Monday, June 18, 2012

Past Glories - the Haystack Pluton

Lisa put up some Father's Day pictures yesterday. This one was actually a sort of geology excursion from a different era.
Will getting excited about geology
Actually he liked being swung and thrown above my head

This trip was to the Cascade River area in the North Cascade Range. The peak behind Will's head is The Haystack. The Haystack is an epidote bearing pluton. Epidote is not a rare mineral but it is a bit unusual to see it as a magmatic mineral - that is it crystallized within a magma. This means that the Haystack Pluton crystallized from a magma to a solid at great depth something like 25 km below the surface Zen

To the right of the Haystack on the ridge with snow patches is the edge of another magma body, the Eldorado Pluton. There is no epidote in the Eldorado and it solidified at a much shallower depth something like 10 km even though the metamorphic rocks around the perimeter of the Eldorado all indicated very deep burial. The Haystack is approximately 75 million years old and the Eldorado is approximately 90. An early interpretation of this 15 km off set suggested a major fault structure which had the Eldorado faulted against the otherwise much deeper rocks.

The problem was finding the contact of the fault had proven difficult. My graduate work was to hunt down the fault line and try to figure out the direction and timing of motion. Indeed finding the fault line proved to be impossible and the answer as to why it was impossible was on the other side of that ridge behind us. The fault did not exist. The Eldorado had melted its way into the surrounding country rocks 90 million years ago. Afterwards the area somehow got deeply buried or pushed downward to 25 km or more in depth and the Haystack magma intruded and then solidified within these deep rocks.

Another nearby pluton provided a time for when the area was back at shallower depths by 65 million years. Hence this chunk of the North Cascades was deeply buried pushed deep into the earth sometime after 90 million years ago and sometime before 65 million years ago. 

Other parts of the North Cascades have been deeply buried as well. However, those areas were deeply buried at different times. For geologists trying to figure out the North Cascades (and the south end of the BC Coast Range) this deep burial at different times always has to be part of the discussion as to how the range evolved.

There is another aspect of the ridge that is not geologic. While working on this ridge, we encountered a lot of bears. A couple of the bears sure looked like grizzlies. At the time I thought they were too small. But apparently North Cascade grizzlies are smallish so we very well may have seen grizzlies. Will is a much larger object these days. Two summers ago he hiked up to the Haystack area and encountered lots of bears as well. It is a very tough hike in with over 5,000 feet of elevation gain from the valley floor and then some tough going up and down and the weather does not always cooperate.

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