Credit: The Virginian-Pilot

What if the February 15, 2013 meteor strike hit the United States, and not Russia? What if it were the size of the meteorite that struck off the coast of Virginia during the age when modern mammals began to appear? What if . . .

A Blast from the Past

About 35 million years ago, an enormous meteorite, a mile or two in diameter and traveling up to 134,000 miles per hour slammed into the shallow Atlantic Ocean where eastern Virginia is today. It vaporized billions of tons of ocean water, melted rocks, cut through a few thousand feet of sediment, and with a catastrophic explosion, created a 24 mile-wide crater. It created a devastating tsunami, scattered tons of sediment along much of the East Coast, and caused glassy particles of solidified melt rock to rain down as far away as Texas.

This explosion was equal to about 10 trillion tons (that’s 10,000,000,000,000) of TNT and is known as the Chesapeake Bay impact crater, the largest crater in the U.S. and the seventh largest in the world. Some of the ejected material, known as ejecta, fell back to the earth, including some that melted and recrystallized into small glass globules called tektites, after the Greek word for “melted.”

“It used to be that people thought that impact craters were extinct volcanoes, even the ones on the moon. But USGS scientists proved there were minerals in these craters that could have only been made by impacts,” said David Powars, a USGS geologist who has been studying the Chesapeake Bay crater for over 25 years.

What was the Effect?

The space debris penetrated through several hundred feet of ocean water and a couple thousand feet of wet sediments. “The enormous blast-splash was about 30 miles high before it collapsed back down,” said Powars.

A map showing the location of the Chesapeake Bay impact crater

The resulting 56-mile diameter complex crater has the shape of an inverted sombrero, with a deep central hole in the crystalline rock. A 1,000- to 4,000-foot-high steep slope marks the outer rim of the crater. In addition there is an approximately 22-mile-wide surrounding area full of faults from the impact, which brings the entire impact crater structure to a diameter of 96 miles. Over the last 35 million years since the catastrophic impact, eastern Virginia has mostly been covered with shallow ocean waters that have buried the crater with fine grain sediments.

Discovering the Impact Crater

The puzzle of the impact crater may have started 135 years ago when the water-well drillers for the Union Army at Fort Monroe, Virginia, worked for five years to drill a 907-foot-deep well, only to find undrinkable, brackish water. But the initial evidence for the crater wasn’t obtained until the mid-1980s, when a core sample was recovered that included 135-35 million-year-old particles of weathered rock and marine plants and animals. Definitive evidence of the crater came in the early 1990s with the release of marine seismic data (a kind of large-scale sonogram) from Texaco and Exxon that provided imagery of the 56-mile complex crater.

The Impact Crater’s Effects on Southeastern Virginia Today

“I think it’s safe to say that without the Chesapeake Bay impact crater the Bay would not look the same today” said Powars. The crater draws the great rivers of the Chesapeake Bay Delta to it, leading to the distinctive shape of the Bay.

The signal from the meteor on February 15, 2013 in Russia was picked up by seismometers in the region that are monitored by the USGS. The attached graphic shows the signal recorded on three seismometers.Small ground motion was observed as far as 4,000 km away from the
explosion.

The crater has affected daily life for area residents – from the early settlers at Jamestown to those who live there today. Until the crater was discovered there was no satisfactory explanation for a bulge of salt water that intrudes into the underground freshwater aquifers of central Virginia. Also historical earthquake data show that four earthquakes align 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 impact crater is affecting us today.

As frightening as the meteor incident in Russia must have been for those affected, another event like the one that led to the Chesapeake impact crater would have a profound impact on the entire globe. The good news is that scientists are now constantly monitoring objects in space and the effects such objects could and do have when they approach our planet.

A development along the mouth of the Elk River, a tributary of the Chesapeake Bay.

The Chesapeake Bay Watershed covers over 64,000 square miles and is home to more than 17 million people. As human populations and activities in the watershed have grown, the Bay’s ecosystem has been degraded by sediment and excessive levels of nitrogen- and phosphorus-based nutrients.

Nutrients flow into the Bay from wastewater treatment plants and from fertilizers spread throughout the watershed. Meanwhile, excess sediment comes from development, runoff from agricultural land, and erosion of stream channels. Too many nutrients rob the Bay of oxygen needed for fish and, along with sediment, cloud the waters, disturbing the habitat of underwater plants that are crucial for aquatic life and waterfowl.

