The Impact Crater
How was the crater found?
| Courtesy of Virginian-Pilot|
Identifying the impact crater has been a puzzle that has consumed thousands of hours of research. Since the crater is hidden from sight, tangled clues began to emerge during a search for water for thirsty Virginians, which slowly led to this unexpected discovery far out in the Chesapeake Bay.
Understanding the impact crater requires understanding the geology of Virginia. Virginia's Coastal Plain is a slopping series of sedimentary beds of sands and clays that were laid down in orderly fashion over millions of years. This subterranean layer cake has been studied for the past 200 years.
Within these layers lie aquifers. Nine aquifers have been identified and tapped for water for hundreds of years. It is from these aquifers that the freshwater needs of Virginians have been quenched.
After World War II, Virginia's growing population demanded more freshwater. Existing reservoirs and wells were not predicted to meet the growing demands. During the 1950s and 1960s, the state of Virginia began to sink test wells to determine the location and extent of the aquifers in the Coastal Plain.
T. Scott Bruce of the Virginia Department of Environmental Quality was on a well drilling team that in 1983 began examining the Newport News area of Virginia. The team expected to see neat layers of sand, clay, and the associated fossils in the core samples it collected at various depths. Instead, the samples showed layers that were jumbled and mixed. Figuring that these samples became mixed during collection, the core drilling team moved eastward looking for more favorable sites.
At about the same time, evidence of a bolide impact off the East Coast of the U.S. came to light. C.Wylie Poag was chief scientist on a drill ship, the Glomar Challenger. Ninety miles east of Atlantic City, New Jersey, Poag was working at a core site known as DSDP 612 . During routine examination of core samples, he began to recognize evidence of a meteorite. Analysis of the cores revealed the telltale evidence. Within the cores samples were shocked quartz, which are battered crystals that show multidirectional patterns of parallel fractures and are hallmarks of impacts from space. In addition, Poag's team detected microtektites, which are droplets of molten rock vapor blasted into theatmosphere after a massive impact. There were also microfossils from the late Eocene Epoch found in the debris of the impact indicating that the meteorite collided with the Earth 35 million years ago!
Poag reasoned that the core samples of microtektities were associated with the North American Tektite Strewn Field and that the source of these tektites was presumably a meteorite impact now buried to the west of the drilling site.
|Courtesy of Virginian-Pilot||
|Microtektites - Wylie C. Poag|
He furthered reasoned that the tektites were from an impact, but where? Not on the East Coast at the surface or on the Atlantic sea floor since these areas had been surveyed. The crater must be buried, hidden, and covered with sediments. But where? Poag began to use the USGScollection of offshore seismic reflection profiles to search for buried craters. Poag first hypothesized that the USGS seismic studies pointed to a small impact structure off the coast of New Jersey, now known as the Tom's Canyon crater. The crater is near DSDP 612, which was where he was examining the core samples at the time. Intrigued by the evidence, he learned about the work of David Powars of the U.S. Geological Survey and Bruce of the Virginia Department of Environmental Quality.
|Dave Powars and Matt Smith examine a core sample.|
Powars was part of a research team drilling cores to study the subsurface layers across Virginia's Coastal Plain. The team was gathering hard data and was systematically drilling cores from Fredericksburg to the Atlantic Ocean. T. Scott Bruce's team needed a test hole drilled. Powars's team needed the core from such a hole. The groups joined forces in 1986 and,on a sultry August night, a very special core sample was brought to the surface. This was a core sample that none had every seen before.
Powars and Bruce noted the abnormalities, which were easy to spot, in the Exmore core sample. Instead of the expected clays and uniform layers of the Coastal sediments, the teams saw a twisted and confused mix of layers. It was the most exciting thing the teams had ever seen!
Powars thought that he had read of this jumbled layer before. In 1913, Samuel Sanford and John Cederstrom had recorded in careful detail the first report of Virginia's groundwater in the Coastal Plain. Drawing contour lines connecting known wells that contained equal concentrations of salt, he noted an "inland bulge" around thelower Chesapeake Bay, centered around the tiny town of Cape Charles on Virginia's Eastern shore.
