America's Volcanic Past -
Maine

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Volcanic Highlights and Features:
[This list is just a sample of various Maine volcanic features or events and is by no means inclusive.]

• Maine
• Maine Regions
• Maine - Brief Geologic History
• Maine's Volcanic Rocks
• Acadia National Park
• Baxter State Park
• Lobster Lake and Seboomook Lake
• Portland Head and the Cushing Formation
• Seven Hundred Acre Island
• Vinalhaven Island

Maine's Rocks:¹
An interesting aspect of Maine's bedrock is that it records a wide array of geologic environments that have been present. All three major rock types, igneous, sedimentary, and metamorphic, are represented. Within each of these major types there is a large variety as well. Igneous rocks include both volcanic and plutonic types generated in rift environments, oceanic environments, island arc settings, and continental collision zones. Sedimentary rocks include terrestrial breccia and conglomerate with vascular plant fossils, shallow marine sandstones, carbonate reefs, anoxic black shales, variously mature and immature wackes, submarine channel and turbidite deposits, and deep sea chert. Metamorphic rocks are found in contact aureoles around plutons as well as in widespread regional metamorphic zones ranging from sub-chlorite zone in the north to sillimanite + K-feldspar zone in the southwest. Several episodes of crustal deformation have produced folds, faults, and shear zones now preserved in the bedrock. This variety of geologic features has attracted geologists with a diversity of backgrounds to study in Maine.

Maine's Granite:¹
Granite is an igneous rock that forms by slow solidification of magma at depths of several kilometers below the earth's surface. Yet, hundreds of granite bodies are exposed in Maine at the ground surface. Therefore, the gradual process of erosion has, over geologic time, removed several kilometers of rock that used to rest above what is now the Maine landscape. Maine granite comes in many colors and textures, with each quarry yielding its own variety of stone. Activity and competition in the granite industry were highest in the late 1800's, reaching a peak in 1901. Many large public buildings such as libraries, post offices, customs houses, and museums built at that time in the eastern U.S., including New York City, Washington, D.C., and Chicago are made of Maine granite. Although they are almost all inactive, the old quarries still dot the landscape, mainly in the coastal region from Penobscot Bay to Washington County.

Maine's Pegmatite:¹
As with any branch of natural history, bedrock geology is driven by an innate curiosity to know more about the world in which we live. What is it made of? How did it form? Many people enjoy discovering, photographing, sketching, or collecting rocks, minerals, and fossils. While each type of mineral or fossil can be expected from only certain kinds of rock, there is enough uncertainty in our knowledge that specific discoveries are difficult to predict. For example, the Oxford County region is well known for the occurrence of a rock called pegmatite (an intrusive igneous rock) that has yielded spectacular mineral discoveries such as tourmaline, beryl, apatite and lepidolite, but it is hard to know which pegmatite body will produce the next big discovery, or which exotic minerals will be found. Both beginners and experienced collectors who put in some time may find treasures, large or small.

Maine's Traveler Rhyolite:¹
Much of the northern part of Baxter State Park is underlain with rhyolite, a light-colored volcanic rock that is similar in composition to granite, in this case the Katahdin granite, but that is much finer grained. In many outcrops the grains are too fine to see with the naked eye. Most of the peaks (The Traveler, North Traveler, Black Cat Mountain), the cliffs around Upper and Lower South Branch Ponds, and the mountains you passed on the way into the park (Horse Mountain) are underlain with this rock. Most of the unit formed from large eruptions of volcanic ash rather than by flows of molten magma. The Traveler Rhyolite is probably 3,200 meters thick and tilts northward at a moderate angle. In a close up of the Traveler Rhyolite, the dark streaks in the rock are flattened pieces of pumice that formed as a rain of volcanic bombs that accompanied the ash eruption.

Maine's Vinalhaven Rhyolite:¹
The geology of the northern part of Vinalhaven, from Middle Mountain to Browns Head, is less famous than the Vinalhaven granite but in some ways more interesting. A variety of volcanic rocks give clues to an ancient past when this part of the earth's crust was being formed. Collectively, these rocks have been named the Vinalhaven Rhyolite, after the most common type of volcanic rock there. We know that this small part of Maine's crust was formed by volcanic activity much like that which occurs today in the Caribbean Antilles or the Aegean Sea of the northern Mediterranean. Unfortunately, the age of the Vinalhaven Rhyolite is difficult to determine, but the best estimate is that it formed in the Silurian Period, more than 420 million years ago.

There once were Volcanic Islands:¹
During the protracted geologic history that has brought Maine to the current stage, the movement of continents by the mechanism of plate tectonics has caused the earth's surface to evolve. Volcanic rocks and distinctive fossil shells show that some of Maine's rocks formed on volcanic islands in a wide ocean that no longer exists. Geologic similarities between parts of the Maine coast and parts of Newfoundland and Wales suggest that these areas formed together on a small continent far away from North America.

Cranberry Island Formation:³
About 400 million years ago, volcanoes belched out their contents of ash which came to rest on a sea bottom. During the times when seaweeds dominated the plant world, pressure and heat transformed these sediments into rocks known as the Cranberry Island formation.

