Environmental Issues of the Northeastern U.S.

 

Geology affects where we live, how we live, and how we use the land. In the Northeast, earthquakes, landslides, land subsidence, and radon are important issues tied to the type of rocks found at the surface and underlying the region. They are "issues" only because they disrupt human lives and constructs. Whether directly caused by human activity (such as landslides and land subsidence in some cases) or simply a natural process (such as earthquakes or the production of radon gas), the significance is magnified because of the presence of people. Ideally, growing knowledge of environmental issues and an understanding of their foundation in geology, will help us to make wiser and more informed decisions on land use and planning. In this chapter we will discuss the Northeast region as a whole.

Earthquakes

Ninety-eight percent of earthquakes occur at tectonic plate boundaries. As the plates collide, pull apart, or move past each other, their grinding and shifting build up stress. When these stresses are released suddenly at the plate boundary or at faults near the boundary, the crust shifts and seismic waves are released, causing an earthquake. In the US, most earthquakes occur west of the Rocky Mountains, where there is currently an active plate boundary between the North American and Pacific Plates. During the break up of Pangea and the preceding mountain-building events, there was an active plate boundary at the margin of the east coast of North America. The eastern margin of the continent no longer is at an active plate boundary. Now the active plate boundary lies thousands of kilometers to the east at the Mid-Atlantic Ridge, where the North American and Eurasian plates are pulling apart and new crust is forming.

Though large quakes are not a common event in the Northeast, earthquakes do occur, most likely caused by old faults formed when the eastern margin of North America was an active plate boundary. Stress upon the old faults may force them to shift suddenly, causing an earthquake. Geologists have not had much luck, though, relating earthquake events in the Northeast to known faults. Unlike the west coast, where there is a clear relationship between earthquakes, faults and the active plate boundary, there is no clear relationship in the Northeast between earthquakes and known faults.

Figure 8.2: Earthquakes with a magnitude greater than 3.0 in the Northeastern U.S. from 1975 to 1999. Image courtesy of Allan Kafka, Weston Observatory.

Most earthquakes in the Northeast are minor, rarely causing any damage. Minor earthquakes occur in every state throughout the Northeast, though relatively few have been located in the Inland Basin region, where the crust experienced little deformation relative to the rest of the Northeast (Figure 8.2). Occasionally, large earthquakes actually do occur in the region. One of the largest was on November 18, 1755 off Cape Anne, Massachusetts. The vibrations from the quake were felt over 450,000 square kilometers.

Figure 8.1: The Modified Mercalli Intensity Scale.

Figure 8.3: Earthquake risk in the Northeast. Earthquake-induced ground movement is expressed as a percentage of the force of gravity (%g). The map illustrates the amount of ground shaking that is predicted in a given period of time. AFter the National Seismic Hazard Maps, United States Geological Survey.

Due to the vague relationship between earthquakes and faults in the Northeast, it is difficult to assess the risk of earthquakes in the region. Using historical records of the Northeast dating back to the 1500’s, geologists predict that future earthquakes are most likely to occur in the same general areas as past earthquakes. Despite such attempts to assess the level of risk in the region, it is still not possible to predict the place and time of individual earthquakes on either the west coast or the east coast (Figure 8.3).

The Northeast has a lower risk of earthquakes than California or other states west of the Rocky Mountains. However, the more densely populated east coast makes the infrequent large quake possibly more damaging than similar earthquakes in the West. Many buildings in the Northeast were not built with earthquakes in mind and could potentially be damaged by stronger tremors. Additionally, seismic waves travel further in the eastern US. The active plate boundary on the west coast makes near-surface rocks west of the Rocky Mountains warmer than rocks east of the Rocky Mountains. Heat absorbs seismic waves and they are unable to travel as far. Cooler rocks, like those of the Northeast, are less of an impedi- ment to seismic waves, allowing them to travel further and potentially cause more damage.

Land Subsidence

Land subsidence is an issue in the Northeast, though more often in the Appalachian/Piedmont and Inland Basin regions. Mines and carbonate rocks are the primary causes of land subsidence. The large amount of underground excavation during mining, especially the coals of western Pennsylvania and Maryland, has left large areas beneath the surface empty or filled with loose sediment. The empty spaces and sediment fills are sometimes unable to wholly support the weightofoverlyingrocks. As a result, the over lying rocks and soil sag or sink downward to create a depression in the land surface. In extreme cases, the overlying rocks collapse completely or the subsidence causes a landslide.

Areas of the Northeast underlain by carbonate rocks, in particular the Valley and Ridge region of the Appalachian/Piedmont, are susceptible to land subsidence. Rainwater, which is naturally slightly acidic and becomes more acidic after passing through acidic soils, is capable of breaking down carbonate rocks. Within a carbonate rock layer might be caverns, widened fractures, and spaces that make the layer an unstable support for overlying rocks. Similar to mine collapses, the overlying rock in a carbonate area has a high potential for collapsing and creating sinkholes (Figure 8.4).

