landscape for vast distances. The flood waters were still around when the eruptions took place, and they deposited sediment, as well as wood (now petrified) and gravel, between some of the lava flows. The individual flows followed each other so quickly that there was not much erosion between them. But when the lava finally stopped, as the waters of Noah’s Flood receded into the ocean they eroded deep valleys into the basalt complex.

Molten rock, while it is inside the earth, is called magma, but once it erupts onto the surface is called lava. Rocks that harden from magma are called plutonic (after the Greek god of the underworld, Pluto), and usually have large crystals, while rocks hardening from lava are called volcanic, and usually have very fine crystals. The composition of the magma depends on the source rock and how much of it melted.

Magma that is rich in magnesium and iron is described as mafic. It is highly fluid (thin, runny) and gushes out of fissures in the ground at

over 1,000 °C. This lava solidifies into a black rock called basalt (if the magma cools inside the earth, it forms gabbro). Like a fountain, basaltic eruptions in Hawaii and Iceland spray red-hot lava into the air, which then flows in glowing red streams into nearby valleys or the ocean. These eruptions are placid and predictable, and popular as tourist attractions. The lava forms large, flat cones called shield volcanoes.
Magma with less iron and magnesium is less fluid, and can solidify into a grey rock called diorite. If this type of magma becomes lava, it will solidify into andesite.

With even less iron and magnesium the magma is thick and tacky. It is called felsic magma because it is rich in elements that produce feldspar and silica minerals. Felsic magma can erupt explosively or ooze like toothpaste to form a blob. The lava solidifies into a yellow, pink or pale-grey rock called rhyolite (the plutonic equivalent is granite). Mt St. Helens erupted into a lava dome of dacite, between andesite and rhyolite in composition." 

CMI