Northern Virginia is moving towards China, at the same speed your fingernails and hair is growing. The North American and Eurasian continents are drifting together, independently of the political systems that come and go on the surface of the earth.
How did the rocks of Northern Virginia get here, and why are they headed west?
Northern Virginia, like the rest of the planet, condensed into a planet about 4.5 billion years ago. The heavy elements such as iron and nickel sank to the center of the planet, leaving lighter elements such as silicon and calcium near the surface. As the earth cooled, a crust formed that today varies between 3-30 miles thick in different places - read Science Daily description.
The crust is formed of lighter elements, but these are not mixed evenly on the surface of the earth. Magma has erupted/oozed from below, cooled into solid rock, then been pushed down again towards the center as continental "plates" have been pushed underneath each other.
Over the years, lighter silica-rich rocks have been squeezed out of the mix and concentrated into continents. As the plates were remelted, the lighter elements and minerals stayed near the surface and clumped together into continents - Africa, South America, Antartica, etc. Think of these continents as Cheerios floating in a bowl of milk, bouncing together as the molten rock currents swirl below in the mantle beneath the crust.
All of the rocks in Northern Virginia have been recycled through this melting process. No Northern Virginia rocks are 4.5 billion years old. The minerals in our rocks have been remixed and remelted at least once since the earth originally developed a hard rock crust. The oldest rocks east of the Blue Ridge and north of the Rappahannock River cooled into their current solid form "only" about 450 million years ago.
Below tall mountains, the earth's crust is thicker; mountains have "roots," while the bottoms of the oceans are the thinnest crust. Under Northern Virginia, the crust will be thinner than below the Himalaya Mountains, but thicker than below the ocean. The oldest rocks in the Blue Ridge on the western edge of Northern Virginia are about 1 billion year old granites, formed when silica-rick rocks melted and then cooled.
In contrast, oceans have a thin crust formed primarily of iron-rich basalt rocks. The basalt that forms the floor of the Atlantic Ocean, from the east coast of Maryland/Virginia to the coastline of France/Spain, is relatively young. That oceanic bottom basalt just east of Ocean City, Maryland did not solidify into its current form until 200 million years ago.
Why the age difference? Molten, iron-rich basaltic rock is emerging (from perhaps as far down as the mantle) at the Mid-Atlantic Ridge, where it solidifies. As more molten rock emerges from the crack in the ocean floor, the recently-cooled solid rock is pushed to the side. The engine pushing Northern Virginia towards China is the push from the Mid-Atlantic Ridge.
Most of the ridge is under the surface of the Atlantic Ocean, but Iceland straddles it. The geothermal heating systems of that country tap into the same heat source that is pushing Northern Virginia towards China, and driving continental plates to drift all over the earth. The youngest rock near Northern Virginia emerged at the Mid-Atlantic Ridge just a few moments ago... and pushed us a little further to the west.
Half of Iceland is being pushed westward towards China. That half is travelling together with the bottom of the Atlantic Ocean between Iceland and North America, plus the North American continent itself. The ocean floor and the North American continent are joined together in one "plate" that essentially floats on the molten mantle underneath the crust. The western edge of the North American Plate is at the Pacific Ocean coastline of washington, Oregon, and California. (A portion of California, on the west side of the San Andreas fault, actually belongs to a different plate.)
Look at plate boundaries in A Cartoon Guide to Geology, by Callan Bentley at Northern Virginia Community College.
C.M. Bailey & Chad Roberts at the College of William & Mary have created a rich website on Virginia geology. Refresh your earth sciences expertise and check out:
- Simplified Geologic Map of Virginia
- A Brief Tectonic History of Virginia
Similarly, L.S. Ficher and Steve J. Baedke at James Madison University developed a great description of the Geological Evolution of Virginia and the Mid-Atlantic Region. Check out:
- Structure of the Earth (just to understand the context... Northern Virginia is a minor blip on the outer edge of a very complex planet)
- Plate Tectonic Theory: Plate Boundaries and Interplate Relationships (The Mid-Atlantic Ridge is a "divergent plate boundary/rifting center," as shown in the middle of the graphic. Northern Virginia is located on the edge of a continental "craton," which is a more technical term for stable continental crust than "Cheerio.")
