The sand collection of Dr. Richard Sternberg is on display in the lobby of the Ocean Sciences Building. Each quarter our sediment group plans to highlight and briefly discuss groups of interesting samples--so keep your eyes out for the “Sediment Stories Quarterly” posted on the collection. Below is the newly posted Winter quarter sediment story.
SEDIMENT STORIES, Autumn 2022: Evidence of a furiously catastrophic past
This Sediment Story describes the abundance of pumice stones found during a picnic excursion to a beach on the Columbia River in May 2022. It traces this material to the eruption of Mount St. Helens (MSH) at 08:32 on May 18, 1980, 42 years ago. It summarizes the first four hours of the eruption in terms of the origin of the pumice, its transport to the Columbia River and its historical past.
Sediment example (Fig. 1)
Highlighted sample: pumice sediment from the beach face of Skamokawa Vista Park, Washington.
Volcanic events on Friday morning of May 18, 1980*
Eruption Phase: (08:32) earthquake occurs and causes the north face of MSH summit to collapse to become a huge debris avalanche of pumice-rich, gaseous pyroclastic material ejected outwards horizontally and upwards. Thought to be “the largest known debris avalanche in recorded history”.
Explosive Phase: (08:32- 09:00) pyroclastic material mixed with melting ice forms liquid debris flows (Lahars) which find their way into the Toutle and Cowlitz rivers and reach 100 km/h as they roar down the river valleys and enter the Columbia River.
Ash Cloud Phase: (09:00-12:15) lithic ash and pumice ejected in a vertical column rising to 20-27 km and spreads rapidly. Coarser ash and pumice ashfall add to the debris flow stratigraphy in the river valleys and Columbia River. Within two weeks the ash cloud encircles the world.
Post Eruption: The eruption begins to subside through the afternoon.
Comments
For anyone interested in Mount St. Helens, an excursion to the beach at Skamokawa Vista Park on the north side of the Columbia River 33 miles west of Longview, Washington is of great interest. It provides a close-up look at pumice stones presently midway between MSH and the Pacific Ocean.
Furthermore, if one wanted to begin a sediment collection from this beach, a more meaningful sample would be hard to find!
These beautiful pumice stones resting in the Columbia River are deceiving. They might appear to be small particles eroded and transported down river by ‘normal’ sedimentary processes (e. g. seasonal floods over months/seasons/years) but they are not! This pumice originated during the first minutes of the Mt. St. Helens eruption, the most notable geologic event in Washington modern history, and was transported in several hours to this location by some of the largest liquified debris flows ever studied and via ashfall of volcanic ejecta from a huge ash cloud which ultimately circled the earth. A furiously catastrophic past!
*Based on studies of USGS and Criswell (1987)
