"Rock of the Month # 130, posted April 2012" ---
These views, and the associated samples, illustrate a segment of the southern shore of the Bay of Fundy known as the Digby Neck, a series of promontories and associated islands extending from the sheltered harbour of the Annapolis Basin southwest to Long Island and Briar Island. These samples are from a small private property southwest from the Prim Point lighthouse, which stands upon the western side of the entrance to Digby harbour, safeguarding the many vessels that pass, including the ferry that plies the Bay from Digby north to St. John, New Brunswick. The lighthouse, like most of the shore here, is underlain by basalt flows that vary in their grain size, internal textures, mineralogy and degree of post-crystallization alteration.
Three basalt samples from the study area, although texturally distinct, share a common mineralogy. The mineralogy of the basalt can be summarized as follows, while noting that mineral proportions do vary from one sample to the next. In decreasing order of abundance, the basalt contains four main magmatic minerals, glass, and alteration minerals formed during or after the cooling of the magma, as follows:
The "spine" of the Digby Neck is composed of a prominent rock formation known as the North Mountain Basalt. The basalt outpourings represent an extensive volcanic episode that occurred 200 million years ago. This "LIP" (large igneous province) formed during rifting in a precursor event to the opening of the North Atlantic Ocean, which later saw similar basaltic magmatism in northwest Scotland, northern Ireland and coastal Greenland approximately 61 to 52 million years ago, and most recently the birth of Iceland, which has grown appreciably in the past 7 million years, with some 35 volcanoes active in the past 10,000 years alone.
Some relevant features of the North Mountain Basalt include:
Selected Recent References
Kontak,DJ, De Young,MYDW and Dostal,J (2002) Late-stage crystallization history of the Jurassic North Mountain basalt, Nova Scotia, Canada. I. Textural and chemical evidence for pervasive development of silicate-liquid immiscibility. Can.Mineral. 40, 1287-1311.
Webster,TL, Murphy,JB and Gosse,JC (2006) Mapping subtle structures with light detection and ranging (LIDAR): flow units and phreatomagmatic rootless cones in the North Mountain basalt, Nova Scotia. Can.J.Earth Sci. 43, 157-176.
Kontak,DJ (2008) On the edge of CAMP: geology and volcanology of the Jurassic North Mountain basalt, Nova Scotia. Lithos 101, 74-101.
Kontak,DJ and Dostal,J (2010) The late-stage crystallization history of the Jurassic North Mountain basalt, Nova Scotia, Canada. II. Nature and origin of segregation pipes. Can.Mineral. 48, 1533-1568.
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