By Duane and Ruth Braun
In this article we will explain and explore geologic features on Mount Desert Island and other nearby locations. Following newsletter articles will be devoted to a geologic feature – how it relates to Maine’s geologic history, what to look for, and how it can be interpreted.
Let’s start with a short synopsis of Maine geologic history.
Maine did not exist 500 million years ago (mya). At that time, the edge of the North American continent (called Laurentia) was basically the Canadian border. Over the next 100 million years four terranes (an island arc, Gander, Avalon, and Maguma) were plastered on to this edge and formed Maine.
To begin the story, we must go back a billion years ago when all the continents were concentrated around the South Pole as supercontinent Rodinia.
By 500 mya the supercontinent had split up and Laurentia had moved northward toward the equator. About 480 mya an island arc collided with the edge of Laurentia forming the western part of Maine. Thirty million years later (450 mya) a slice of continental material called the Gander terrane collided with the Laurentian edge and most of Maine was formed.
The Gander terrane was originally part of the large southern continent of Gowanda. Gander broke away from what is now South America (Venezuela) and moved north across an ancient sea to collide with Laurentia. The collision zone (active volcanism) was in northern Maine. MDI and the rest of today’s coast of Maine were on the trailing edge and experienced little activity.
In another 30 million years (420mya) another slice – the Avalon terrane – collided with Laurentia. The coast of Maine and MDI were now in the collision zone. The collision produced enough energy to form a series of volcanoes along the present coast of Maine. MDI was the largest one. These were active for several million years.