Note: no "armchair caching", please: we believe a player must visit a cache site in person in order for it to count as a find.
One can picture a volcano with a fiery maw of lava. The lava cools. The crater becomes a lake and eventually a grassy plain ringed with a mountainous rim. If you stop here to look and think that’s what you are seeing, you’ve missed it. That is too simple, and too small. Yes, that would still be an awesome mental picture to explain the spectacular view before you. The true story is even more colossal. As you begin to realize the real scale of Valles Caldera, and the small portion that is Valle Grande, it becomes clear that it’s going to take more than a stop or two at the scenic turn-outs along the south rim to get the whole picture.
We encourage you to explore the Valles Caldera National Preserve to find out more. Visit the Preserve’s office in Jemez Springs. (Jemez is pronounced like the Scottish name, Hamish: “Hame’-ish”.) Even better, make reservations and attend one or more of the recreational activities that interpret the rich natural and cultural heritage as well as financially support this living laboratory. Meanwhile, here is a start at understanding what you are seeing when you look across the Valle Grande (Valle rhymes with “hi ya”; Grande is “grahn’-day”).
This lush, grassy valley did indeed begin with magma (molten rock) surging upwards from deep within the earth’s crust. As the eruption began, a single vent exploded. Stage 1 sent material high into the air, where it cooled rapidly into frothy lumps, blanketing the surrounding landscape two meters deep in ash, crystals, and pumice. These airborne layers raining down out of the sky are called tephra. Pick up any of the light gray stones around your feet. You may be surprised at how little they weigh. Put them in a cup of water; pumice floats! (At least until the air spaces fill up with water.) Some of the material ejected was flung hundreds of miles away, landing in what is now Kansas and Oklahoma. /p>
It did not take long, maybe only hours, for multiple vents to open in an ellipse shape 16 kilometers (10 miles) across. In Stage 2, the multiple vents connected in a ring-fracture vent system. Tremendous amounts of magma shot out of these vents, so much material emerging so quickly that the eruption could only roll along the ground, filling valleys and building broad plateaus. /p>
Imagine sticking pins through a piece of paper in a ring shape; you can guess what happens if you push on the disk of paper. In reality, it was not pressure, but lack of pressure that broke the center of the caldera (collapsed crater). As the huge magma chamber beneath the crust emptied out onto the landscape, the underground void could not support the weight of the rock above. The “roof” of the chamber collapsed catastrophically, subsiding as much as 1500 meters on the east side. The western side sank only 300-400 meters. The pressure of the collapsing rock on the magma chamber during Stage 3 was like squeezing the last of the toothpaste out of a tube. The pyroclastic material coming out of the caldera now began to cool into an orange and beige rock called tuff.
Amazingly, when the Valles Caldera eruption happened 1.25 million years ago, it took place almost in the same place as a previous eruption, the Toledo Caldera eruption of 1.6 million years ago. The two eruptions, the Toledo Caldera eruption and the Valles Caldera eruption produced the Upper and Lower Bandelier Tuffs that cover the surrounding landscape. That frothy layer of tephra thrown out in Stage 1 is part of the Cerro Toledo Interval that separates the Upper and Lower Bandelier Tuffs.
In Stage 4, rain and snow filled the 20- x 24-km. (12- by 15-mile) hole to make a lake very much like Oregon’s Crater Lake. If that had eventually turned into the grass-covered plain you see before you, you would not be looking a valley just 3-4 km. (2-5 miles) across. Valle Grande would have been Valle Muy Muy Grande indeed! Instead, within 50,000 years, a blister began rising in the center of the lake, pushing the water into a ring-shaped moat. Most calderas produce a resurgent dome, and Redondo Peak surged 900 meters (3,000 feet) above the floor of Valles Caldera.
In Stage 5, the moat began to fill in with small new mountains, as volcanic activity resumed. The first of the new intra-caldera ring-fracture volcanoes was Cerro del Medio, on the northeast side of Valle Grande. One by one, new mountains popped up in a counter-clockwise circle. Things were quiet for a long time, half a million years or so. Then the volcanic eruptions started up again. The latest activity was at El Cajete, to the west of Valle Grande, about 50,000 years ago. What we see from this vista is the largest remaining portion of Valles Caldera’s original caldera floor. As spectacular as it is, Valle Grande is only a small fragment of the original caldera! Is Valles Caldera finished? Will ring-fracture mountains eventually emerge here in Valle Grande someday? Who knows…?
Over time, eruptions and erosions broke down the edges of the caldera. The interior lakes drained away. Now the water meandering across the rich meadowland of the Valle Grande are creeks and streams that become the East Fork of the Jemez River. Visit the Jemez Falls to see its departure from the caldera.
To log this earthcache, post a photo of yourself or your GPS receiver, taken at this location with Valle Grande in the background. Email us the answers to these 2 questions:
1. Look on the sign, at the map. Valle Grande, which lies before you, is the flat area mostly south of the "bear paw" shape of Redondo Peak and the other newer mountains rising from the floor of the caldera. Compare Valle Grande to the whole Valles Caldera region. What percentage of Valles Caldera would you estimate is taken up by Valle Grande?
2. We encourage you to explore further, if you have the time. How close to this point will you find the start of the nearest trail that may be used by visitors who arrive without reservations for the recreational opportunities available at the preserve?
References: Kempter, Kirt, PhD. and Dick Huelster, Valles Caldera Map and
Geologic History of the Southwest’s Youngest Caldera, High Desert Field Guides, 2007.