The Shatter Zone
Early this morning I ventured out to collect photos at Little Hunter's Beach in Acadia National Park in preparation for our first Nature Links geology class. Geologists from all over the world come to this special place to study a phenomenon known as the "Shatter Zone." To a geologist, rocks tell stories that span hundreds of millions of years. The story of the Shatter Zone goes like this:
500 million years ago, rivers carried sand and volcanic ash to the ocean, where they settled at the bottom. Under pressure, these deposits became a sedimentary rock layer that further metamorphosed into what we now call Ellsworth Schist, the main bedrock of this area. (This was a very slow process - the sedimentary rock built up by one inch every 100 years!) Later on, intrusions of magma broke up the bedrock, creating a sort of soup of molten granite and suspended chunks of schist (some shards remained intact, some melted, and some warped). The whole mix cooled and solidified underground, to be later revealed on Mount Desert Island's eastern shores by surface erosion from waves, wind and glaciers.
The Shatter Zone at Little Hunter's Beach off the Park Loop Road in Acadia National Park.
A close-up of the Shatter Zone. Sedimentary rock (the gray striped rock) warped by molten granite (lighter-colored rock) - almost looping back on itself!
The Shatter Zone can lead the amateur geologist into many investigations - the story I just told leaves out a lot of detail! In today’s Nature Links class, we decided to take a closer look at granite, one of Maine’s most iconic coastal rocks.
All rocks are made of combinations of minerals. Minerals are combinations of elements that organize in a lattice structure - or, in other words, all minerals are crystals. Granite is one of the best rocks for observing mineral composition. Because it is formed from magma that cools slowly underground (an "intrusive igneous" rock), larger crystals have time to form. This is why granite appears so coarse and its minerals are visible to the eye.
Challenge: Find a piece of pink granite and try to distinguish its different minerals! The black flecks are composed of hornblende or mica, the pink parts are feldspar, and the shiny white parts are quartz.
The final geological event we discussed as a class was the succession of ice sheets that covered North America during the past 2-3 million years. The glaciers that shaped Mount Desert Island as we know it occurred around 18,000 years ago - a blink of the eye in geological time! These ice flows carved out the mountains, valleys and lakes of the island, as well as Somes Sound, a deep body of water that nearly splits the island in half. Take a look at a topographic map of MDI, and it almost appears as though a giant hand raked the island from top to bottom, leaving u-shaped valleys between parallel mountain ridges.
The glaciers also explain another common phenomenon in the area: Have you ever seen a large granite boulder perched on the shore or tucked away in the woods and wondered how it got there? As glacial ice flows melted, rocks trapped in the ice were randomly deposited. These rocks are known as glacial erratics. If you feel inspired to begin your geologist’s journey and seek out strange new rocks, check out this summer hiking challenge offered by Blue Hill Heritage Trust called Beautiful Boulders!
Bubble Rock, a famous glacial erratic in Acadia National Park.
Below are some additional photos I took at the Shatter Zone at Little Hunter’s Beach in Acadia National Park. There are 2 glacial erratics represented in these pictures - can you find them? (Hint: They are both made of pink granite!)