The billion year old rocks on the western edge of Northern Virginia are igneous (once-melted) granites, located in the core of the Blue Ridge. These rocks are believed to have been formed (i.e., cooled into solid form from a liquid state) when two continents collided in the "Grenville Orogeny." (Orogenies are mountain-building events). The Grenville Orogeny pushed up a major mountain range, comparable to perhaps the Alps or Andes. The eroded roots of those mountains are exposed in the center of Canada today, as well as along parts of Skyline Drive in Shenandoah National Park.
After the Grenville Orogeny, Northern Virginia may have resembled Tibet or Nepal. At some point, however, the mountains eroded away, the continent split up, and an ocean formed east (in modern orientation) of Northern Virginia. The ocean is called the Iapetus Ocean, because in Greek mythology Iapetus was the father of Atlantis. (As you will see in a moment, the Iapetus Ocean will disappear as continental plates shift around, but later a younger ocean - the Atlantic - will form.)
About 400 million years ago, the "Cheerios" of crust collided again. Island arcs like Japan bumped into what is now the eastern coast of North America in the Taconic Orogeny, pushing up mountains again. Underneath those mountains, crust melted to form more granite. That ancient granite rock is exposed now at the Vulcan "Graham" quarry, where Route 123 crosses the Occoquan River. You can also see it at Lake Jackson in Prince William County. This 400 million year old rock dates from the era when plants first appeared on land.
Between 400-200 million years ago, two island arcs and then the continent of Africa were pushed into North America. The orogenies are called the Taconic, the Acadian, and finally the Alleghenian orogeny. In the course of the collisions, the soft muds and sandy sediments that had accumulated on the shoreline of the Iapetus Ocean were scrunched together, folded, and metamorphosed into hard rock. Some sand grains were melted into blobs of silica that ended up as blobs of quartz surrounded by the metamorphosed sediments.
Those quartz nodules, especially the crystalline versions known as jasper and chert, were prized by Native Americans before the Europeans arrived. The rocks could be hammered together and split to form sharp edges. You will probably create comparable conchoidal fractures if you drop a glass Coke bottle and the thick bottom breaks. Glass bottles, like quartz, are made from silicon dioxide (silica).
Through trading networks, Northern Virginia natives also aquired rhyolite (volcanic rock) from Maryland's Blue Ridge, and copper from the Great Lakes area. Using the local rock resources, skilled artisans flaked stones to create a wide range of scrapers and points (including "arrowheads") for perhaps 15,000 years before John Smith sailed to Little Falls in 1608.
He was the first European to visit Northern Virginia. With the permanent settlement of Jamestown, the Native Americans in Northern Virginia gained steady access to iron and copper/brass tools. It was far easier to trade deer/corn for such tools. One measure of the challenge of making useful tools from local rocks: few historical interpreters become skilled enough today to demonstrate "flint knapping."
Gold also melted and flowed through the sediments, ultimately congealing into veins that were mined in Fairfax, Prince William, and Fauquier counties until the 1930's.
At the end of the Alleghenian Orogeny, Northern Virginia was in the middle of a supercontinent known as Pangaea - and elevated high in the sky in a mountain range equivalent to the modern Himalayas. Changes in oxygen levels, as mountains were pushed up, may have triggered mass extinctions. Geologists measure different eras of time based on the mass extinctions as recoirded by changes in fossils. Northern Virginia may have been in the middle of the mass die-off of many species at the end of the Permian Era.
The sediments that were scrunched by the collision of Africa with North America are now exposed in modern time. Erosion stripped away the overlaying mountains and exposed the metamorphic rocks formed 200-250 million years ago. You can see the baked/scrunched rocks at Great Falls in the Potomac River, where the Potomac River is carving away at the hard crystals formed by a continent-sized pressure cooker. The rocks have been quarried as building stone ("Rock Creek" in DC is lined with old quarries), and "Potomac bluestone" is still used for decorative walls throughout Northern Virginia.
Great Falls has been described by the US Geological Survey:1
Not very poetic, but it does explain why the falls exist. Waterfalls exist where the bedrock is harder than whatever is downstream. Falling water cuts a relatively deep hole/channel downstream, but is unable to carve an equally-deep channel into the hard bedrock. The Potomac River originally carved a channel into the Coastal Plain sediments, and the waterfall was 9 miles downstream from its current location.
