O Where, O Where Are The Volcanic Rocks?

Posted by on November 9, 2010 in Igneous Rocks | 1 comment

One of the most common questions I receive is “Were there ever volcanoes in York County?” After all, today we think of active volcanoes in the states of Hawaii, Alaska and Washington, as well as Iceland.

What you have to realize is that our part of the country has been through many changes over geologic time. One of the earliest geologic events that occurred here were volcanoes. Approximately 600 million years ago, we were not residing on dry land. York County was located on or near the boundary where an ancient continent known as Rodinia was splitting apart. This was similar to today’s mid-oceanic ridge, the longest mountain range on Earth, found in the Atlantic and Pacific oceans. Iceland, which made news early this year for its tremendous volcanic eruptions, is only the top of a volcano whose base lies some 30,000 feet under the Atlantic Ocean. Rodinia was also located well south of the equator.

Today, there are four areas where you could go to see volcanic rocks. A greenish rock that produces a dense sound when struck with a hammer is known as metabasalt. Today, basalt originates as lava on the ocean floor where two crustal plates are pulling apart.

Ancient sea-floor volcanic rocks can be seen at several areas in York County. The largest is at Accomac, Hellam Township. The large roadcut on the south side of Accomac Road near the Accomac Inn is composed of basalt. A similar rock can be seen in several roadcuts along Beaver Creek Road near the York-Adams County line in the Pigeon Hills. The metabasalt in the Pigeon Hills shows some cavities in the rock that were once gas bubbles in the lava.

Metabasalts along Accomac Road

Vuggy metabasalt along Beaver Creek Road

Basalt can also be observed along the York County Heritage Rail Trail between Glen Rock and New Freedom. The best location is found just north of a picnic rest area north of Taylor Hill Road in Shrewsbury Township. Here , similar to the basalt in the Pigeon Hills, you can find preserved gas bubbles. The fourth location and the most obscure is found adjacent to the Holtwood Dam spillway in Lower Chanceford Township. Here the basalt is similar in appearance with the surrounding rock known as schist. However, careful observation will show that the basalt is greenish in color and slightly more dense than the surrounding schists.

Metabasalt showing gas bubble cavities along the York County Heritage Rail Trail

Another volcanic rock in York County is known as rhyolite. Similar in composition to the more-famous rock known as granite, rhyolite is associated with the breakup of continental crust (remember basalt is associated with the rifting of oceanic crust). Since the rhyolite is about the same age as the basalt (about 600 million years old), this rock is directly associated with the splitting of the Rodinian continental crust. Rhyolite is present on the property of the Boy Scouts of America Wizard Ranch north of Accomac, Hellam Township. Rhyolite is also found at the southeastern corner of Rocky Ridge County Park in Hellam Township. The rock is a pale reddish or pink color and, like its sister, basalt, produces a dense ringing sound when struck with a hammer.

The rhyolite once was quarried as an ornamental stone just north of Highmount in the Hellam Hills. Although the quarry is no longer present, pieces of rhyolite can be seen scattered in the fields along Deer Forest Road.

If you read any geologic literature concerning our basalt and rhyolite, you will see “meta” placed in front of the words. Because our area has been through at least two major mountain building events (to be discussed in a future post), these rocks have gone through much heat, pressure and burial. As a result the mineralogy and/or texture of these volcanic rocks have changed.

Finally, you will be surprised to learn where the most volcanic rocks in Pennsylvania occurs. The oldest rocks in South Mountain of neighboring Adams, Franklin and Cumberland counties are metabasalts and rhyolites similar to the above mentioned. It is believed that South Mountain was a major rift of Rodinia about 600 million years ago.

1 Comment

  1. Jeri – any chance of Obsidian showing up in Central PA. We live in Bowmansdale, PA, and recent heavy rainstorms have washed up 2 pieces of what looks like obsidian rock. One small, one larger. Any input? Thanks

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