Quick Refresh on types of rocks:
Igneous Rocks Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things: composition (what they are made of) and texture (how big the crystals are).
Sedimentary Rocks In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks:
Metamorphic Rocks The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.
Virginia has a long history of eruptions and volcanic activity, particularly the basalt flows in the Triassic basins east of the Blue Ridge and similar-aged dikes in the Shenandoah Valley. The highest spot in Virginia, Mount Rogers, is rhyolite that erupted 750 million years ago. The Catoctin basalts flowed over the core of the Blue Ridge about 575 million years ago, and are now well-exposed in Shenandoah National Park.
However, Virginia also has two very young volcanos, Trimble Knob in Highland County and Mole Hill in Rockingham County. Young is a relative term - Mole Hill is 48 million years old, and Trimble Knob is 35 million years old.1 They were created in the Eocene Epoch, at a time when tectonic shifts may have created a temporary weak zone in the middle of the normally-quiet North American Plate.
Those are not the only two locations where recent volcanic activity can be documented. Outcrops similar to Trimble Knob splatter the geologic map of Highland County, and a 12-inch layer of lava was intruded into the limestone at Natural Chimneys at roughly the same time when the Mole Hill diatreme formed.
Mole Hill appears to be "diatremes," formed when a shaft of magma intercepted shallow groundwater. Near the surface, the water flashed into steam and erupted through the overlying sediments, creating a thin volcanic tube in the ground. Today, we do not see the original surface where the volcanic rock erupted. Over the last 35-48 million years, 1,100 feet or more has eroded away.2 In Canada, Wyoming, and Arkansas, some diatremes include kimberlite "pipes" with diamonds that formed deep at the crust-mantle boundary, perhaps 500 million- 1 billion years ago.3
forest-covered Mole Hill, west of Harrisonburg (Rockingham County)
Source: US Geological Survey (USGS) National Map
distinctive geology of Mole Hill, west of Harrisonburg in Rockingham County
Source: Virginia Department of Mines, Minerals and Energy,
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1. Look at the rocks around you, large and small. What are their colors and textures?
2. Which of these rocks do you think are volcanic(Igneous)?
3. Do you notice a difference in vegetation from the land around you and the vegetation on the hill? Why do you think this happens?
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