Schoodic Peninsula:³
Far below the earth's surface, a huge molasses-like plug of magma at least 8 miles in diameter moved upward. And, as it undermined the overlying bedrock, the heavy roof bedrock began to sag, eventually sinking and melting into the magma. The fiery mass incorporated material from the earlier granites, from diorite, and from the weak rocks. Eventually it reached the surface, and gradually cooled to form the coarse-grained granite. After the various granites developed, other minor intrusions of molten material occurred. The most conspicuous of these consisted of black diabase dikes that spread themselves into open fractures of older rock. The most obvious examples of these formations in the park are on Schoodic Peninsula.

Mount Katahdin:¹
An area of outstanding natural beauty, Baxter State Park is home to some of Maine's most unique geology. Mount Katahdin's huge granite massif yields northward to lower peaks underlain with volcanic rhyolite and valleys underlain with sandstone and fossiliferous shale. For those willing to put in a little effort, a fabulous cross section of this geology can be toured along South Branch Pond Brook between South Branch Pond and Trout Brook.

Traveler Rhyolite:¹
Much of the northern part of Baxter State Park is underlain with rhyolite, a light-colored volcanic rock that is similar in composition to granite, in this case the Katahdin granite, but that is much finer grained. In many outcrops the grains are too fine to see with the naked eye. Most of the peaks (The Traveler, North Traveler, Black Cat Mountain), the cliffs around Upper and Lower South Branch Ponds, and the mountains you passed on the way into the park (Horse Mountain) are underlain with this rock. Most of the unit formed from large eruptions of volcanic ash rather than by flows of molten magma. The Traveler Rhyolite is probably 3,200 meters thick and tilts northward at a moderate angle. In a close up of the Traveler Rhyolite, the dark streaks in the rock are flattened pieces of pumice that formed as a rain of volcanic bombs that accompanied the ash eruption.

Trout Valley Formation:¹
The Trout Valley Formation underlies Trout Valley, the most significant valley in the northern part of the Park. It consists of a variety of rock types, including conglomerates at the base (probably of volcanic origin) followed by sandstone and finally fossiliferous shale along Trout Brook itself. The Trout Valley Formation is about 500 meters thick. The Conglomerate --- The clasts are almost all rhyolite, with some being recognizable as sections of the columns seen farther upstream. Most are sub-angular boulders with a bit of ash between, but many cobbles and boulders lay directly on others rather than being supported by the ash. A Lahar at the base of the Trout Valley Formation --- is made up of angular pieces of the underlying volcanic columns and is cemented by volcanic ash. The rocks were deposited by a lahar, a violent eruption and flow of ash that rips up pieces of the underlying rocks as it travels rapidly down a mountain slope.

Lobster Lake:¹
Along the shores of this beautiful lake northeast of Moosehead Lake is some of the most spectacular geology found anywhere in the state. In outcrop after outcrop, the shore of the lake reveals a complex geological story that begins with deep-sea sediments, is punctuated by several periods of igneous activity and folding, and ends with shallow marine sediment that is profusely fossiliferous.

Lobster Mountain Volcanics:¹
Much of Lobster Mountain on the west side of the lake and Big Island is underlain by a variety of volcanic rocks collectively called the Lobster Mountain volcanics. These are all Ordovician in age and probably represent volcanic islands created when part of the oceanic crust was subducted. Lobster Mountain itself is underlain by a light greenish gray rhyodacite: an otherwise very fine-grained rock with large angular crystals of quartz. It weathers a distinctive light gray and is well exposed on the trail to the summit that begins at Jackson Cove.

Big Island Diabase Dikes:¹
Big Island is underlain by a collection of volcanic rocks and volcanically related sedimentary rocks. These include pillow basalt, siltstone, and sandstone. Some of the sandstone beds contain fossils. Most of the prominent points and knobs on the island are underlain by diabase dikes which are more resistant to erosion. These formed from molten magma which forced its way through cracks in the overlying sedimentary and volcanic material to solidify at relatively shallow depths. A most interesting aspect of these dikes is that they have features which indicate that they intruded soft, wet sediment, not hard rock: isolated blobs of diabase are surrounded by sedimentary rock; edges of dikes are unusually convoluted. All this speaks to a very dynamic geologic environment in which sand and mud were being deposited in a marine basin as magma was forcing its way up from depth.

Seboomook Lake, Northwestern Maine:¹
The rocks of Maine record a rich history of geological events that formed our landscape. Four hundred million years ago, Maine was a vastly different place, with an ocean covering most of the northern part of the State, massive young mountains along the present-day coast, and volcanoes punctuating all. Sand and mud -- materials eroded from the young mountains -- were deposited by ocean currents in layer upon layer. Lava erupted from fissures into the seawater to form oddly shaped deposits within the sand and mud. All of these features are well exposed on Seboomook Lake, a long narrow lake a few miles northwest of Moosehead Lake.