The actual contact between the soft sediments and the hard bedrock is on the upstream end of Teddy Roosevelt island, between Rosslyn and the Kennedy Center in DC. The waterfall has moved in the last 20,000 or so years, as it gradually cut into the hard bedrock upstream. This migration could continue until the Potomac River etches upstream to Seneca Creek. There the bedrock changes to a softer sandstone, and the Great Falls could turn into a series of rapids without dramatic individual drops .
Read The River And The Rocks: The Geologic Story of Great Falls and the Potomac River Gorge
- The Falls Today
- The River And The Land
- The origin of the Potomac River valley and the carving of Great Falls
- The origin of the rocks (note that granite could have been intruded into the some of ocean sediments long before the continental collision baked the mud/sand into hard metamorphic rock)
Read, from "Geologic Map of the Potomac River Gorge: Great Falls Park, Virginia, and part of the C&O Canal National Historical Park, Maryland:"
- Landscape Evolution And Photo Gallery to see how the current hills/valleys have been carved by recent erosion (HINT: might be a good place for a field trip...)
Read Fall Line, to see how that geologic feature will affect human settlement patterns in Northern Virginia. Draw a line from Fredericksburg, through Occoquan to Georgetown in DC, and you have a rough approximation of the Fall Line... but it is really a zone rather than a line (and now you know why it is 9 miles wide at the Potomac River).
What happened to the tall mountains, the Himalayas of Northern Virginia that were pushed up at the same time the ocean sediments were metamorphosed? Water and wind etched the mountains away, rapidly in major storms and gradually at other times. In addition to the physical weathering, the crystals in the mountain rocks absorbed atoms and molecules, changed shape, and altered into forms that dissolved in water.
Between physical and chemical weathering, perhaps 25,000 feet of rock has been stripped away as the once-towering mountains eroded to the curent flatness of Northern Virginia. It took many years of erosion, and the proces continues today. The Potomac River currents that tear at the bedrock of Great Falls, and the energy of other streams in Northern Virginia, are still cutting away particles of rock and carrying them downstream.
All the rocks of Northern Virginia, not just the bedrock underneath rivers, are also decaying or weathering. The now-scrunched and metamorphosed ocean sediments are exposed on the surface today between I-95 and Greenbriar (on Route 50), Centreville (on Interstate 66), and Manassas (on Route 234). In that zone, the minerals of the metamorphosed rock are turning into clay. The decomposing rock, known as "saprolite," is normally so rotten that bulldozers can shove the earth without requiring drilling or blasting.
Grab a handfull of dirt from a construction site on the Fairfax Campus, and note the color of the soil. The saprolite often includes flakes of shiny mica, as well as clay minerals that allow you to squeeze the dirt into a sticky ball. Note the reddish color too. That comes from iron in the soil - the crust of continents is not as rich in iron as the basalts on the ocean floor, but there's plenty of iron oxidixing to rust in the soil of Northern Virginia.
Ever wonder why bricks were red? The iron is oxided in the kiln as the clay is baked. To get yellow or other-color bricks, the manufacturers can adjust the amount of oxygen in the kilns, as well as the temperature and the source of clay that is baked into brick. Of course, owners can always paint brick too, but that creates a major maintainence headache. Unpainted brick does not fade or peel, and need to be repainted.
Take a look at a sidewalk or gutter on campus, or on a city street in Northern Virginia. See any flaked or cracked places? The pieces of concrete, like natural rock, are going to move downhill, downstream, until ultimately reaching the Atlantic Ocean. A particle of sand in front of the George Mason statue next to the Johnson Center could end up at Virginia Beach in a few decades, and could be pushed all the way to the continentalshelf and then the bottom of the Atlantic Ocean. When you measure time in millions of years, solid earth can be reshaped just like a bar of soap can disappear.
Have you noticed that this story is still missing a key piece? The mountains washed away over 200 million years, but where did that rock go? Look east of Interstate 95, or west of the Shenandoah Valley, and you can see the sediments. In West Virginia, roadcuts expose the layers of sediment that piled up as the mountains eroded. Some of the rocks of the ancient Appalachian Mountains stretch all the way to Illinois.
To the east, the rocks that were once part of the Appalachians now form the Coastal Plain, the flat land between I-95 and the Atlantic Ocean. Sediments that eroded from the ancestral Appalachian Mountains, which were once 20,000 feet higher than today, stretch over 30 miles to the east.
The hard crystalline rock exposed at Great Falls lies underneath the sediments, buried too deep to make quarries economic. (In the Coastal Plan, quarries extract sand and gravel, and blasting is not required.) The Coastal Plain also includes some other sediments that were deposited from the ocean, when sea levels were higher and the Atlantic Ocean coastline was roughly the current location of I-95.
