Welcome to Rock of the Month Club!
We are delighted to offer this space to explore geology and celebrate rocks. This page is an accompanying space to our monthly rock displays created and curated by local geologist Bob Stewart.

This month’s display features rocks and minerals crystallized from melts (magma) that cooled and solidified at depths of a few tens to several hundred kilometers below the Earth’s surface.  The hot magma bodies are less dense than the enclosing older rocks and thus rise through the latter until solidifying as plutons, which consist of granites and other rock types.  Plutons vary in size from 10’s to 100’s of square kilometers, known as stocks, to many 1,000’s of square kilometers, known as batholiths. 

Intrusive igneous rocks crystallize slowly over hundreds to thousands of years or more, which allows large crystals to form.  The minerals that make igneous rocks reflect the chemical composition of the magma, which is commonly rich in silicon, aluminum, iron, calcium, magnesium sodium and potassium, plus other trace elements, as well as gases such as carbon dioxide, fluorine and water vapor.

All the specimens in this exhibit were transported to Wellfleet by glaciers, except as noted in the rock descriptions.  The large sheet of mica was likely mined from a pegmatite, which formed very slowly from a granitic magma rich in water and gases.  Pegmatites are common in New England and historically were mined for mica used as a heat-resistant glass in ovens.  During the cold war, pegmatites were mined for the mineral spodumene (LiAlSi2O6), an ore of lithium.  Lithium has two stable isotopes, lithium-6 and lithium-7, and the lighter isotope, lithium-6, is used as a fusion fuel in hydrogen bombs.  Non-military uses for lithium include batteries for electronic devices, electric vehicles, and some non-rechargeable batteries.

Questions or comments?  Email Bob Stewart – iceagestewart@gmail.com

PAST DISPLAYS

QUARTZ

The Foundation of Wellfleet and Cape Cod

* Did you know –

1.Quartz makes up 90% of beach sand in Wellfleet and most of Cape Cod.

2. The hardness of quartz is 7 (Mohs hardness scale), greater than most other rocks and minerals on Cape Cod beaches, which is why it is so widespread.

3. Pleistocene glaciation created Cape Cod, and the outwash plains that make up Wellfleet and the outer Cape consist mainly of quartz sand. The outwash plains were deposited by braided streams of meltwater running off the glaciers.

4. The quartz-rich glacial deposits were derived from rocks from the Boston area, the Gulf of Maine, Maine itself, and northward.

5. All the specimens in this exhibit were transported to Wellfleet by glaciers, except the quarried specimen from Lantern Hill, CT.  Before it closed in the 1990s, the Lantern Hill quarry was a source of high-purity quartz that was crushed and sized for use as a filter sand in swimming pools and municipal water and wastewater treatment systems, among other uses.

Questions or comments?  Email Bob Stewart at iceagestewart@gmail.com

References:

Mineralogy and provenance of Pleistocene outwash-plain and modern beach sands of outer Cape Cod, Massachusetts, USA  Marine Geology Volume 130, Issues 1–2, February 1996, Pages 121-137

The Late Quaternary Construction of Cape Cod, Massachusetts: A Reconsideration of the W.M. Davis Model. 1996. Geological Society of America Special Paper.  Edited by Elazar Uchupi, G.S. Giese, D.G. Aubrey, and D.J. Kim.

Crosby, W. O. 1879.  On the Occurrence of Fossiliferous Boulders in the Drift of Truro on Cape Cod, Mass. Boston Society Natural History, Proceedings, Vol. 20, p. 136-140.

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