Franconia Ridge

The White Mountains of New Hampshire were formed about 100 million years ago by magma intrusions. Specifically, the North American plate traveled over the New England hotspot (a volcanic hotspot holding magma), causing widespread eruptions. The cooling of magma underneath formed the granite one sees everywhere today (2).

Additionally, the White Mountains were heavily influenced by the presence of glaciation. As 1/2 mile thick glaciers pulled out 12,000 years ago, they left glacial troughs due to their immense weight. The first image below provides perspective from the top of the summit, whereas the second one emphasizes the u-shaped valley -- carved out by glaciers 12,000 years ago (1). See "A Very Brief History of New England Geology" on the home page for more information on this phenomenon.

As we made our way up Mount Lafayette, we noticed many samples of granite lying along the trail. Known as the Granite State, New Hampshire's landscape is dominated by this rock type. As magma slowly cooled underneath volcanoes millions of years ago, minerals formed and created this rock type. Quartz, feldspar, biotite, muscovite, hornblende, augite, and olivine are minerals commonly found in igneous rock. (3)

Wilder made the excellent observation along the trail of another interesting rock type. At first, we believe it to be granite with high amounts of feldspar -- this would explain the reddish color. However, as we walked along the trail, we noticed that this rock would crunch underneath our feet. In other words, it was much softer than any granite that we had encountered before. At first, I predicted that, with the extreme weather pounding the ridge yearlong, the rock had simply been worn down. Wilder added another perspective: perhaps we were observing sedimentary rock. And eventually, after realizing how easy it really was to break this rock, I agreed with Wilder. See field pictures below:

Upon further research (1), however, we discovered an alternative explanation --and one that did not support our original hypothesis. The 'red-ish' granite turned out to be a sample of 'Conway Granite,' which, just like any other granite type, cooled slowly in the magma chamber millions of years prior. But, in this instance, the rainwater dissolved the potash (potassium-rich) feldspar in the Conway Granite, and thus caused the rock to crumble.

1. NH State Parks: Geology of Franconia Notch

2. Peak Visor: White Mountains (New Hampshire)

3. Common Minerals in Igneous Rocks