Seashells in those sediments resulted in some layers of limestone/marl in the Coastal Plain. Otherwise, Northern Virginia has only a few outcrops of sedimentary limestone, in western Loudoun County. If you visit Statuary Hall in the US Capitol, note the columns - they were carved from Loudoun's "Calico Rock," a form of limestone that resembled marble.
Outcrops of any hard rock are rare east of Alexandria, Dumfries, or Fredericksburg. The "rock" at Freestone Point (now part of Leesylvania State Park) was readily available, but the sand grains in the stone were only loosely cemented together. Further south at Aquia Creek, the builders of the Capitol and the White House found an outcrop of slightly more-solid stone that was easy to transport by boat (imagine hauling big blocks of stone over dirt roads in 8-mulepower wagons, after rain turned the roadbed to mud...). However, the stone was weak, and later covered by other materials to prevent the sandstone from flaking away.
Most of the wealthy gentry in colonial Virginia resorted to baking clay into bricks to create artificial building stones of the desired size. Under the heat of wood-fueled fires, the clay vitrified and became impervious. (Your toilet may be formed through the same chemistry.) Next time you visit Gunston Hall, home of George Mason, note what he sort of material he used. The flexibility, availability, and cost of brick has made it a popular choice for three centuries of construction in Northern Virginia.
George Washington found a very creative solution to meet his desire for building an impressive mansion that was unique in the neighborhood. He had his workmen carve the wooden exterior siding into the shape of blocks. They mixed sand into the paint, and - voila! a stone house emerged at Mount Vernon.
Read: Building Stones of Our Nation's Capital: Washington's Building Stones and Washington's Geologic Setting (just two pages are required reading, but feel free to explore the whole publication)
Northern Virginia has experienced no collisions with continents since the Triassic Era breakup of Pangaea, so the Coastal Plan had no pressure to push up mountains. The mountains were uplifted on what is now the western edge of Northern Virginia, and everything to the east of the Blue Ridge is relatively flat. Water flows downhill, so rivers and streams in Northern Virginia flow from west to east. The Potomac River reaches sea level near Mount Vernon, but tides affect the river all the way to Georgetown. Wind and tides can cause the Potomac River to have a westward current at times.
The Widewater Peninsula in Stafford County is particularly flat. However, not far away the Crows Nest area has significant "topographic relief" (the distance between a nearby high/low point). The hills of Northern Virginia may be just "bumpy" to someone familiar with the Rocky Mountains, but even a little topographic relief can have a major impact on transportation routes. After all, would you want to carry a heavy load up and down a hill, or would you be willing to go slightly out of your way to avoid that work?
Read Rail Route In Tysons An Uphill Challenge
Why the difference? Some sediments are loosely bound, and they erode easily when a stream shifts and cuts into the sediments. Other sediments are more resistant, so they form the bluffs and low ridges along the Potomac River shoreline in Northern Virginia.
The ridges/hills in Northern Virginia within the Coastal Plan reflect how different sediments were "glued together" by heat/pressure of overlying sediments, or by chemical reactions among the minerals, while the Blue Ridge mountains on the western boundary were originally pushed up in an "orogeny" when Africa collided with North America. (The Blue Ridge today has significant topographic relief compared to the Culpeper Basin because, over time, the sediments in the basin erode faster than the crystalline rocks in the Blue Ridge. Differential erosion plays a large part in shaping the topography of Northern Virginia, but the continental collisions/breakups determined which rocks would be located on the east vs. west edges of Northern Virginia.
The eroding sediments have accumulated at the trailing edge of the North American "Cheerio," as the content has moved westward in the last 180 million years since Pangaea cracked up. In contrast, sediments that wash off the Cascade and Coast Range mountains pile up on the Oregon/Washington coast are "run over" as the continent drifts westward.
Now it's time to tell the tale of the Atlantic Ocean.
The supercontinent of Pangaea broke up about 220-180 million years ago. It was not an overnight event - you can see an equivalent breakup underway now in eastern Africa, where a great rift valley in Kenya and the Red Sea are slowly splitting up Africa. A magma-spewing ridge has not formed yet in eastern Africa, but wait a few tens of millions of years and a new crack down to the mantle may be obvious.
