Wil McCutcheon presents - 'Tephra' - in Olalla, BC EarthCache
Wil McCutcheon presents - 'Tephra' - in Olalla, BC
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During a volcanic eruption, clastic materials, pulverized rock, hot gases, and lava are violently ejected.
The bits of rock and clastic materials together are known as 'tephra'. This appears as a large white (pyroclastic) cloud.
During the event, heated materials ascend high into the atmosphere and a sorting process begins.
Heavier (bigger) chunks of the tephra mixture (known as 'bombs' or 'blocks') follow short duration trajectories and immediately fall back to the surrounding area while lighter pieces roll in the atmosphere for a longer duration, suspended by the on-going thermal activity.
Finer particles (dust) saturate the air-space and are carried off several hundred kilometres by the upper-winds.
Falling like snow, the tephra powder blankets large sections of the Earth in various thicknesses.
Land topography (elevation, steep terrain, deep pockets, etc.) effects how much of the fallout will be detained in any given area.
Centuries of weathering, chemical change, gravity, water saturation, and exposure to new materials (dirt, dust, pollen, organic matter, etc.), slowly (or rapidly as in landside activity) result in the white line of tephra being compacted, covered up, and forgotten.
Forgotten, that is, until further natural process or man-made activity (as in road construction) cuts into a hillside, exposing the stratified layer. This is what you’ll find just off highway #3A, a bit north of Olalla, BC.
If you are wondering which volcano this particular tephra layer most likely came from, you need to look toward Crater Lake National Park in Oregon. The crater there was not caused by an impacting meteor, but by an exploded volcano now named Mt. Mazama.
Mt. Mazama was a stratovolcano within the Cascade Range that ascended to well over 3200m. After several thousands of years of activity (and a long period of dormancy throughout the last ice-age) a large caldera formed deep within the mountain. The volcano top collapsed into the hot gas-filled cavity and an explosion ensued.
Large amounts of clastic materials were launched up and out, covering parts of (what is now) Oregon, Washington and Canada’s Southern British Columbia.
*** For further insight, check out the descriptions of 'tuff' as it pertains to tephra developement. ***
*** Also Note : some of the tephra lines you'll see throughout BC originate from other volcanoes like Mt. Mazama. If you are further interested, check out the geological/volcanic history of Mt Meager (north of Harrison Lake), Glacier Peak & Mt. St Helen's (Washington), .
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(Please don't climb up to the Tephra line and pick away at it. Lots of samples are findable at the hillside baseline.)
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To maintain credit for discovering this Earth Cache, you must accomplish the following;
Post (1) picture - of the tephra line discussed in this write-up. (Show your GPS in the pic as well).
Post (#2) picture looking down the highway (either direction) showing the parking pull-out along with some of the local steep valley terrain (mountain side) slope.
Feel free to pose with your group or geo-partner (if you like) in either picture.
E-mail your answers to the 2 following questions in to the cache presenter/host.
#1) -
- Some of the rocks have rolled down to parking pull-out level. These have some of the tephra still attached/fused to them. Try scratching the tephra. Is it solid/hard crystal-granular, slightly chalky, or mushy - like toothpaste?
#2) -- What is the approximate range of thickness, of this exposed line of ancient tephra?
***Please 'POST PICS' - it's easy!! I hate vanquishing 'smilies' - so please please please - get your super-cool pics up asap!***
Additional Hints
(Decrypt)
Jngpu bhg sbe Zbhagnva Furrc!