Geology Past, Present, and Future


By Robert C. Smith II and John H. Barnes, Pennsylvania Geological Survey

This article was published for Earth Day 1996 in Pennsylvania Geology, vol. 27, no. 1.

"The present is the key to the past" has been a key concept used by geologists in interpreting their field observations since it was first proposed by the pioneering Scottish geologist James Hutton in 1785. It is a reminder to geologists that, by studying modern geologic processes, we can understand what went on in the past to create the features that we find preserved in ancient rocks. This concept is not much different than the "history repeats itself" concept of historians, who usually deal with events that occurred hundreds of years rather than hundreds of thousands or hundreds of millions of years ago. Taken together, these concepts suggest that Earth will, at times, be a scary place in the future.

hemlock
An old Eastern Hemlock (Tsuga canadensis), Pennsylvania's state tree, awes visitors to the Rothrock State Forest with its beauty, size, and age. Yet its height is only about three tenths of one percent of the deepest point in the ocean, and its age is one-millionth the age of the earth.

How scary? Not as scary as in its earliest days. Drawing again upon analogy with human history, many details have changed with time. When the Sun was new, much debris cluttered the space around it. Some of that debris coalesced to form the planets, including Earth, but many thousands, if not millions, of small fragments were left to circle the Sun. In its earliest days, Earth was something of a target, colliding with debris that circled the Sun in orbits that intersected that of Earth. In doing this, Earth swept out a path through space so that giant meteorite impacts should no longer be of immediate concern to us. After all, it has been 65 million years since the last really big collision occurred, perhaps spelling doom for the dinosaurs and clearing the way for mammals. The most significant collision between Earth and another body in recorded history, one that flattened forests around Tunguska, Siberia, in 1908, was very minor by comparison - only a hit from a cosmic snowball with the energy of 10 million tons of TNT.

As Earth has matured, some other scary things have also become much less of a threat than they were when Earth was younger. About 2 billion years ago, there was enough naturally occurring fissionable uranium on Earth to cause the reactions that take place today only in the controlled environment of nuclear reactors to take place in nature. At Oklo, Gabon, on the west coast of equatorial Africa, the remnants of such a natural reactor can be observed. Thanks to millions of years of radioactive decay, there are no longer natural concentrations of fissionable uranium. Similar radioactive decay has allowed Earth's internal heat engine to run down to the point that volcanic activity has decreased. The destruction of life by volcanoes over an area of more than 100 square miles is now unlikely in any given decade, but in the distant past such eruptions were common. Fears of meteorite impacts, natural nuclear reactors, and volcanoes will not interfere with our celebration of Earth Day 1996.

But is all well with our mature "Mother Earth"? Not necessarily. Without any help from man, and despite the over 4 billion years that Earth has had to settle down, earthquakes such as those at New Madrid, Mo. (1811, magnitude greater than 7), and Charleston, S.C. (1886, magnitude greater than 6), will continue to occur. They will be infrequent, but even with present technology there will be no warning. At present, we cannot even do a very good job of predicting where east-coast earthquakes will occur.

flood72In 1972, the Pennsylvania Geological Survey underwent a soggy but educational experience when its headquarters was flooded.

Floods! You say Pennsylvania has had enough once-in-500-year, once-in-200-year, and once-in-100-year floods in the last 25 years? Well, we agree. Our soggy experience at the Pennsylvania Geological Survey's former headquarters on a floodplain during the 1972 flood that accompanied tropical storm Agnes was educational but not satisfying. But more floods will come, and there is nothing sacred about a 200-year-old record book on a 4.5-billion-year-old planet. Do you know if you are living on an area that receives once-in-10,000-year floods? Do you know the depth to the water table beneath your property? How much that water table fluctuates? Whether you are in a landslide-prone area? Whether you live in a limestone or dolomite sinkhole-prone area? This is not intended to be a commercial for the Fearmonger Shop, as "advertised" by humorist Garrison Keillor on his radio program, A Prairie Home Companion. It is just a reminder, as we pause to pay respect to our home planet on this Earth Day, that the geological processes that have shaped and modified Earth are guaranteed to continue, and that they will occasionally be destructive.

What does the future hold for an Earth dominated by mankind? There will be more landslides and sinkholes than would otherwise be the case because we have a constant need to rearrange the landscape so that we can construct buildings and highways to accommodate an ever-increasing population. Many of these landslides and sinkholes are preventable, but not all will be prevented. Better news coverage in the electronic era will spread news of sinkhole collapses and call attention to municipalities that employ poor storm-water management plans, thus providing disincentive for further development in such places.

Floods downstream from dikes and levees will be more severe than they would be if there were no dikes or levees, but government-run facilities such as the Raystown Dam will ameliorate effects in some basins. Catastrophic dam failure, such as in Johnstown in 1889 and 1977, will become less frequent because generations of horror-stricken visitors to the Johnstown Flood National Memorial will learn from history and will demand independent dam inspections. Periodic droughts have occurred in the past and are guaranteed to occur again in any part of Pennsylvania except the part under Lake Erie. This should suggest that continued groundwater investigations are in the "essential," not just in the "it would be nice" category.

The composition of the atmosphere and the climate have varied dramatically over Earth's 4.5-billion-year history, but the rate of change has typically been extremely slow. Are we now creating a risk of catastrophic change by burning fossil fuels? We do not know, but we would not want to be living in Pennsylvania without access to such fuels, especially not after the frigid Januarys of '94 and '96! For most of us, the risks of living near well-designed and independently monitored nuclear power plants are acceptable. However, few of us can afford to pay to build more such plants, and fewer still are keen on flooding all of our river valleys to build more hydroelectric dams or covering half of the state with opaque photovoltaic cells to capture solar energy.

A potential risk associated with the burning of fossil fuels comes from the liberation of carbon dioxide into the atmosphere. If the atmosphere contained too much carbon dioxide, heat that normally is radiated into space would become trapped in the atmosphere. With time the heat might build up, resulting in changes in climate and the permanent flooding of coastal areas as the polar ice caps melted. Some scientists suggest that Earth has natural mechanisms that correct for such changes in the composition of the atmosphere. One such mechanism is the removal of excess carbon dioxide through the formation of additional deposits of limestone, which is made up partly of the elements in carbon dioxide. Other scientists maintain that the process of limestone formation could be too slow to protect us from rapid changes in the atmosphere.

Although many studies that scientists undertake sound strictly academic, they can have important practical applications. The study of the process by which limestone forms is clearly one of these. Another, which is related, is a careful worldwide study of the circulation and temperature of the oceans. The oceans, collectively, are the greatest reservoir of heat that drives the weather and affects climate, yet very little is known about their thermal properties. Meanwhile, practical considerations require us to continue to identify sources of fossil fuels and groundwater, and to use existing resources wisely.

On this Earth Day, we pause to think of the many ways in which Earth's natural processes affect us. Geology has taught us that Earth has changed and will continue to change. Through further geological investigations we can learn what the processes of change are, how they have affected life on Earth in the past, and how they are likely to do so in the future. Our studies will always lead to more questions and still more studies, but it is only through this process we can learn to better adapt to the forces that continually act to shape our planet.