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The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Intrusive Igneous Rocks
Intrusive igneous rocks, primarily granites, make up the bulk of Mt. Desert Island. All of these rocks intruded and crystallized during the Devonian Period in a span of time from approximately 360 to 380 million years ago. The ages of these intrusive rocks can be determined in two different ways. First, since a granite can be no older than the rocks it intrudes, its maximum age can be determined if the age of the host rock is known. In this case fossils in rocks related to the Bar Harbor Formation and Cranberry Island Series indicate a Silurian-Devonian age; this, in turn, provides a Silurian-Devonian (or younger) age for the intrusive rocks (Figure 1).
The second way to obtain the age of intrusive rocks (and some volcanic rocks) is to use isotopic dating techniques. These techniques make use of the fact that naturally occurring radioactive elements (parent isotopes) decay to another element (the daughter product) at a fixed rate. By measuring the amount of daughter product in a rock relative to the parent isotope, it is possible to determine approximately how long since the intrusive rock cooled and crystallized. Based on these techniques, the best ages estimated for the granites on Mt. Desert Island range from approximately 365 million years old for the granite of Cadillac Mountain to 377 million years for the granite on Schoodic Point and 373 million years for the granite of Southwest Harbor. Thus these data indicate a Devonian age for the intrusive rocks.
Volcanic rocks of the Bar Harbor Formation on Mt. Desert Island have also been dated with isotopic techniques, and the age obtained (408 million years) agrees well with the Siluro-Devonian ages assigned to these rocks based on correlations with fossil-bearing formations elsewhere. The predominately volcanic Cranberry Island Series has been dated by this method, and the age is determined to be 378 million years.
Gabbro and diorite are both igneous rocks that are poor in silica, alumina, sodium, and potassium, and rich in iron, magnesium, and calcium. They differ slightly in their composition, but this difference is difficult to distinguish visually in an outcrop. They are frequently intermixed over short distances and cannot be shown separately on a map of the size associated with this website, hence the hyphenated term.
The gabbro-diorite (Dgd on the bedrock geologic map - pdf format), found predominantly on the northwest side of Mount Desert Island, is the oldest of the intrusive rocks. It was intruded in turn by younger granitic rocks. The gabbro is a dark gray, coarse-grained rock in which you can see individual black and gray mineral grains (pyroxene and plagioclase feldspar crystals). If the rock is lighter gray due to the abundance of light gray feldspar, it is called diorite. Some exposures show an irregular layering of light and dark colored rock, indicating that the two types are closely related in origin. Good exposures of the lighter colored diorite are found along Route 3 just west of Salsbury Cove on the north side of the island. Gabbro-diorite can also be seen in Hulls Cove, three miles northwest of Bar Harbor, and on the summit of Great Head on the east side of the island. Gabbro-diorite is also exposed on the Porcupine Islands where it occurs as thick layers, called sills, which intruded into the Bar Harbor Series.
Granite of Southwest Harbor
In contrast to the dark gray to black gabbro-diorite, granite is a light-colored rock. It is rich in silica, alumina, sodium, and potassium, and poor in iron and magnesium, the chemical elements that make up many dark-colored minerals. Small differences in chemical composition and percentages of accessory minerals can produce significant differences in color and texture of granites, however, and a number of different types can be distinguished on the island.
The Southwest Harbor granite (Dshg on the geologic map) is a fine-grained, light gray granite, commonly with a tan or pinkish hue. Outcrops are visible along the shore at Southwest Harbor and near West Tremont, but inland exposures are not abundant. Quartz and alkali feldspar are the major minerals in all granites, but due to their small size in this particular granite, the grains are usually not easily identified except with the aid of a magnifying glass. Rock similar to the Southwest Harbor granite makes up the southern end of Schoodic Peninsula and is beautifully exposed at Schoodic Point.
Granite of Cadillac Mountain
The bald ledges of Cadillac Mountain are made of a pink to greenish-gray, coarse-grained granite (Dcg on the geologic map). In contrast with the Southwest Harbor granite, individual grains of translucent, gray, glassy-looking quartz and pink or gray feldspar are easily seen. The black mineral is hornblende. The rock is uniform in appearance and you will recognize it easily as you hike the high ridges.
One of the most spectacular geologic features on the island is the shatter zone (Dsz) that surrounds the Cadillac Mountain granite on nearly all sides and ranges in width from a thousand feet to more than a mile. Within this zone, angular pieces of rock (in some cases, gabbro-diorite; in other cases, Ellsworth Schist or volcanic and sedimentary rocks from the Bar Harbor Formation) up to hundreds of feet across are surrounded by fine-grained "granite" (Figure 6).
Though the details of its formation are not fully understood, the shatter zone is undoubtedly related to the intrusion of the Cadillac Mountain granite. Close to the main body of granite, the light colored matrix that surrounds the angular pieces appears to be a mixture of ground-up and recrystallized country rock plus fine-grained Cadillac Mountain granite. Farther away, the light colored matrix is believed to be exclusively ground-up and recrystallized country rock. This suggests that the shatter zone resulted from severe fracturing of the country rock as the granite was intruded. One theory, favored by C. A. Chapman, is that the shatter zone represents the outermost edge of a collapsed central region and that blocks of crushed country rock were intruded by the magma which crystallized to become the granite of Cadillac Mountain (Figure 7).
Excellent exposures of the shatter zone occur at Otter Cliffs and at Western Point south of Black Woods Campground. It is also well exposed on both sides of the entrance to Northeast Harbor; look for it as you leave the harbor on your way to the Cranberry Isles. Other good exposures can be seen at the east end of Sand Beach and near the parking lot for the overlook at Schooner Head.
