Fiery Rock Dust Cloud, an Ignimbrite Eruption EarthCache
Fiery Rock Dust Cloud, an Ignimbrite Eruption
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This Earthcache is located in City of Rocks State Park. There is a day-use fee of 5$ per car to enjoy the park. Camping is also available at numerous camp-sites throughout the park.
Imagine an enormous cloud of volcanic pumice, ash and gas, belching from a vent hundreds of kilometers away. Even so far away the cloud is so thick, dense and hot that it blankets the surrounding countryside in a thick layer, tall enough to bury tall trees. This was the main phase of the Kneeling Nun eruption, and was known as the ignimbrite eruption. In fact, the term ignimbrite was coined by New Zealand geologist Patrick Marshall and means ‘fiery rock dust cloud’. 1000s of cubic kilometers of ash were deposited in this area 34.9 million years ago. It was a cataclysmic event, one that makes the 1980 eruption of Mount St. Helens seem tiny by comparison. To get some perspective of how large this eruption was, it ranks 26th (by volume) of the earth's largest volcanic eruptions.
The rock formations in City of Rocks State Park a result of this material. The columnar landforms would have begun forming shortly after the main explosion of the Kneeling Nun Tuff. The errosive forces of freeze-thaw action, wind and vegetation have acted on this landscape to form the pillars. Given another several millions of years, these forces would eventually reduce this landscape to a flat plane. While the columns would be gone from this location, another nearby area could have the Kneeling Nun Tuff exposed, creating a new "City of Rocks".
To log this Earthcache, visit the posted coordinates and answer the following questions:
- Facing South-East you will see a formation divided in two. Describe what divides this rock and in what direction it runs.
- The cooling Kneeling Nun Tuff would have formed joints perpendicular to the cooling surfaces.What is this surface?
- The rock faces are often pocked with holes, often called "huecos". These are formed by wind and acidity weathering away softer portions of the Tuff. On the rock formation seen from question 1 above there is a distinct line of these huecos going from top to bottom. About how many huecos make up this line? Describe how over time, this process will change the shape of the formation.
- About 20 feet north from GZ, there are some additional rock "hole" formations on the rock floor. Make some observations about these, are they natural or man-made? How can you tell?
- (Fill in the blank) Examine the Kneeling Nun Tuff closely, and describe how it appears. The particular abundance of ___________ is indicative of a zone of dense welding which gives the rock a granitoid appearance. Because of this, it's easy to see how one might mistake this rock for granite instead of volcanic Tuff.
Additional Hints
(Decrypt)
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