Land subsidence can cause major problems, particularly in urban areas where sewer systems, water lines, and gas pipes can be damaged due to the sagging ground surface. In places where the surface collapses, buildings and roads can be badly damaged. Sinkholes also provide a fast drainage from the surface to groundwater, increasing the chances of polluting groundwater supplies.

Figure 8.4: Naturally acidic rainwater creates cavities within carbonate rock, making the overlying rock unstable (left). As the cavities enlarge, the overlying rocks collapse to form a sinkhole (right). Figures by J. Houghton.

Landslides

Intense rainfall, rapid snowmelt and steep hillsides are prime conditions for a landslide. Landslides range from watery mud to thick mud carrying rock, boulders and trees, to toppling rocks off steep slopes. Moving as fast as 60 kilometers/hour, landslides can potentially cause millions of dollars of damage to buildings and roads as well as human fatalities. In the Northeast, landslides are common in the Appalachian Mountains, New England, and the Appalachian Plateau due to steep slopes, a fairly rugged landscape and clay-rich soils. Though the Appalachian Plateau is called a plateau, it is deeply dissected by river valleys that have formed steep slopes along which landslides are common.

Clay and clay-rich soils that become saturated with water have drastically reduced friction, allowing other rock layers or sediment to slide rapidly upon it. In the glaciated areas of the Appalachian Plateau, including northern Pennsylvania and most of New York, clays were deposited at the bottom of glacial lakes occupying glacially carved valleys. Though many of the lakes no longer exist,the clay deposits still remain and are a source of landslides in the region.

Other causes of landslides, especially in the Appalachian Plateau region, are the result of the activities of humans. Underground mining, common in the bituminous coal fields of western Pennsylvania and Maryland, reduces the stability of overlying rock and often results in landslides. When older mines are filled, the settling rocks and sediments can cause landslides. Poorly engineered fills on slopes can also trigger landslides.

Recognition of landslide-prone areas is important for land use planning and zoning decisions. Damage from landslides can be prevented by not building in areas that commonly experience landslides or show evidence of past landslides.

Radon

Figure 8.5: The radioactive decay of Uranium-238.

Radon is a chemical element: an odorless, colorless, radioactive gas that commonly forms from the breakdown of the element uranium. Radon first came to wide public attention as an environmental issue in the mid 1980’s when high concentrations of the gas were found in houses overlying the Precambrian rocks of the Reading Prong in southeastern Pennsylvania. Though scientists continue to debate the health risks of radon, it is clear that smokers exposed to high levels of radon gas have an increased risk of lung cancer.

Uranium-238, the uranium isotope from which radon originates, is a radioactive substance. When a radioactive substance decays, the nucleus breaks down by the loss of protons, electrons or neutrons, forming another element. The decay process continues until a stable (non-radioactive) isotope is reached. The decay of uranium-238 produces a series of unstable elements, including radon-222 (Figure 8.5).

Radon-222 is also radioactive, decaying to eventually produce a stable form of lead. Though it takes 4.4 billion years for half of a given amount of uranium-238 to decay, it takes radon-222 only a few days.

Both uranium and radium are solids and incapable of moving through rocks and soil. Radon, however, is a gas. Where soils and rocks are porous and permeable, radon can migrate upwards towards the surface. We are naturally exposed to low levels of radon in the air and water around us with no ill effects; radon, however, can become concentrated at high levels indoors. Poorly sealed house foundations with inadequate air flow allow the radon gas to enter homes, becoming concentrated and possibly inhaled. Radon may also be naturally dissolved in well water and released indoors when the tap is turned on.

Figure 8.6: Radon potential in the Northeast. After the United States Geological Survey, Generalized Geologic Radon Potential Map.

Most susceptible to high radon levels, are those areas with uranium-rich rocks. Though most rocks have a small amount of uranium, certain types of rocks have higher concentrations of the radioactive element, such as light-colored volcanic rocks, granites, dark shales, sedimentary rocks with phosphates and some metamorphic rocks. Rocks that have pathways such as fractures, faults and connected pore spaces between grains allow radon gas to move upwards to the surface. Likewise, thin, permeable and porous soils with cracks aid in the upward migration of radon. Additionally, because moisture inhibits the movement of the gas, radon moves more quickly in dry, well-drained soils. The igneous and metamorphic rocks of the Appalachian Mountains and Adirondacks are uranium-rich and sliced by numerous faults, resulting in an area with the potential for high levels of indoor radon. The mineral glauconite, found in parts of the Coastal Plain sediments, is also uranium-rich. For the most part, however, the Coastal Plain has one of the lowest levels of radon risk in the country (Figure 8.6).