The methods of core analysis were much cruder at the time. Coredrilling was expensive. Cederstrom had to content himself withliterally cataloging chips and pieces that were flushed up and out of wells that were being dug. He noted that the orderly layers of the Coastal Plain at one point became mangled and mixed. He called this strange batch of sediments the Mattoponi formation from a local Indian name. He also noted something else unusual, a low spot, perhaps afault, where southeastern Virginia subsurface layers seemed to dropaway.
What Cederstrom didn't find was freshwater. Aquifers flowing downfrom the west contained salt. Drilling 52 wells and connecting the salinity contents, Cederstrom noted what Powars would later recognize -- the "inland saltwater wedge."
Fellow scientists at that time did not accept Cederstrom's conclusion that the Mattoponi formation was somehow involved with the salinity in theaquifers, and Cederstrom's work was scorned as being inaccurate.
Saltwater aquifers had been known in the area since the Civil War. Union soldiers stationed at Fort Monroe were plagued by cisterns that dried upduring long hot summers. In 1864, the fort began to drill a well. Forfive years, the drillers labored. Finally, they reached water, but it wastoo salty to drink. They drilled deeper still, until they reached 907 feet. But the water was still too salty to drink, and they eventually gave up.
No one knew why the water was salty or why the James River took a sharp 90-degree turn to the northeast near Fort Monroe. Neither thewell nor the course of the river was following the usual patterns.
The normal course of rivers flowing from the mountains to the sea along the East Coast is predictable. They gently incline toward the Atlantic. However, the James, Rappahannock, and York rivers flow in this fashion until they near the coast. There, they bend abruptly and turn their open mouths north and east to face the tiny village of Cape Charles.
Poag, noting the findings of Powars and Bruce, analyzed some samples of what is now known as the Exmore breccia. Poag found age-scrambled rock fragments and shocked quartz in the breccia. Early on, he thought that the breccia was produced by a tsunami from the smaller impact he referred to as the Tom's Canyon Crater. However, additional data from the release of Texaco's seismic analysis of the Exmore area revealed exhilarating information. The Texaco seismic analysis revealed to Poag a huge crater buried beneath the Chesapeake Bay. Poag then collaborated with the National Geographic Society and the Lamont-Doherty Earth Observatory of Columbia University for new seismic surveys of the area. Through the use of gravity surveys, a much more detailed structure and the shape of a crater were revealed.
The hypothesis that a large meteorite impacted the Atlantic Ocean during the Eocene Epoch was tested and, through dogged research, scientists came to the inescapable conclusion that Virginia had been impacted and affected by an Earth-changing event.
What does the impact crater look like?
The Chesapeake Bay Impact Crater looks like an upside-down sombrero, with an upturned outer rim, a trough, then a high peak in the center.
It is a complex crater, as opposed to a simple bowl shaped crater. Its shape was affected by landing in the water. The space debris penetrated through several hundred feet of ocean water and a couple thousand feet ofwet sediments to create a crater 56 miles in diameter. The crater appears to be greatly influenced by an enormous blast-splash (about 30 miles high) that collapsed back down, vaporizing billions of tons of seawater, which possibly caused subsequent steam explosions. Trains of gigantic tsunamis finished off the event. The wet sediments (mostly clays and sands) appear to have been easier to disturb than the crystalline rock and most likely contributed to the width of the flat-floored rim of the inverted "sombreo." A 1,000- to 4,000-foot high steep slope marks the outer rim of the crater. An approximately 22 milewide outer fracture zone of faulting appears to surround the outer rim,bringing the entire impact structure to a diameter of 96 miles!
|Wylie C. Poag|
How does the crater affect Virginia today?
After the meteorite hit, water blasted out of the area and then sucked in whatever would fill its enormous chasm. Sediments laid down over 35 million years filled in the area in a jumble and compacted the crater debris. Despite all of the matter that was brought into the crater, it continues to be a low spot in the floor of what is now the Chesapeake Bay.
|Courtesy of Virginian-Pilot|
Water flows to the lowest ground, and rivers are no exception. The Rappahannock, York and James rivers turned toward the crater, as they continue to do today. Without the crater, the Virginia port of Hampton Roads would not exist. Without it, the shores of the world's greatest military harbor would simply have been cut through by the relentless drain of the James on its way to the sea.
