Report: The Geology of the Northport, Maine Amphibolite

Amphibolite / Rock #5142

Eons ago, the featured stone was part of western Europe. It was part of the European plate which, according to plate tectonic scientists, crashed into the North American plate. When this occurred, a small portion of the European plate broke off and was left stranded and attached to the North American plate when the two plates subsequently parted again. This plate movement slowly increased the distance between Europe and North America, which created the Atlantic Ocean. As these plates continued to move apart, there was a ripping of the ocean floor in the Atlantic. This explains the volcanic activity that is still going on in Iceland and the mid-Atlantic Ocean area. So, what about this stone? The featured stone is from that part of the European crustal plate which rammed into what is now Northport, Maine, where my son Josh lives and I have a small camp. This stone was part of a mountain that I had previously thought to be black granite and, while this stone is related to granite, it is, in fact, called amphibolite.

In layman terms, the amphibolite was formed by the heat and pressure generated by a continental collision between the North American and European plates. While amphibolite is related to granite, it is really quite different. It is a metamorphic rock (one created by heat and pressure) which, in this case, was granite initially but became amphibolite. It is harder, heavier, and denser than the granite from which it was formed, because of the metamorphic changes in the rock caused by the heat and pressure referred to above. And because of this, “geologists and lapidary workers have discovered that some amphibolite produces a shimmering effect when it is polished for various pieces of jewelry” [ and].

Amphibolite / Rock #5146

“Amphibolite is a rock from convergent plate boundaries where heat and pressure cause regional metamorphism” []. This metamorphosis results in a rock with a hardness between 5 and 7 on the Mohs scale. Its predominantly black color is primarily from hornblende which, in granite, is a minor component, with feldspar and quartz being more major components. The piece of featured amphibolite mostly shows black hornblende with small white/yellow/orange bits of feldspar that survived the metamorphic process. While Maine has large amounts of granite throughout the state, the appearance of amphibolite is much more limited in scope, due to the fact that its creation required one geological step beyond granite to be created, e.g., amphibolite is metamorphosed granite and other materials.

On a personal note, it is interesting that I have built upon and walked over this mountain for over 55 years and only now discovered the mountain’s gemstone hidden in plain site all along: amphibolite.