Thunder Hole
Introduction
Thunder Hole is located on the eastern side of the island along Acadia’s Park Loop Road up the coast from Sand Beach. It’s an inlet with a cave carved into the granite that, when the weather is stormy and the waves are violent, creates a thunderous roar that attracts many tourists.
Figure 1. Aerial Imagery of Mount Desert Island
Figure 2. Aerial Imagery of Thunder Hole
Figure 3. Geologic Bedrock of Thunder Hole
North
Thunder Hole Field Site
Figure 4. Coarse-grained phaneritic crystals in the Cadillac Granite
Figure 6. Epidote forming along the contacts between a quartz vein and the Cadillac Mountain Granite
Cadillac Granite: pink with phaneritic grains of feldspar, hornblende, and quartz. The granite at Thunder Hole also surficially contains a green mineral called Epidote (Fig. 7), which forms when water flows along a surface and precipitates it. This causes some of the quartz crystal veins to turn greenish. This can also be seen at Sand Beach, although this process is more obvious at Thunder Hole. The potassium feldspar causes the abundant pink hue of the Cadillac Mountain Granite which also contains little specks of black hornblende, and gray-white quartz crystals
Figure 9. Parallel fractures in the Cadillac Mountain Granite at Thunder Hole
Figure 7. Epidote forming along a fracture
Figure 8. Epidote forming on quartz parallel to a fracture
Fractures: Fractures are cracks in the rock caused by stress. At Thunder Hole, they are trending south-east and west. Fractures are important because they inform the way the landscape is shaped. For example, many of the island’s u-shaped valleys were formed by glaciers that carved out the fractures. Everything we do from day to day is effected by the shape of our landscape.
Figure 10. A different view of parallel fracture sets in the Cadillac Mountain Granite at Thunder Hole
Fractures Measurements:
Set 1. 147, 153, 150, 154 | Average: 150
Set 2. 262, 264, 258, 257 | Average: 260
These averages are similar to other averages of fracture sets found around MDI.
Observations: Two interesting things that caught our eye are the color change in the granite near fractures, and the Epidote. When viewing certain cracks, there was this significant color change on both sides of the fractures. When looking at the epidote, it was is cracks and what possible could have been a dike. Both of these formations in the granite have a similar explanation, which is water run off. A long time ago when the Cadillac Granite was underground, water would seep through the cracks. For the Epidote case, the water dumped minerals through a length of time. The run off was slow enough to deposit the Epidote. As for the washed out cracks, the opposite we think happened. The water was running fast enough and must have either been hot or acidic to seep the surrounding minerals out, like the Feldspar. This site has wonderfully large examples of both these processes.
Figure 11. the pink granite has lost some of its original color
Figure 12. a strip of leached Cadillac Granite
Landscape: Wave impact is slowly breaking apart massive pieces of the rock due to Thunder Hole’s location. Waves and the freeze/thaw cycles break down the rock even faster in coastal areas. These types of erosion eventually cause chunks to break off and fall, leaving boulders of granite rolling around in the water below under the rough waves. These boulders catalyze the erosion of this formation by grinding away at its sides and base. Because of the relatively quick erosion, this site is quickly deteriorating and in the next few centennials or so it could be non-existent.
Figure 13. Savier admiring the fractures
Figure 14. Tourists on 'floating rocks'
Figure 15. Waves crashing against the rocks
References cited:
Gilman, Richard A, and Carleton A Chapman. “Bedrock Geology of Mount Desert Island.” 1988.
Braun, D.D., and Braun, R., 2016, Guide to the geology of Mount Desert Island and Acadia National Park: Berkeley, CA, North Atlantic Books.