Until the crater was discovered, there was no satisfactory explanation for the saltwater bulge in the aquifers. Scientists now theorize that the slightly salty to brackish groundwater that occurs in the aquifers adjacent to and with the crater (beneath the lower portions of the York-James and Middle peninsulas) is due to the differential flushing of seawater related to the structure of the buried crater. The need for water across the Hampton Roads region has led several municipalities to develop brackish-water desalination plants. In addition, previously unexplained geologic mapping of concentric zones of facies and structures within and surrounding the crater and coincidental location of elevation breaks over the outer rim suggest a continual subsidence over the crater. This subsidence plays a role in the location of the rivers and lands of eastern Virginia by maintaining either a topographic or bathymetric low, thereby flooding the land first as sea level rises or diverting the rivers toward land when sea level drops.
Incidentally, there have been four earthquakes aligned with the outer rim of the crater,including one that occurred in 1995 in York County. All of these factors point to the fact that this 35 million-year-old meteorite is still affecting us today.
Asteroid -- a rocky body orbiting the sun, usually greater than 100m in diameters. Most asteroids orbit between Mars and Jupiter.
|Courtesy of Virginian-Pilot|
Bolide -- an extraterrestrial impact mass.
Cape Charles, Virginia -- the location of a huge peak-ring impact crater whose center is located near this Eastern Shore Virginia town.
Central peak -- a small mountain that forms at the center of a crater in reaction to the force of the impact.
Chesapeake Bay Meteorite -- one of the largest known impact structures found in North America. This event has been dated at about 35 million years ago during the Eocene Epoch of the Cenozoic Era. The nature of the impact substantially affected the geology of the Atlantic continental crust and is suspected to affect the nature and quality of groundwater in southeastern Virginia. Estimated at 90km, the crater may be about 1.3km deep.
Chicxulub -- very large impact structure off the coast of the Yucatan peninsula. It is suspected to have contributed to the demise of the dinosaurs.
Crater -- the result of an asteroid, comet, or planetary body hitting the surface of another planetary body. The resulting explosion leaves a round hole or crater.
Dinoflagellates -- unicellular protests, which exhibit a great diversity of form. Many are photosynthetic, and some are capable of producing their own light through bioluminescence. Others cause concern when they “bloom” and produce a neurotoxin that can affect susceptible organisms.
Eocene Epoch -- 58-33.8 million years ago. This time period is marked by the emergence of mammals as the dominant land animals. The fossil record reveals many mammals quite unlike anything we have today. However, there were increasing numbers of forest plants, freshwater fish, and insects that were much like those today. In fact, the term Eocene means “dawn of the recent.”
Ejecta -- the debris that shoots out of the impact site when a crater forms.
Fault fractures -- caused either by tectonic movement or impact events.
Floor -- the bottom part of an impact crater. It can be flat or rounded and is often lower than the surrounding surface of the planet or moon.
Glomar Challenger -- a ship that can drill into the Earth's core and was used to locate evidence of a bolide impact.
Impact breccia -- a “rubble” sediment that contains the mix of debris resulting from an impact event.
Iridium -- a very hard and brittle metal, atomic number 77, often associated with meteorite impacts.
Mass -- the measure of an object’s inertia, i.e. how heavy it is. Mass is not the same as weight, which measures the gravitational force on an object.
Meteor -- a bright streak of light in the sky caused by a meteoroid or small icy particle entering Earth’s atmosphere. It is also know as a “shooting star.” Meteor showers occur when the Earth passes through debris left behind by an orbiting comet.
Meteorite -- small rocky remains of meteoroids that survive a fiery journeythrough Earth’s atmosphere and land on Earth.
Micrometeorite -- a very small meteorite with a diameter of less than one millimeter.
Ocean Drilling Program -- funded by the U.S. National Science Foundation and 22 international partners to conduct basic research into the history of the ocean basins and about the overall nature of the crust beneath the ocean floor using the scientific drill ship, JOIDES Resolution.
Ray bright -- lines of debris projecting from the edges of craters.
Reflection profile -- the seismic “sonogram” created by the graphic representation of sonar waves.
Rim -- the highest point along the edge of a crater hole.
Rubble bed -- the jumbled sediments and age dated fossils that are associated with the Exmore beds of the Chesapeake impact structure.
