Sleeping Giant State Park in Hamden Connecticut is an excellent place to view an example of an igneous intrusion.

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        Around 200 million years ago, at the beginning of the Jurassic period, enlarging rifts created great fissure systems across the landmass of Pangea (which, by then, was gradually beginning to break apart). These conduits allowed molten magma to flow towards the surface, where it poured out as lava over very extensive areas of land. Widespread volcanic eruptions were relatively common at this time, occurring on a global scale that has been observed in the rock record of our planet’s history on only a handful of occasions. 

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        A series of three separate lava flows—occurring over a vast geographic area and all within a period of less than a million years—flooded the Central Lowlands of Connecticut, as well as adjacent basins up and down the eastern seaboard. During the first eruption, some of the molten magma did not reach the surface, but was caught in a vast underground magma chamber.  This chamber was formed by hot molten rock intruding into the overlying sandstone, some of which was actually melted and incorporated into the magma.  The magma cooled to form a dark-colored igneous rock called diabase. 

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        Over the course of tens of millions of years, the layers of soft sandstone above the Sleeping Giant were gradually eroded away. When the diabase forming the Giant was finally exposed, it eroded at a much slower rate. It is for this reason that the now-exposed magma chamber stands so much higher than the surrounding landscape.

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