Using Cereal Bowls and Car Crashes to Understand Virginia Geology

topographic view of Virginia
Source: ESRI, ArcGIS Online

Some of the hydrogen, helium, and lithium now on Earth may have been created in the Big Bang 13.7 billion years ago. All the other atoms in the earth were created since then within stars. Elements heavier than iron were created when those stars exploded as a supernova or when stars collided, with fusion forming the heavier elements.

Within the Milky Way, gas clouds distributed the atoms until gravity caused them to coalesce and trigger formation of new stars. The sun in our Solar System is 4.6 billion years old, and it is fusing helium into hydrogen. Later in its life cycle, fusion processes will create the elements with more protons in the nucleus ranging from lithium (three protons) to iron (26 protons).

Astronomers classify three populations of stars based upon the percentage of heavy elements within them. The Sun is a Population I star, relatively enriched with heavy elements. Population I stars assembled from gas clouds that were formed as older, Population II stars died and disseminated elements which had been created through fusion. The very first stars, formed after the Big Bang led to the creation of matter, are classified as Population III stars. Those oldest stars contained the lowest percentage of heavy elements because their source material was almost exclusively hydrogen and helium.

the earliest Population III protostars formed from gas clouds with a low percentage of elements heavier than helium
Source: National Aeronautics and Space Administration (NASA), NASA’s Webb Uncovers Star Formation in Cluster’s Dusty Ribbons, NGC 346 (NIRCam)

The elements within the rocks of Virginia may have been through three cycles or more of star formation, death, dispersion, and coalescing. All matter which is part of Virginia today is destined to go through yet another cycle.

In another five billion years, the sun will run out of hydrogen, become unstable, and morph into a red giant. The core will shrink into a white dwarf. In about seven-eight billion years, the gas cloud enveloping the core will dissipate and create a planetary nebula expanding into space. The elements in that planetary nebula may coalesce later into another sun or planet, repeating the cycle underway since the first stars formed perhaps 100 million years after the Big Bang.¹

elements that now form rocks in Virginia were created within stars and dispersed through space, many via plantary nebula as previous stars have died
Source: National Aeronautics and Space Administration (NASA), Helix Nebula - Unraveling at the Seams

All the atoms that form elements, minerals, and rock formations in Virginia were created before the Earth formed 4.6 billion years ago. The Earth initially formed from the gas cloud and fragments of already-assembled particles surrounding the sun. The planet was originally a molten mass, until the crust cooled and the first basalt rock crystallized. During the Late Heavy Bombardment 4.5-3.8 billion years ago, large portions of the surface of the Earth remelted due to the energy of fragments from space impacting the planet.

At the center of the earth, the temperature at the core exceeds 10,800°F At the boundary between the mantle and the outer core, the temperature is roughly 5,800°F Closer to the earth's surface, the temperature 60 miles deep is 2,400°F From that point up to the surface, the cool crust and a portion of the molten mantle form a layer known as the lithosphere. The lithosphere is fragmented into tectonic plates which move at slightly different speeds and directions. The top 20 miles of a tectonic plate, roughly, is composed of solid crust.

the center of the Earth is 10,000°F hotter than the surface
Source: Wikipedia, Earth radius

Since the original assembly of atoms to create the earth, the planet cooled and a solid crust formed on the exterior. Today, there are seven major tectonic plates of hard crust and a dozen or so smaller "chunks." They move around the surface of the globe in slightly different directions and at slightly different speeds.

Tectonic plates can move because below the crust, at depths varying from 5-20 miles, the heat and pressure are sufficient to make the rock move like honey or molten plastic. The "asthenosphere" below the crust is the upper layer of the mantle. Even where the rock in that layer has not melted, it still can flow.

Continuing geneation of heat from the steady decay of radioactive elements keeps the mantle hot enough for deeper rock to flow in convective currents up to the asthenosphere. In smaller planetoids, just the heat from radioactive uranium and thorium is sufficient to melt the interior and create a heat flow from the center to the surface.

On Earth, the flow of molten material between the core and the crust generates electromagnetic forces in space around the planet. Those forces deflect the solar wind, the plasma particles constantly streaming from the sun. On Mars and the moon, however, the core of Mars is apparently a cold and solid mass now. That plant may once have had a molten interior and electromagnetic forces around it, but after the interior of Mars "froze" the solar wind stripped away most of the atmosphere and water molecules from the surface of Mars.

On earth, motion within the mantle has been moving and reshaping tectonic plates on the crust for 3.3 billion years; no other planet in our Solar System developed a crust with tectonic plates. The process by which the initial crust cracked into separate tectonic plates, and what powers their motion, is poorly understood.

It is possible that tectonic plate movement may be related to formation of the moon. The moon was created when Earth and a Mars-sized protoplanet called Theia collided 4.5 billion years ago. Two "blogs" of thicker rock in the mantle located below West Africa and the Pacific Ocean may be remnants of the mantle of Theia. Creating places with different densities in the Earth's mantle could have affected heat flow from the hot core, spurring convection that cracked the crust into tectonic plates and still moves them.

convection flow in the mantle may be related to the collision with Theia 4.5 billion years ago
Source: Representacion posible de la creacion de la Luna (by Rufus Gefangenen)

What may have once been a homogenous crust has differentiated into oceanic crust and continental crust. Oceanic crust most closley resembles the composition of the mantle, with a high percentage of iron and manganese. Continental crust is physically lighter, with a higher percentage of lighter elements such as oxygen, silica, aluminum, phosphorous, and sodium. Those elements combine to form a variety of minerals which weigh less than the minerals in oceanic crust.