Shocked minerals -- minerals, especially quartz, that show the result of tremendous forces, such as those found in impact events that alter and distort the normal optical qualities of a quartz crystal.
Tektites -- millimeter- to centimeter-sized glass beads derived from sediments melted by an impact.
Tsunami -- a very large ocean wave usually associated with underwater earthquakes or volcanic eruptions. Tsunamis may also be associated with large meteorite impacts in the oceans.
United States Geological Survey -- recognized as a world leader in the natural sciences through scientific excellence and responsiveness to society’s needs. The mission of the USGS is to describe and understand the Earth, minimize loss of life and property from natural disasters, manage water, biological, energy, and mineral resources and to enhance and protect our quality of life.
United States Geological Survey Coastal and Marine Center at Woods Hole, Massachusetts -- one of three marine teams that conduct research, explore, and study many aspect of the underwater areas between shorelines and the deep ocean off the East Coast, Gulf of Mexico, and in parts of the Caribbean and Great Lakes.
Virginia Department of Environmental Quality -- a department of the state government of Virginia that is dedicated to protecting Virginia’s environment and protecting the health and well-being of the citizens of the Commonwealth.
Wake material -- material left along the trajectory of a meteor after the head of the meteor has passed.
Wall -- the sides of the bowl of a crater.
Check out this Web page on "Meteorite Types and Facts."
Meteorite Impact Activities
ACTIVITY NO. 1
TITLE: GRADED SEDIMENTS AND IMPACT EVENTS
|Core sample from David Powars|
One of the guiding principals of geology is a concept known as superposition. It is the common sense observation that what is at the bottom of a group of sediments is the oldest.
One of the geographic regions of Virginia is the Coastal Plain. The Coastal Plain is an eastward dipping series of sediment layers that range in age from 120 million years old to the present. These layers have been studied for more than 200 years and, though new information is constantly being brought forward, the general characteristics and fossils that are found within the Coastal Plain sediments are quite familiar to the student of Virginia geology. These sediments were laid down in orderly fashion, one on top of the other as the sea advanced and retreated over millions of years.
In the early 1980s, the state of Virginia began to sink research wells throughout southeastern Virginia. The cores brought up from the drill sites were studied and the ages of the sediments correlated with known fossils. All went as expected until 1986. At that time, researchers found a confusion of sediments jumbled and tumbled in a fashion never before seen in the Exmore bed.
Or so the researchers thought that this had never before been seen. Careful research into the mystery beds revealed that in the 1940s the USGS had sent geologists to hunt for water supplies in the area of the lower Chesapeake Bay. The region's massive build-up of support and supply facilities for World War II demanded more freshwater. A geologist at that time reported an unusual superposition of sediments during the digging of wells. The sediments were mangled, mixed, and overturned. The search was not successful; instead of freshwater sources the geologists encountered saltwater! In 52 test wells, only saltwater was brought up.
By simply connecting the location of the wells on a map, the geologist at that time noted an "inland saltwater wedge." However, his work was scorned. Fellow geologists couldn't accept the fact that the sediments as found might be stopping the outward push of freshwater aquifers flowing from the west. In general, sedimentary layers just don't look or act like the ones that the geologist reported in the 1940s.
Now we know that these early observations were correct. Those jumbled and mixed sediment layers were the result of an Earth-changing event. Approximately 35 million years ago, a meteorite slammed into the Atlantic and in a matter of seconds changed the layers into a jumble of old and new. The impact took seconds to change what had taken millions of years to lie down.
A surge of water and debris rushed outward. In what is now thought to be 30 minutes, the orderly layers of Coastal Plain sediments were a chaotic mix of rocks, mud, and sediments. In the next 30 minutes, there was a return surge and rocks and debris began to be sorted by size.
Today, geologists recognize three distinct layers caused by the impact of the meteorite. The first and lowest are the mega blocks of continental shelf, this area is known as crater unit A. The next, crater unit B, is made of house-size blocks of rock. The last and thickest is crater unit C or the Exmore beds. As Dr. Lucy Edwards, a USGS scientist who is an expert in dinoflagellates said, " No one had ever seen anything like it. Beds of that thickness that are sorted are just not seen" - unless they are created by a huge force, such as that of a meteorite, and located in a crater.
