These are the deposits of volcanic eruptions that occurred some 10
million years ago, one of up to 15 such deposits which have been
identified in the Ellensburg/Yakima area. This particular deposit
is the product of a violent eruption in the Cascades about 60 km
(40 miles) west, though the exact source is not positively known.
During the volcanic eruption, large quantities of ash were
expelled by the upward force of hot gases, creating a dense
column-shaped cloud that may have been several kilometers high.
Eventually, the weight of the ash debris could no longer be
suspended by the upward push of the eruption, and the cloud
“collapsed” into a pumice flow. The kinetic energy of this
falling mass flowed down the side of the volcano at speeds likely
in excess of 100 km per hour (60 mph). Try to imagine a huge
avalanche-like flow of hot gas, ash, and rock fragments roaring
down a mountainside!
As if this wasn’t already bad enough news for anything living in
the area at the time, the pumice flow mixed with water, probably by
entering a river. The result: A slurry similar in consistency to
wet cement. Despite its thickness, this mixture (known as a
lahar) likely travelled down the river valley at a rapid
speed. Lahars are incredibly destructive and will demolish
everything in their path. If you’ve ever seen video footage of the
lahars generated by Mt. St. Helen’s 1980 eruption, you may recall
buildings and a steel bridge floating away like they were made of
popsicle sticks! Evidence of this power is right in front of you:
Partly sticking out of the outcrop is a large pink boulder that was
carried along by one of these lahars.
Eventually, the lahar came to rest over a large area (it was
much flatter here then) including where you now stand. This violent
event occurred not once, but several times between 8.5 and 11
million years ago, resulting in this outcrop. Now, it’s your turn
to examine it!
To log a find on this Earthcache, you’ll need to look carefully
at the rocks from a wider perspective, so I suggest standing on the
far side of the road from the outcrop to better take in the “big
picture”. If you haven’t noticed it already, take a moment to look
at the horizontal layers. Is it all the same, or is there
variation? As you scan the outcrop from the bottom to the top, do
you notice any repeating patterns?
A handy-dandy type of diagram that geologists are fond of is the
stratigraphic column. The concept is simple and not as scary
as it sounds: Take a cross-sectional (sideways) view of a
rock outcrop showing relative thicknesses, textures, and
compositions. Think of it as just a sideways drawing of the
rocks.
Provided below is a stratigraphic column depicting a
generic lahar deposit at this location. I’ll help you decipher it:
The bottom layer (a) is less than a foot thick and sandy. Above
that is the bulk of the lahar (b), a mixture of everything from ash
to gravel to large chunks of rock, though most of it is sand-sized
or smaller. The top layer of the lahar deposit (c) is partly sandy,
partly ashy, deposited in crossbeds, which looks kind of
like long hair flowing in water. That’s the sequence, and it
repeats each time a new lahar was deposited. Keep in mind that this
is a generic sequence, and there may be variations. Using the
stratigraphic column below and your own power of observation,
ponder the following:
1. How many lahar deposits are visible here?
2. Is the pink boulder suspended at the top of a lahar, or was it
dragged along the bottom?
3. Is the layer that is visible at ground level (the first 6 feet
or so) the top or bottom of a lahar?
4. The funny patterns you see in the crossbedded layers (c) are the
result of sandy material depositing into ripples, just like you
have probably seen in a stream or at the beach. How tall do you
suppose the ripples have gotten? You may have to get closer to
see the ripples.
Per GSA
Earthcache requirements, please email the answers to these
questions to me to log your find. Go ahead and log it after you've
done so. Additionally, take a photo of yourself or group and upload
it to your log (optional)!
(Image source: Schmincke 1967)
Rating Information
While this Earthcache will not involve any difficult terrain,
the road shoulder may not be suitable for folks in wheelchairs,
especially after rain.
References:
Schmincke, H. "Graded Lahars in the Type Sections of the
Ellensburg Formation, South-Central Washington". Journal of
Sedimentary Petrology, June 1967.
Camp, V. "How
Volcanoes Work"
Tabor, R.W., et al. "Geologic Map of the Wenatchee 1:100,000
Quadrangle, Central Washington". USGS Publication.
Washington State Department of Natural Resources, GIS Data.