Mount St. Helens

Mount St. Helens is an active stratovalcano in Skamania County, Washington, in the Pacific Northwest region of the United States. It is located 96 miles south of Seattle and 53 miles northeast of Portland, Oregon. The mountain is part of the Cascade Range. It is most famous for a catastrophic eruption on May 18, 1980. That eruption was the most deadly and economically destructive volcanic eruption in the history of the United States. 57 people were killed, and 200 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. The eruption blew the top of the mountain off, reducing its summit from 9,677 feet to 8,364 feet in elevation and replacing it with a mile-wide horeshoeshaped crater.

Like most of the other volcanoes in the Cascade Range, St. Helens is a great cone of rubble, consisting of lava rock interlayered with ash, pumice and other deposits. Volcanic cones of this internal structure are called composite cones or stratovolcanoes. Mount St. Helens includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit; another formed Goat Rocks dome on the northern flank. These were destroyed in St. Helens' 1980 eruption.

The first recorded sighting of Mount St. Helens by Europeans was by Royal Navy Commander George Vancouver and the officers of HMS Discovery on May 19, 1792, while they were surveying the northern Pacific Ocean coast from 1792 to 1794. Vancouver named the mountain for British diplomat Alleyne Fitzherbert, Baron St. Helens on October 20, 1792.

According to geological evidence, St. Helens started growth

37,600 years ago with dacite and andesite eruptions of pumice and ash. Mudflows were very significant forces in all of St. Helens' eruptive cycles. Starting around 2500 BC eruptions of large amounts of ash and yellowish-brown pumice covered thousands of square miles. This eruptive cycle lasted until about 1600 BC. After 400 years of inactivity, St. Helens came alive again around 1200 BC. This cycle, which lasted until about 800 BC, is characterized by smaller volume eruptions.

Mt. Saint Helens woke up on March 20, 1980, with a Richter magnitude 4 earthquake. Steam venting started on March 27. By the end of April, the north side of the mountain started to bulge. With little warning, a Richter magnitude 5.1 earthquake triggered a massive collapse of the north face of the mountain on May 18.