Most of the oceanic and continental crust that originally formed on the surface of the earth has cooled and been carried (subducted) back into the mantle over the last 4.6 billion years.

Oceanic crust in particular has been recycled regularly. It is rare to find any oceanic crust older than 200 million years, though the bottom of the Heroditus Basin in the eastern Mediterranean Sea may date back to 340 million years ago.

Continental crust, because it is lighter, resists being subducted and remelted at depth. Nonetheless, most continental crust has been subducted, melted, and re-emerged on the surface, transformed over time. About 35 fragments ("cratons") of the continental crust formed in the Archean eon, and now at least 2.5 billion years old, were never subducted. The Slave craton in northern Canada is oldest craton, at 4 billion years, though other cratons contain zircon grains as old as 4.4 billion years.

The rock formations in the 35 cratons have been metamorphosed by heat and pressure, but those chunks of continental crust have not melted at the edge of the mantle and been recycled into new crust. Those cratons appear to be welded to a buoyant piece of the mantle which allows them to avoid subduction in geodynamic cycles.

New pieces of continental crust have emerged from the mantle over time, as chemical and physical processes create more fragments that are rich in silica and lighter than the heavy basalt that forms the bottom of the oceans. Younger chunks of continental crust have attached themselves to the cratons to form continents. The edges of those continents may get pushed back into the mantle though the subduction process and emerge again as new fragments of continental crust, but the cratons are stable.²

Source: University of Texas at Dallas, Plate Tectonics Basics 1

The other fragments of crust that today includes Virginia has evolved dramatically over the last 2 billion years. Volcanoes have spewed lava onto the surface, oceans have deposited sediments, continental collisions have added clumps of land, and sediments have eroded from uplands to form new land at the edge of the waters (or underneath the water). About 600 million years ago, the core of North America (the "craton") was much smaller than today's continental plate.

Though the North American plate has grown in size, Virginia also gotten smaller at times. Continental collisions pushed rocks over each other and folded up Virginia like an accordion, pushing the Blue Ridge west and compressing the Shenandoah Valley. Throughout the entire time, erosion has carved into the land and ocean currents have carried sediments away.

Virginia's current topography reflects over a billion years of geologic evolution
Source: USGS Earth Resources Observation and Science (EROS) Center, National Elevation Dataset - Shaded Relief Map of Virginia

The process by which the landmass has expanded/contracted is fascinating, but geology terminology can be as challenging as computer manuals. Here's a different way to grasp the formation of Virginia's rocks and ridges:

- Imagine you have a bowl of milk in front of you.

- Add Cheerios, so you have one clump floating in milk on one edge of the bowl.

- Watch some Cheerios dissolve and sink into the milk, then add several clumps of Rice Krispies on the other edge.

- Swirl the milk a little with your spoon and watch the clumps collide together.

- Swirl the milk again. The clumps separate, but some Rice Krispies stay behind with the Cheerios lump.

That's the basic process, and it affected the entire eastern coast of North America from Maine to Florida. Geologists refer to the North American plate (the Cheerios), plus the Taconic, Avalon, and other island arcs plus Gondwana (the Rice Krispies). When all the cereal clumps were together, they formed the supercontinent Pangea.

The limestones in the Shenandoah Valley and the igneous rocks in the core of the Blue Ridge are part of the original clump of Cheerios. Much of the eastern third of Virginia was added later, when clumps of Rice Krispies smushed into the Cheerios. That part of Virginia east of Great Falls on the Potomac River to Danville on the North Carolina border is "Rice Krispies" bedrock, formed from slices of crust left behind after collisions of island arcs and North Africa with the old continental core (the Cheerios).

the bedrock of Virginia was assembled over the last billion years by accretion of major terranes
Source: Virginia Department of Energy, Mapping Seismic Hazards in Virginia and Seismic History of Virginia (Figure 6)

The crystalline bedrock underneath the more-recent limestone in Florida was once part of Africa. That bedrock was part of the African plate (one of the clumps of Rice Krispies) before it smashed into the North American plate (the Cheerios). When the continents split up and the Atlantic Ocean formed, the Florida chunk of that African plate broke off and was left behind.³

The original chunk of Cheerios is about one billion years old. It expanded gradually on the edge, as limestone deposits accumulated in the adjacent ocean, for another 600 million years. Much of central and eastern Virginia, like the Florida bedrock, has been attached to North America for only the last 300 million years.

• The Grenville Event and Virginia Geology
• The Orogeny Zones and Virginia Geology
• The Rest of the Story About Virginia Geology

Great North Mountain (Shenandoah County)

(Whew, did you look at all three? Long description, lots of words... and it's only a short, not-exactly-perfect abbreviated version. Considering that you covered 1.8 billion years in a few minutes, that's a pretty good compression ratio.) Now check out:

"Once Upon a Time..." (The Story of the Rocks in Northern Virginia)

Tectonic History of Northern Virginia

much of Virginia has been added to the core "craton" of North America over the last 600 million years, as limestone reefs grew on the continent's edge and through collisions with other tectonic plates
Source: US Geological Survey (USGS), The North American continent

the boundaries of different geological provinces were recognized by Virginia's state geologist, William Rogers, before the Civil War
Source: Huntington Library, Map of Virginia (by Jedediah Hotchkiss, 1875)