Last updated: September 29, 2020
Article
Why do the granite blocks on Mount Desert Island have such rectangular shapes?
NPS/S Stalker
While the answer is simple in many ways, the vast span of time over which these blocks formed can make it confusing.
To understand, we need to go back in time 450 million years, to the beginning of the island’s geologic history. Mount Desert Island began as a bit of land on a tectonic plate that was struck from behind by another larger piece of land. This can be compared to a pile up on the freeway, but instead of happening in seconds, this pile up took around 60 million years. Instead of resulting in wrinkled hoods, mountain ranges formed and volcanos began erupting.
Most people associate volcanoes with magma, and rocks are formed when magma cools. When magma passes through existing rock, it crystalizes and slowly cools to form an intrusion. Molten rock that comes to the surface through volcanic eruption is called extrusive rock. In the case of Mount Desert Island, the volcano had a caldera (crater) more than 10 miles in circumference. Inside the erupting volcano there was intrusive molten granite and more liquid molten rock that was extruded out from the volcano.
Eruptions helped to grow the volcano to over two miles in height by the time it became inactive. When the volcano stopped erupting, two processes began. The first process occurred when the granite inside the caldera began to cool and harden, which took place over millions of years. The second was the process of weathering and erosion, wearing away all the ash and rock from the eruptions. Weathering and erosion continue even today here on Mount Desert Island.
Throughout this time, movement of the tectonic plates continued. The collision of land masses that created the Mount Desert Island volcano continued forming the supercontinent Pangaea, where the dinosaurs roamed. Later, the movement broke Pangea apart to form the continents as they are today.
Now, finally, it is now time to answer the original question. During all this moving of land masses, erosion continued. The granite which had formed from magma had finally cooled and hardened.
Granite is a hard, brittle rock, and the movement and collision of land masses caused the rock to fracture, forming small vertical cracks. The process of erosion then wore away two miles of soil, ash, and rock. Without the weight of two miles of debris, the downward pressure on the granite was released. Even though the rock was solid, this compression and expansion caused it to fracture horizontally again, leaving small cracks.
During the ice ages, water seeped into these vertical and horizontal cracks. The processes of freezing and thawing and refreezing expanded these cracks, forming the rectangular chunks of rock we see today. While there is no longer a volcano on Mount Desert Island, it is fascinating to see evidence of the island’s geologic history right in front of our eyes.
To understand, we need to go back in time 450 million years, to the beginning of the island’s geologic history. Mount Desert Island began as a bit of land on a tectonic plate that was struck from behind by another larger piece of land. This can be compared to a pile up on the freeway, but instead of happening in seconds, this pile up took around 60 million years. Instead of resulting in wrinkled hoods, mountain ranges formed and volcanos began erupting.
Most people associate volcanoes with magma, and rocks are formed when magma cools. When magma passes through existing rock, it crystalizes and slowly cools to form an intrusion. Molten rock that comes to the surface through volcanic eruption is called extrusive rock. In the case of Mount Desert Island, the volcano had a caldera (crater) more than 10 miles in circumference. Inside the erupting volcano there was intrusive molten granite and more liquid molten rock that was extruded out from the volcano.
Eruptions helped to grow the volcano to over two miles in height by the time it became inactive. When the volcano stopped erupting, two processes began. The first process occurred when the granite inside the caldera began to cool and harden, which took place over millions of years. The second was the process of weathering and erosion, wearing away all the ash and rock from the eruptions. Weathering and erosion continue even today here on Mount Desert Island.
Throughout this time, movement of the tectonic plates continued. The collision of land masses that created the Mount Desert Island volcano continued forming the supercontinent Pangaea, where the dinosaurs roamed. Later, the movement broke Pangea apart to form the continents as they are today.
Now, finally, it is now time to answer the original question. During all this moving of land masses, erosion continued. The granite which had formed from magma had finally cooled and hardened.
Granite is a hard, brittle rock, and the movement and collision of land masses caused the rock to fracture, forming small vertical cracks. The process of erosion then wore away two miles of soil, ash, and rock. Without the weight of two miles of debris, the downward pressure on the granite was released. Even though the rock was solid, this compression and expansion caused it to fracture horizontally again, leaving small cracks.
During the ice ages, water seeped into these vertical and horizontal cracks. The processes of freezing and thawing and refreezing expanded these cracks, forming the rectangular chunks of rock we see today. While there is no longer a volcano on Mount Desert Island, it is fascinating to see evidence of the island’s geologic history right in front of our eyes.
NPS/S Stalker