When Pangaea split about 200 million years ago, it cracked in multiple places, like a souffle as it cools. If you don't cook souffles these days, try pulling at two edges of a block of Jello. You'll see multiple cracks form in the surface of the gelatin, until one finally becomes dominant and the block of Jello splits along that line.
As Pangea split into Africa/Europe vs. North America, the crust became thinner and thinner as the crust pulled apart. One of the cracks became deeper than the others, and intersected the molten rock underneath the crust. What we now call the Mid-Atlantic Ridge formed, but initially there was no Atlantic Ocean. The magma welled up at the new expanding ridge (or "divergent center"), pushing Europe/Africa away from North/South America. Cooling magma oozing from the crack in the crust formed the basalt (iron-rich) floor of the Atlantic Ocean. Each year, the ocean gets a few centimeters larger to the west of the ridge, pushing the North American Plate closer to China. That's why the ocean bedrock is so young, compared to the billion-year old silica-rich crust in the Blue Ridge and Canada.
Did I mention that the continental breakup created more cracks than just the one that formed the Atlantic Ocean? One crack failed to become the center of the "new" ocean, but it still is obvious on the Northern Virginia landscape. Go to Centreville on Route 29 and look west, and you'll see a vista... because there is a valley between Centreville and the Blue Ridge mountains. That valley is one of the cracks, formed in the Triassic Era when Pangaea split up, that could have become the Atlantic Ocean.
The crack in the valley west of Centreville could have become the Mid-Atlantic Ridge, if the magma upwelling had continued there. In that case, the boundaries of the continental plate would have been in a different place. Everything to the east of Centreville would have become part of Europe, and what is currently Northern Virginia would have started at Route 15. (If you like old Marlon Brando monvies, "It could have been a contenda...")
The valley in Northern Virginia formed by the thinning crust that cracked 180 million years ago is known as the Manassas Basin, or the Culpeper Basin. The crack opened up in the Triassic Era, a time when dinosaurs wandered around Northern Virginia. We find their tracks in the sandstones of various Triassic-era basins along the east coast of North America, stretching from Georgia to Nova Scotia.
The basin has a fault on the western edge, and the land dropped furthest down at the base of the Blue Ridge. (A "fault" is a line where, on either side, the earth has moved in different directions.) On the east side of the basin (marked roughly by Greenbriar shopping center on Route 50, Centreville on Route 29, and the armory near Manassas on Route 234), the land tilted as the basin formed to the west. Streams poured sediments into the low-lying area and the weight of new sediments compacted older deposits into what we see today, purple-red sandstones.
Some are used as building materials, such as the Smithsonian "castle" on the National Mall in DC. Both the Stone Bridge over Bull Run and the Stone House at Manassas National Battlefield Park used Triassic sandstone. If you look downstream from the modern Route 29 bridge over Bull Run, you can see how the layers of sediment that washed into the Culpeper Basin dip down towards the west.
The "brownstown" houses in New Jersey/New York are made of Triassic-era sandstone. The brown color develops as the iron in the sandstone oxidixes, after stones are cut and the interiors are exposed to the atmosphere.
One factor to consider when buying a home in Northern Virginia - radon. It is a radioactive gas emitted as uranium decays. A tiny percentage of uranium is mixed in with the rocks in Northern Virginia. (What may be the largest deposit of uranium in North America is located at Coles Hill in southern Virginia, near where Route 29 crosses the North Carolina border.) Accumulation of radon inside houses, where people could breathe enough to cause lung damage, seems to be a particular concern in the zone with Triassic sandstones - perhaps because the sedimentary rock is permeable enough to permit the gas to migrate. When we build air-tight houses and block the escape of the radon, we may trap enough to cause a health risk.
As the earth's crust stretched in the Triassic Era, it became thinner in the basin. Hot magma broke through the sediments and at times flowed out on the surface of the valley - just as magma erupted in what became the Mid-Atlantic Ridge. At other times, the hot magma was injected into layers of sandstone and squeezed sideways, without erupting on the surface. (Imagine using a needle to inject chocolate from the bottom into a layer cake, and having the chocolate push between the layers of cake without squirting out the top.)
Geologists refer to the magma flows that cut through rock layers as "dikes." The layers of magma that flowed sideways are called "sills." The cooled magma, known as "diabase" or "traprock," is rich in iron, and is much darker in color than the surrounding sandstones.