Look closely at the map in the region where the Cadillac Mountain granite is exposed, and you will see small symbols similar to those used in the Ellsworth Schist. These show the orientation of tabular shaped inclusions of older rock within the granite. Good examples of these inclusions can be found on the top of Great Hill, west of Bar Harbor. Study of these inclusions shows that they are essentially horizontal in the interior of the granite body; but, toward the outside margin they tend to be tilted downward towards the interior. This, plus evidence from the shatter zone, suggests that the magma filled the space left by the collapse of the roof above the magma chamber, a process geologists have called cauldron subsidence. As the magma crystallized, the inclusions apparently settled to the floor of the magma chamber producing a saucer-shaped arrangement as shown in Figure 7.
Granite of Somesville
The head of Somes Sound and the region to the west are underlain by a fine to medium grained, pink and gray granite, which at first glance may not look much different from the Cadillac Mountain granite. However, the Somesville granite (Dsg on the geologic map) has smaller mineral grains and a slightly different type of feldspar. The feldspar in the Cadillac Mountain granite is all pinkish perthite (greenish where fresh), whereas the feldspar in the Somesville granite is of two types: pink or cream colored K-feldspar and light gray plagioclase. Further, the black grains in the Somesville granite are biotite mica, not hornblende as in the Cadillac Mountain granite. This is not an easy distinction to make, so don't be surprised if you have trouble telling one granite from the other.
The map shows a separate type of granite in the center of the Somesville intrusion (Dsg1). This fine-grained granite is probably closely related to the medium-grained granite that surrounds it. The difference between these two types is rather subtle but can best be seen at exposures along Route 102 south of Somesville. Here, north of Echo Lake, the ledges along the road are of the fine-grained type, and many show abundant phenocrysts (larger grains that are set in the fine-grained host rock). Farther south along the road, particularly near the southernmost of the two roads to the village of Hall Quarry, the granite is coarser grained and has no phenocrysts.
The relationships between the two varieties of the Somesville granite are not yet well understood, but it appears that the fine-grained variety is younger than the medium-grained type, and may even be a separate intrusion.
There are several other examples of granite on the island; all are pink, fine to medium grained, and contain black flakes of biotite mica. Examples of these granites are well exposed at Baker Island (Dg1) and Bass Harbor Head (Dg2). The Baker Island granite is distinctive because it has a faint "grain" or alignment of darker minerals in a northeasterly direction. The granite at Bass Harbor Head (also exposed at Wonderland) has a distinctive fine-grained granular texture similar to a lump of sugar.
These dikes are composed of a fine-grained intrusive igneous rock called diabase and formed when magma was forced into fractures in the host rock. Diabase is very closely related to the black volcanic rock called basalt, and you may see black dikes referred to elsewhere as basalt dikes. Diabase and basalt are also very closely related to gabbro. Chemically, the three rocks are similar, but they represent different modes of origin. Whereas basalts erupt onto the surface, diabase dikes represent magma that did not reach the surface, but cooled quickly in fractures some distance below the surface. Diabase dikes are "transitional" between the deep-seated gabbros and extrusive basalts.
Most dikes in the park are less than ten feet wide, but some exceed 60 feet. Not all of these dikes formed at the same time, as evidenced on a small scale at Schoodic Point where dikes of a younger age cut across older ones. On a larger scale, an interesting relationship is found if the directions of the dikes are compared with the type and age of the rocks through which the dikes cut. Such a study reveals three general trends: (1) 45 to 75 degrees east of north, (2) due north to 30 degrees east of north, and (3) 10 to 30 degrees west of north. The first group (trending northeast to east-northeast) is found in rocks believed to be older than the Bar Harbor Formation and is, therefore, the oldest group of dikes. The second group (north to north-northeast trending) is found in all rocks except the Cadillac Mountain granite and the Somesville granite and therefore must have been intruded before these granites, but later than the gabbro-diorite and the Southwest Harbor granite. The last group (north-northwest trending) is found in all rock types and therefore must be younger than all of the other rocks of the area.
This study of the relative ages of the dikes presents an interesting contradiction. On one hand, the Cranberry Island Series is believed to be younger than the Bar Harbor Formation on the basis of radioactive dating. On the other hand, an abundance of east to northeast trending dikes in the Cranberry Island Series suggests that it is the older of the two series. Clearly the geologic history of the area is not completely understood and resolution of the problem will require more study.
Of the numerous places dikes may be found, two localities are especially good: Schoodic Point (to be discussed later) and Cadillac Mountain. At the hairpin turn 1.8 miles from the entrance gate on Cadillac Mountain Road, two five-to-ten foot wide dikes can be found cutting through the granite. This is a hazardous curve, but there is a small parking area just up the hill from the turn. These dikes are probably extensions of the ones that can be seen along the trail on the south side of the mountain that leads to Black Woods Campground. Exposures of the dikes along this trail are the best of any along the high ridges and will provide a hiker with excellent opportunities to study their features. The dikes can be traced from the south side of the road at Blue Hill Overlook to beyond the rock knoll just south of where the trail crosses the Canon Brook trail. In fact, the marker at the trail intersection is placed in granite with black dikes just a few feet away on both sides. Three major dikes and several smaller ones can be found on the rock knoll; in places they merge. The details of their margins are quite complex and tracing out their geometry should provide a challenge for the curious hiker.
Last updated on January 11, 2008
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