Wide-mouthed jar with lid
Sediment (your local creek bed, mud puddle, or any place where dirt is exposed; sediment from the seashore will work, too)
- Scoop up dirt that includes small rocks, sand, and mud
- Place in jar, no more than 1/3 full
- Add water to 1 inch below the top
- Cover with lid
- Shake thoroughly
- Write down/draw what you expect to find, specifically: How long do you think it will take for the sediment in the jar to settle? What will it look like when it settles?
- Wait (1-3 days!)
- Describe your results and compare them to your predictions
- What did you learn?
Big particles settle out before fine ones and it will take several days for the water to clear completely, even though mixed sediments will form in layers according to size.
ACTIVITY NO. 2
TITLE: WHERE'S THE HEAT?
To observe how one can create friction from the movement of materials. (Teacher note: Students should understand how a meteor can become so hot when approaching earth and entering the atmosphere. Space temperatures are below 200 degrees Celsius.)
- two Styrofoam cups
- masking tape
- balance (optional; used to find calories)
(Optional: Calculate the calories. You must find the weight of the sand. See below.)
- Place sand into one of the cups.
- Seal the top of one cup to the top of the other cup with masking tape.
- Create a hole in one of the cups and place the thermometer in the hole. Seal the thermometer.
- Measure the temperature of the sand. Record.
- Shake the cups for a designated time.
- Measure the temperature of the sand immediately after the designated time. Record.
- What is the connection between motion and heat?
- Rub your hands together vigorously. Why do your hands get warm?
- Explain how this relates to a meteorite as it strikes the earth.
Calories: This is the standard unit for heat. One calorie is: the amount of heat required to raise one gram of water one degree Celsius. Calculate the number of calories that were produced in the shaking sand experiment. The sand must be weighed. (Background: To find the calories one needs: the change in temperature, mass of the sand, and the specific heat of sand.) The specific heat of sand is 0.24.
The formula is:
Calories = change in temperature X mass X 0.24
Remember that the temperature must be in degrees Celsius and the mass in grams.
Joule: This is the standard unit of energy. One calorie is equal to 4.186 joules. Convert your calories into joules. The formula is:
Joules = number of calories X 4.186
CLASS DISCUSSION ON THE AMOUNT OF ENERGY AND HEAT CREATED THROUGH IMPACTS.
Look at the data below and discuss how a meteor can create destruction on the earth. The energy is expressed in kilojoules, which is 1000 times a joule.
Create a way that you can compare the amount of energy between each event. Is the amount of energy between the collision of 2 cars two times the amount of one car and wall?
Find a way to solve this problem.
ACTIVITY NO. 3
TITLE: ANNUAL METEOR SHOWERS
In the data table below are listed some of the annual meteor showers. In the space below your class may want to investigate and add other meteor showers that are smaller in number and brightness than the ones that are listed. This can be a research projects. NASA has large lists in its Web site.
Click on Amazing Space to find Web-based activities for classroom use about comets, galaxies, stars, the Hubble, black holes, and more.
This program produced by PBS and the accompanying Web site has an outstanding, short explanation of tsunamis at http://www.pbs.org/wnet/savageearth/tsunami/index.html. "Scientists at the Los Alamos National Laboratory in New Mexico calculated that if an asteroid three miles across hit the middle of the Atlantic Ocean, the tsunami would swamp the upper East Coast as far inland as the Appalachian Mountains and drown the coasts of France and Portugal," states the Web site. Evidence of this is found in the Chesapeake Crater.
Space Science Education Resource Directory
The Space Science Education Resource Directory is a convenient way to find NASA space science products, ideas, and related Web sites for use in the classroom. Type "meteorite" in the search box and find several outstanding activities for classrooms. For example, http://cse.ssl.berkeley.edu/AtHomeAstronomy/ has 10 activities that would work in the classroom as well as at home for grades 4 to 8. One activity at http://cse.ssl.berkeley.edu/AtHomeAstronomy/activity_05.html investigates the formation of craters. Students can learn how the size, angle, and speed of a meteorite's impact affect the properties of craters. In addition, students will become familiar with the terms meteor, meteoroid, and meteorite.