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Modoc Plateau Flood Basalt Layers Earthcache

A cache by PathfinderMark Send Message to Owner Message this owner
Hidden : 8/11/2009
1.5 out of 5
1.5 out of 5

Size: Size: other (other)

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Geocache Description:

This rural flood basalt layers earthcache is just north of rural Alturus, CA. Why not fulfill the pletoria of challenge caches that need a smiley in this remote location of NE California with an earthcache?

The Modoc Plateau in the Northeast corner of California is nearly totally volcanic. If there is any exposed solid rock on the plateau that is not volcanic it is a rare occasion. The entire plateau is covered in a thick accumulation of basalt flows, pyroclastic flows*, volcanic mudflows* and volcanic ash. The valleys have been filled with a combination of the above and topped with thick deposits of alluvium.

The plateau today is roughly bounded by the High Cascades to the west, the Oregon border on the north, the Warner Mountains on the east and the Sierras to the south. The Modoc Plateau has a very interesting geologic past that created this remarkable landscape.

The plateau was once nothing but Pacific Ocean far off the ancient coastline of North America. Plate tectonic crustal movements gathered and stacked several micro-plates against the North American continent from California to Alaska. Once in place, these micro-plates are known as exotic terranes. A terrane, as geologists spell it, is a fault-bounded crustal fragment with a geologic history that differs from adjacent fragments. When terranes "dock" against a continental plate they are referred to "accretions" and, in this case, they served to extend the continental coastline westerly.

The Klamath Mountain geomorphic province to the west is an accreted superterrane. It was initially comprised of a series of separate micro-plates that formed somewhere out in the Pacific Ocean. As the small plates collided with others they first formed into a composite terrane and finally into a superterrane just off the North American coast. This superterrane eventually collided with and accreted to the North American continent. As it was arriving, it jammed and jumbled up the continental shelves and seafloor between the two crashing titans thereby creating a basin.

This basin became the Modoc Plateau. Further compression and erosion off the two continental bodies filled and raised the plateau until it was well above sea level. If that wasn't enough, the invasion of four mountain building operations capped the entire Modoc with thousands of feet of volcanic materials.

Pacific plate subduction activity continued after the Klamaths arrived. Subduction zones typically create volcanic mountain ranges known as volcanic arcs. The Western Cascades arc in Washington and Oregon intruded into the Modoc as did the Ancestral Cascades from the south, a name given to the ancient Sierra Nevada volcanic range. These two old mountain ranges supplied massive amounts of volcanic rock, ash, pyroclastic and volcanic mudflows to cover the original volcanic and sedimentary rock of the Modoc. These ranges eventually died out and have all but completely eroded away or been buried under new mountain development. The Western Cascades and the Ancestral Cascades were active at approximately the same time during the early to mid Cenozoic Era.

As the old ranges faded away two new players stepped in to continue supplying volcanic materials to this day. The High Cascades is the name given to the current Cascade Mountains volcanic arc from Vancouver, BC south through and substantially reducing the size of the original plateau. This range includes Mts. Shasta and Lassen, both recently active. The Basin and Range was created by a process of crustal extension creating fault block mountains and "basin" valleys. GPS station data suggests that this process is still in motion. This is a very large province that stretches from Oregon into Mexico and roughly between the Sierras and the Wasatch Range in Utah. Basin and Range spreading also produces volcanic activity and it has intruded into the Modoc. The High Cascades and the Basin and Range have been volcanically active since the late Cenozoic Era, specifically the late Miocene Period to the present.

This all brings us to what you see across the highway at this location. Every solid rock along Highway 395 from Susanville to the Oregon border is volcanic. Here we have a basalt lava flow on top of a rhyolite ash flow or more likely an ash fall. Ash from an air fall arrives airborne and consolidates in place while an ash flow is, in part or singly, a pyroclastic flow.

From the 2010 California State Geologic Map it is apparent that this ash is from the Pliocene period and so probably from the Western Cascades or Ancestral Cascades or possibly early Basin and Range activity but I think that is much less likely. The basalt flow shows as being from the younger Quaternary period. It appears that not a lot of time passed between the two formations since there is little or no soil development between the two. It is also possible that there is a greater age difference and the soils that might have developed had been eroded prior to the arrival of the basalt flow.

The ash here was originally white and is probably part of the same formation as that up the road at Chimney Rock. The basalt flow covered the then cold ash and baked it red from the top down due to the heat of the basalt.

The cache location:

There is a broad pull-off on the west side of the road directly across from the road cut that contains the distinctive layered pattern under discussion. You do NOT need to cross the road, but rather can answer the logging requirements from the western shoulder.

Logging Requirements:

Send the answers to #1-#4 to me through my geocaching profile (Since the advent of the Message the Owner feature, I prefer messages through that venue).

1. List the name “GC1X4B5 Modoc Plateau's Volcanic History Earthcache” in the first line of your email. Also, list the number of people in your group.

2. What two distinctive colors are present in the road cut?

3. Based on the information you’ve read above concerning the forces at work in the formation of the Modoc Plateau, what do you think caused the lower layer’s distinctive coloring, since rhyolite ash is not typically this color?

4. Based on the information you’ve read above, determine what major event caused the creation of the Modoc Plateau. Explain briefly.

5. (Per current guidelines, photos are no longer allowed to be required. HOWEVER they are encouraged, since they can help clarify that you have visited the location if your other logging requirement answers are vague). Post as part of your log, if so inclined, a picture of yourself and your GPS with the VALLEY to the west of Hwy 395 plainly visible behind you. DO NOT show the road cut in your picture as it contains visible answers to logging requirements. Failure to follow directions precisely will result in your log being deleted!

I will only respond if you have incomplete logging requirements. Go ahead and log your cache.

*Pyroclastic Flow: A combination of very hot rock, debris and ash flowing across ground or water surfaces.

*Volcanic Mudflow: Ice and snow melted by the heat of an eruption run downhill carrying ash particles and other debris. These volcanic mudflows are called lahars.


Roadside Geology of Northern and Central California, "Second Edition", 2016, Alt/Hyndman
Roadside Geology of Oregon, "Second Edition", 2016, Marli B. Miller
Fire, Faults and Floods, 2005, Marge and Ted Mueller
California State Geologic map, 2010, Alturas Quadrangle
Unavco map - Tectonic Motions of the Western United States

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