The Dikes of Odiorne Point
In New Hampshire, United States
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Chiselled by the waves and lying in depressions partly covered by boulders, several trap dikes can be explored along the beautiful shores of Odiorne State Park.
A great variety of rocks can be seen at Odiorne State Park. The oldest of these is called the lower member of the Rye Formation and is at least 460,000,000 years old. They were first laid down as flat lying muds and sands in a great inland sea during the early part of the Paleozoic Era. Over the years, these original sediments were covered by many thousands of feet of new sediments which piled on top of them in a great trough.
After several million years the area underwent a mountain-making movement, called the Acadian Revolution. As the rocks were squeezed and folded into great mountains, terrific heat and pressure resulted. Because of the great temperature and pressure, the original muds and sands were changed and recrystalized into metamorphic rock. Molten granite from great depths was squeezed into the folding rocks.
Eventually the mountain-making movements ceased. Millions of years passed while erosion, or wearing down of the mountains, took place. There was a time about 200 million years ago, during the Triassic Period, when molten volcanic material, called magma, was pushed into the rocks in the form of fine-grained black trap (basalt) dikes. The trap dikes entered along planes of weakness, cutting parallel to or across beds deep below the present rock surface. As the molten material came into contact with the cold rocks, it cooled more quickly than the magma at the center of the mass. This differential cooling resulted in fine grained borders and coarser grained centers. Erosion continued resulting in the exposure of the folded Rye Formation which is what can now be seen at Ordiorne Point.
There are several large black-trap (basalt) dikes cutting across the landscape of this location. Some are twelve feet wide, and one at the northeast end of the point near the ocean is twenty to thirty feet wide. You may observe pitting where weathering has occurred and decayed small mineral crystals consisting of feldspar or where gas bubbles in the magma later filled with calcite and weathered quickly.
Your mission, if you chose to accept it:
1) Locate the largest basalt dike and record the orientation.
2) Locate one of the basalt dikes which appears to be the skinniest and estimate the length.
3) Chose a dike and follow it towards the water’s edge. Describe how it changes.
Exploring Odiorne Point: A Guide to the Natural and Social History of Odiorne Point State Park
Edited by Juila Steed Mawson
(No hints available.)
Last Updated: on 10/8/2017 4:17:36 PM Pacific Daylight Time (11:17 PM GMT)
Coordinates are in the WGS84 datum