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Big Denali EarthCache

Hidden : 05/23/2013
5 out of 5
5 out of 5

Size: Size:   other (other)

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Geocache Description:

Denali is the highest mountain in Alaska, the United States and North America ... but it is not a volcano.

The Alaska Range and therefore also Denali are lifted by tectonic pressure from the Pacific plate and the North American plate. You will find similar Mountain Ranges all over the world like the New Zealand Alps (Pacific and Australian plate), the European Alps (African and Eurasian Plate) or the Himalayas (Eurasian and Indian plate).

Lifted by tectonic pressure means the plates collided and since there is no way for them to go left or right, they have slide under or over each other. Pending on the rock, the weather (erosion) and a couple of other factors you will get a higher or a lower mountain range.

In this case the Pacific plate is subducting (which means sliding under) the North American plate. This line where the plates are sliding under each other is called a fault line. And since the pacific plate makes a sharp bend under Denali the fault line which is called "Denali Fault is particular seismic active. Or to put it in other words: A lot of earthquakes can be measured here.

Now you might think "Earthquake - Tectonic Plate - Volcano!". Well despite being a seismic active area, Denali is not a volcano. A simple definition of a volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from the magma chamber below the surface. So this doesn't apply to Denali.

However there are a type of volcano which is associated with subduction zones: A stratovolcano, also known as a composite volcano (see graphic to the left). It is a tall, conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic explosive eruptions and quiet eruptions. You will find the closest active volcano about 200km to the south of Denali: Mount Spurr is a stratovolcano.

What does the volcano has to do with Denali? Well just because the magma or "lava" didn't make it to the surface it doesn't mean it's not there. A blob of magma or “lava” which was created due to the subduction of the colliding plates; it cooled and solidified beneath the surface at perhaps tens of kilometers down. Over thousands of years that blob got uplifted by the tectonic pressure. Now this blob is a special type of rock. Very sturdy and normally quite old by the time you see it: Granite. Why old? Because the blob of granite was created under the surface and all the layers above it are covering it up. These layers could be any other type of rock for example something called Flysch. Funny name, isn't it? Flysch is a sedimentary rock which means it is layer which got formed by the deposition of material in water. It is a deep marine sediment so it gets created on the bottom of the ocean. Over time the bottom of the ocean gets pushed up by our blob and sooner or later the less weather resistant Flysch erodes away and leaves the granite.

Where is the difference between between the lava-blob-rock (granite) and the ocean-bed-rock (flysch)? Both are heterogeneous rocks. So they look like a salami however Flysch can be made of multiple different type of rocks. Remember: The material got collected deep in the ocean and therefore it can come from anywhere. Granite on the other side was on liquid blob which cooled down and solidified. It's still looking like a salami but granite is much harder. The crystalline structure gives it its name: Granite which comes from the Latin word granum and that means - grain.

Alright. Since Denali is not a volcano and there are constant earthquake underneath it, will it be "shaken down"? Does it grow or not? It actually grows! The Pacific plate is rushing under the North American plate at a speed of 5cm - 7.5cm per year and thus lifting the Alaska Range another 1mm per year further up. The Alaska Earthquake Information Center (AEIC) is maintaining an active monitoring and research program in Denali in order to provide answers by careful analysis of the seismic observations of the earthquakes and associated deformations.

In order to log this cache please answer the following questions and perform the tasks:

  • Grab a small rock above 4500 m (If you're on the West Buttress Route don't take the one at N 63 04.868 W 151 03.855 but that's the type of rock we're looking for) and describe it (Texture, Color, ...). What kind of rock could it be?

  • Standing on top of Denali, is there any former seabed left under your feet? Compare the rock you grabbed earlier with the rocks you find close to the summit. Is it Flysch or Granite? What is the conclusion for finding that rock-type? As a climber you might be able to tell the major difference between the two types anyway but Granite is very hard and makes a good climbing rock while Flysch is softer and crumbles. Scratch the two rocks against each other and if one or both crumbles you probably have Flysch in your hands.

  • (Bonus Question - Optional) What kind of rock will you find under your feet if you're standing on the North Summit? Compare its appearance and relative strength to rock found on the South Summit/West Buttress route.

  • (Picture - Optional) Include a picture of yourself at GZ.


sources / links:
National Park Service
Little Denali

This EarthCache is created with express written permission from the National Park Service.
Finds prior to the publication of this cache do not count.

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