As modern erosion cuts through the sediments, the magma sills are exposed. The sills are slower to erode, because the diabase (an igneous rock) is much harder than the sandstone (a sedimentary rock). In addition, some sandstone next to the dikes was baked/metamorphosed into harder "hornfels" as the magma was initially injected.
The sills form modern ridges within the Culpeper Basin. Drive north on Route 28 from Dulles airport to Route 7, and the ridge on your left (west of Route 28) is a biabase sill. Commuters in Prince William County near Gainesville who drive Devlin Road between Linton Hall and Wellington will see a similar sill on their west.
At one time, that ridge was planned to be developed as a quarry. Almost all the quarries in Northern Virginia (except the granite quarry near Occoquan) are excavating the diabase and crushing it to the desired sizes, before mixing the rock with cement/sand to form concrete - or with asphalt, for road pavement.
You might think that quarries would quickly fill up with water, but in diabase quarries pumping costs can be minimal. The diabase rock is impervious (watertight) as well as resistant to pressure, which is why the material is valuable for construction. (Local sandstone, if mixed with asphalt, would crumble under the weight of tires and cause cracks in a road.) Rainwater and an occasional fault in the diabase are the primary sources of water entering a Northern Virginia quarry.
how to sort rocks into desired sizes: rock crushing equipment at diabase quarry on Route 29, western edge of Fairfax County
The diabase quarry behind Stonewall Jackson High School in Manassas is actually located on top of the dike, where molten rock spurted to the surface at what could have been near-supersonic speeds around 150-180 million years ago. The quarry owners could excavate forever into that source material, unlike other quarries where the sills are of a imited thickness. However, the costs of hauling the rock to the surface are a factor in the economics of quarry operations. Vulcan, the quarry owner, is seeking to open a new quarry near Nokesville with rock at the surface, and planning to close the existing quarry in Manassas in the next 20 years or so.
Quarries are located where the resource is located, of course. There are no diabase quarries in Northern Virginia outside the Culpeper Basin, because the diabase is not exposed near the surface in other areas.
One other key factor is quarry location is the distance to market. Diabase is a high-volume, low-value product compared to, say, computer chips or oil. Hauling large amounts of rock in dump trucks is expensive. In areas where population is growing, such as Northern Virginia, the construction industry wants to be able to buy from different quarries in order to get the most-competitive price. However, in many cases the closest quarry will get the order because transportation costs are such a high percentage of the total cost. A construction site equidistant from two quarries may be able to stimulate competition.
One advantage of the diabase quarry behind Stonewall Jackson High School in Manassas is that it is located on a rail line. The rock can be shipped to some construction sites inside the Beltway without the costs of navigating through traffic jams. Each carload of rock is the equivalent of up to 10 dumptrucks, so the cost savings are obvious.
For more, see Simplified Geologic Cross Section of the southern part of the Culpeper Mesozoic Basin at the William and Mary "Geology of Virginia" site. The green color in the graphic ("Triassic-Jurassic sedimentary rocks") equates to the purple-red sediments in the Culpeper Basin. The red color shows magma that formed both a dike and sills.
The bedrock of Northern Virginia was disturbed by volcanic dikes and sills of diabase, but never interrupted by a kimberlite "pipe." That rock, believed to be squirted up from the top of the mantle, is the source rock ("mother lode") of diamonds. Since Northern Virginia lacks the source rock, there are no diamond mines in our area. (There are some indications of ore with diamond potential in Warren County, west of the Blue Ridge... but don't quit your day job and go prospecting.)
Northern Virginia lacks coal and oil/gas resources too. It is possible that these existed at one time, then eroded away in the periods of time for which we have no remaining rocks. Gas exploration under other Triassic basins in Virginia have produced some "shows" of natural gas, but none were productive enough to develop into producing fields.
Any oil that existed prior to the continental collisions was presumably baked away in the metamorphism of that time. Since then, Northern Virginia has not had the necessary conditions for formation of oil or coal deposits.
Bottom Line: fossil fuels are not present here. Commuters in the region depend upon oil and other energy resources from outside the region. Residents and industry depend upon electricity generated by coal, natural gas, and uranium that was extracted elsewhere. In the absence of fossil fuels, and with limited hydropower and wind resources, solar and geothermal energy are the most likely local resources that might make Northern Virginia energy independent, someday.
1. America's Volcanic Past: Washington D.C., http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_washington_DC.html (last checked January 23, 2008)