Seboomook Lake Pillow Lavas:¹
Around the western part of Seboomook Lake there are some volcanic rocks (Canada Falls Member of the Frontenac Formation). They formed when fissures opened in the ocean floor and molten rock (lava) spewed onto the bottom of the sea. How do we know this? Within the volcanic rocks are distinctive shapes, called pillows by geologists, which form when lava is suddenly cooled by contact with cold water. The water immediately cools and hardens the outer skin of lava into a rind, but the pressure of the eruption pushes more lava out until a partially hardened blobby shape (pillow) forms and breaks off. The process continues as more lava erupts, in this way forming a deposit made up almost entirely of pillows.

Portland Head and the Cushing Formation:¹
The bedrock that is exposed to view at Portland Head (southern entrance to Portland Harbor) belongs to the Cushing Formation. This formation, named for Cushing Island, underlies an area that stretches northeasterly across Casco Bay from the Delano Park area of the Cape Elizabeth shore through Cushing Island, Peaks Island, most of Long Island, part of Chebeague Island, the Goose Islands, and Birch Island to the Brunswick shore just west of Harpswell Neck. The Cushing Formation is more resistant to weathering than the neighboring rock formations, which is why it forms such prominent rocky headlands and islands.

A close look at the rocks at Portland Head reveals clues to its long geologic history. For safety reasons, access to the rock cliffs below the chain link fence is strictly prohibited. You can see many of these features easily from behind the fence or from the trails leading to the beach north of the lighthouse. Seen best on a clean surface, the rock is a light gray gneiss composed mainly of the minerals quartz and feldspar in very small, almost microscopic grains. Elsewhere, the Cushing Formation contains larger rock fragments in addition to the minute mineral fragments. Such fragments are typical of deposits produced by a series of explosive volcanic eruptions. Certain minerals from the Cushing Formation, carefully collected, processed, selected and analyzed in a U.S. Geological Survey laboratory, have been dated at approximately 471 million years old (+/- 3 million), which is in the Ordovician Period. This means that the volcanic eruptions that produced the Cushing Formation occurred before the Himalayas or Alps had formed, before the Atlantic Ocean existed, before the dinosaurs lived, and before the first land plants had evolved. The volcanoes that must have existed near here at that ancient time have long since been eroded; all that is left is a layer of volcanic debris preserved in the rocks. At a later time, probably in the Devonian or Carboniferous periods, all the rocks of southern Maine were heated, compressed, and twisted causing the rocks to be stretched and deformed like putty. This deformation also produced the strong "lineation," or alignment of minerals, that characterizes the Cushing Formation at Portland Head. The lineation is not horizontal, but is tilted down toward the southwest at an angle of several degrees. This lineation angle is what gives the rock an overall sense of jutting out to the northeast into the sea.

Portland Head's Diabase Dike:¹
Beyond the end of the fence below the foghorn, there is a deep, steep-sided cleft in the rock. You can see the cleft from across the cove or look down into it at the fence by the foghorn. At low tide, a different type of rock is barely visible in the bottom of the cleft. It is a dark colored rock called diabase or basalt. It formed when molten rock (magma) intruded along a vertical fracture and solidified underground forming a thin sheet of rock called a dike. The dike rock is more fractured and therefore weaker than the Cushing Formation, so through the centuries the cleft has been eroded down almost to low tide level. Such dikes of black, basaltic rock are common along the Maine coast. Some are better preserved in the cliffs just on the other side of the lighthouse, toward the gift shop. Some of the larger basaltic dikes in New England have been dated at about 200 million years old, indicating that they were intruded during the Mesozoic Era at the time when Africa and Europe were splitting away from North America to begin opening the modern Atlantic Ocean.

Seven Hundred Acre Island:²
Approximately 650 million years ago: episode of metamorphism and pegmatite intrusion in unknown geologic setting (near Islesboro). Some sedimentary and volcanic rocks on Seven Hundred Acre Island were metamorphosed and cut by a pegmatite dated at 647 +/- 4 million years old. The rocks which are cut by the pegmatite must be older, but we don't know by how much.

Vinalhaven Island Granite:¹
Vinalhaven Island is a large island in Penobscot Bay, 1 hour and 15 minutes by ferry from Rockland. The central and southern parts of Vinalhaven are underlain by a massive body of pink, medium-grained to fine-grained granite. The Vinalhaven granite was quarried in the late 1800's and early 1900's as part of the famous Maine granite industry that provided dimension stone and decorative stonework for large buildings and bridges in Boston, New York, Chicago, and elsewhere.

Vinalhaven Island Rhyolite:¹
The geology of the northern part of Vinalhaven, from Middle Mountain to Browns Head, is less famous than the granite but in some ways more interesting. A variety of volcanic rocks give clues to an ancient past when this part of the earth's crust was being formed. Collectively, these rocks have been named the Vinalhaven Rhyolite, after the most common type of volcanic rock there. We know that this small part of Maine's crust was formed by volcanic activity much like that which occurs today in the Caribbean Antilles or the Aegean Sea of the northern Mediterranean. Unfortunately, the age of the Vinalhaven Rhyolite is difficult to determine, but the best estimate is that it formed in the Silurian Period, more than 420 million years ago.

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