Volcanic History during the Jurassic Period in New Hampshire
Volcanic Rock In New Hampshire
Volcanic rocks, not abundant in the state, are only found associated with the granitic rocks of the southern White Mountains.
The volcanic rock at The Flumen and Sabbadary Falls is of the same period as the volcanic rock at the ring dike at Ossippee.
The yellow blobs in a straight line trending north and south are where the volcanics are in New Hampshire on the bedrock map.
The Top of the Flume Where Basalt Can Be Seen
Geology of The Flume
The Flume is a natural gorge extending 800 feet at the base of Mount Liberty in Franconia State Park, on I-93 in New Hampshire. The walls of Conway granite rise to a height of 70 to 90 feet and are 12 to 20 feet apart.
Formation of The Flume
Nearly 200 million years ago in Jurassic times, the Conway granite that forms the walls of the Flume was deeply buried molten rock. As it cooled, the granite was broken by closely spaced vertical fractures which lay nearly parallel in a northeasterly direction. Sometime after the fractures were formed, small dikes of basalt were forced up along the fractures. The basalt came from deep within the earth as a fluid material, and because of pressure, was able to force the Conway granite aside. The basalt crystallized quickly against the relatively cold granite. Because of this quick cooling, the basalt is a fine- grained rock. Had this material ever reached the surface, it would have become lava flows.
Erosion gradually exposed the dikes. As the overlying rock was worn away, pressure was relieved and horizontal cracks developed, allowing water to get into the rock layers. The basalt dikes eroded faster than the surrounding Conway granite, creating a deepening valley where the gorge is now.
The gorge was covered by glaciers during the Ice Age, but the ice sheet did not greatly change the surface. It partially filled the valley with glacial debris and removed soil and weathered rock from the vicinity. After the Ice Age, Flume Brook began to flow through the valley again.
The highly fractured granite and basalt have been eroded by frost action as well as by the brook’s water. As you walk through the Flume, look at the floor of the Gorge and you many notice remnants of the main basalt dike, and on the walls of the gorge, small trees are growing. Erosion is still occurring (modified from http://www.nhstateparks.org/explore/state-parks/flume-gorge.aspx).
Definition of "Flume"
A “flume” is a geological feature formed when a plug of basalt is worn away from between two walls of harder granite—the result is a torrent of water in a steep gorge.
Ice climbers in The Flume
Basalt rock
Clear description of igneous rock types: http://itc.gsw.edu/faculty/tweiland/igrx.htm
Diabase dikes on Schoodic Peninsula are similar to the diabase dikes in the Conway Granite of Sabbaday Falls
Basalt: an extrusive igneous rock
Diabase
The Difference Between Basalt and Diabase
Diabase is actually a variety of gabbro which consists mainly of labradorite feldspar, augite, magnetite, and olivine. The grain size of diabase is on the smaller side. Diabase and gabbro have the same general mineral composition as basalt, but they have a larger grain size.
Basalt is identical to diabase and gabbro, but is very fine-grained. Basalt is from a melt that cooled very rapidly- in other words, on the surface of the earth or in the ocean. When basalt is broken, it leaves very sharp edges. The finer the grains, the sharper these edges can be-- with the most extreme being obsidian (volcanic glass), which can form from either basaltic or rhyolitic lavas.
Aphanitic (a = not, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallizes rapidly on or near the Earth's surface. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. An example of an aphanitic rock is basalt.
Image of a boulder in The Flume which has since fallen down from erosion from a large rainstorm.
Description of Sabbaday Falls
Sabbaday Falls has two sections of waterfalls. The top is a large cascade and the bottom section takes a right angle through a slot in the rock. The waterfalls show this is a site where mass wasting or weathering and erosion by water is still taking place from the time of the melting of the glaciers.
Three tiers of waterfalls at Sabbaday Falls
General description and photos of Sabbaday Falls
Dark dike intruded into granite at Sabbaday Falls
Geologic setting of Sabbaday Falls
At Sabbaday Falls off the Kancamagus Highway in New Hampshire, a diabase dike intruded into the Conway Granite and has been subsequently eroded by large volumes of water melting from the continental glacier that covered the White Mountains. Erosion continues today as the Sabbaday Falls continues to flow over and erode the basalt dike deeper and deeper forming the narrow gorge or flume. Sabbaday Falls is formed similarly to The Flume in Franconia Notch.
Conway Granite
Conway granite formed during the Jurassic period when blobs of magma, or plutons, pushed up through the crust but cooled below the surface forming the Conway Granite. Conway granite as it's name implies can be found in Conway New Hamphsire. Conway Granite can also be found in Franconia Notch. It is the cliff face of Cannon Mountain where the Old Man of the Mountain fell down. The Old Man of the Mountain was made of Conway Granite. Granite generally contains the three minerals of quartz, mica and feldspar. The feldspar in this case is pinkish/orange, or salmon in color, which is what distinguishes this kind of granite from others.
Diabase
Diabase is usually found in smaller, relatively shallow intrusive bodies such as dikes and sills. Diabase dikes occur in regions of crustal extension. Diabase while mafic and dark in color shows some crystal formation which makes it different from basalt.
Dikes
Dikes are an intrusion into an opening or fissure; dikes are always younger than the surrounding bedrock. Dikes can vary in texture and composition, but most are fine grained and diabase or basaltic in nature. Dikes are generally associated with volcanic activity and therefore dikes are found radiating outward from a volcano.
Diabase dikes in New England
Most diabase dikes are found east of central Vermont, where they intruded in northeast-trending fractures during Late Triassic to Early Jurassic time.
Extensional stress directions in the New England crust can be inferred from dike trends. The dikes may delineate extensional fracture zones.
The Role of Melting Glacial Water
Large volumes of melting water from the last ice age carried rocks and sand that eroded the softer basalt and scoured the granite bedrock forming pools at both Sabbaday Falls and The Flume.
Human history of Sabbaday Falls
The falls were once known as Church’s Falls, named after Frederic Church, who was a landscape painter. The area is now known as Sabbaday Falls. The name was allegedly coined by some road builders who stashed their tools near the falls, so they wouldn’t have to carry them home at the end of the building season and could return to them the next year. Before leaving on a Sunday morning they named the area for the “Sabbath Day” or shortened to “Sabbaday”. Later, the Shackford family had a house nearby after the Civil War. They housed guests visiting the White Mountains and ushered them to popular spots, such as these falls.