The Chesapeake Bay Program, a partnership between federal and state agencies, local governments, non-profit organizations and academic institutions, is working to improve water-quality in the Bay by striving to meet the Chesapeake Bay Total Maximum Daily Load (TMDL), a requirement of the Clean Water Act for impaired waters that identifies the maximum amount of a pollutant the waterway can receive and still meet water quality standards. Meeting the Chesapeake Bay TMDL standard requires that stricter practices for reducing nutrients and sediment in the Bay be implemented by 2025.

Start with Science

In addition to other efforts to clean up the Bay, the Chesapeake Bay Program, through a partnership among USGS with EPA, six states and Washington D.C., established a network to monitor changes in nutrients and sediment in the Chesapeake Bay watershed.  Information from this network is used to help scientists and managers assess water-quality conditions and long-term trends as management practices are implemented.

Monitoring water quality is critical to answering key questions: Are TMDL management practices and regulations making the needed reductions in nutrients and sediment? Is water quality in the Bay improving for birds, fish, crabs, oysters, and underwater grasses?

Recent USGS Monitoring Results

The USGS recently released a report showing that nitrogen and phosphorous concentrations have improved at a majority of the sites in the Bay watershed since 1985. However, improvement is less significant over the last 10 years.

Since 1985, nitrogen concentrations have decreased at about two thirds of the monitoring stations. Over the past 10 years, however, improvements occurred at less than half of the sites with the majority of the sites having no significant change. Similarly, over the long-term,  phosphorus concentrations have improved at most monitoring sites and worsened at only a handful. However, in looking at just the last 10 years, phosphorus concentrations improved only at one-third of the sites with a majority having no significant change.

In general, sediment concentrations have shown less improvement overall than nutrients. Over the long-term, sediment trends have improved at about one third of the sites, but they have only improved at a handful of sites over the past 10 years. Sediment trends worsened at about a quarter of the sites both over the long term and past 10 years.

Image of the Chesapeake Bay taken from Landsat satellite data. The Chesapeake Bay watershed is 64,000 square miles and has 11,600 miles of tidal shoreline, including tidal wetlands and islands. The watershed encompasses parts of six states, and approximately 17 million people live there. The Bay itself is the largest estuary in the U.S.

Why is There Less Improvement Over the Last Decade?

The long-term trends indicate that pollution-reduction efforts, such as enhanced controls at wastewater treatment plants and practices to reduce nutrients and sediment from farms and suburban lands, are improving water-quality conditions in many areas of the watershed. However, there is a lag time between implementing water-quality practices and seeing the full benefit in rivers, which is one reason why scientists see less improvement over the past 10 years.

Lag times depend on both the type of practice being implemented and the time it takes for nutrients and sediment to be transported through the Bay watershed. For example, generally speaking, improvements related to upgrades of sewage treatment plants yield quick results. However, improvements from nonpoint sources, such as reductions in fertilizer from agricultural and suburban areas, take longer. In fact, the lag time for nonpoint source improvements can range from weeks to decades. Previous studies by the USGS found that some of the nitrogen applied in the watershed travels through groundwater and thus it may take years to decades to move to streams. Sediment, which also carries nutrients, can take longer to move down streams to the Bay.

USGS annually updates information detailing water quality changes in the Bay watershed. An update with results through 2011 is expected later this year.

Understanding Complicated Trend Results

The runoff of precipitation carries nutrients and sediment from the land surface to streams and rivers. Eventually they are delivered to the Bay. USGS has found that periods of low inputs of nutrients and sediment coincide with periods of low streamflow. Similarly, inputs are higher during years with greater streamflow. These correlations indicate that precipitation and streamflow significantly influence nutrient and sediment concentrations in the Bay and can mask whether management practices are impacting water quality.

The USGS has used “flow-adjustment techniques” that take variations in precipitation and flow into account when assessing water quality. By normalizing streamflow conditions, scientists and managers can paint a clearer picture of whether changes in nutrients and sediment are related to management and restoration activities.

The Chesapeake Bay Watershed. Graphic courtesy of the Chesapeake Bay Program.
http://www.chesapeakebay.net/discover/baywatershed

Nutrient and Suspended-Sediment Trends, Loads, and Yields and Development of an Indicator of Streamwater Quality at Nontidal Sites in the Chesapeake Bay Watershed, 1985–2010, U.S. Geological Survey Scientific Investigations Report —SIR 2012-5093

USGS Chesapeake Bay Activities

Chesapeake Bay Executive Order

Chesapeake Bay Program

USGS scienctists discuss new science efforts to restore the Chesapeake Bay

The Chesapeake Bay has long been an R&R destination for DC residents. However, the watershed’s overpopulation contributes to its decline. Join us when USGS’s Scott Phillips and Peter Claggett discuss new science efforts applied to restoring the Nation’s largest estuary on October 6th.

More information