Earth, Our Environment - Class Notes
Chapter 15, Geologic Hazards and the Environment
Earth, Our Environment - Class Notes
Humans have always had to contend with the natural world. We benefit greatly from Earth's riches and hospitable climate and are quite fortunate to have prospered during a relatively "quiet" geologic time. However, Earth can be a dangerous place and our study and understanding of these hazards is vital to our continued success (as a species). The best way to deal with environmental hazards is to prepare.
Preparation can take on short-term or long-term aspects. The long-term preparations for hazard mitigation are frequently complex and often involve investment of substantial resources. Your responsibility as a citizen and neighbor is to study these problems and share your opinions and insight.
Earthquakes are inevitable in many places throughout the world, but their precise time can be a big surprise. If you happen to be very unlucky and build a house or structure across a fault - it may be ripped apart during an earthquake. If you happen to live in the immediate vicinity of an earthquake, your house may be shaken apart by the seismic waves emanated from near the fault.
Three main factors will influence the shaking intensity that you experience during an earthquake:
Another problem with earthquakes is that they can damage the infrastructure that we normally rely upon. For example, roads and bridges can be impassable, water pressure can fail, and gas lines can break and leak. As a result, fires can spread unchecked and often do much of the damage in earthquakes.
Three main characteristics of the seismic waves affects the potential for building damage experience during an earthquake:
A magnitude 6 earthquake will generally shake intensely for about 4-8 seconds, a magnitude 8 earthquake may shake for a couple of minutes. Like treble and bass, earthquake waves have shake the ground over a range of frequencies. As a rule of thumb, the resonance period of a building is about 0.1 seconds times the number of stories in the building.
The type of rock and soil that we build our structures can have an impact on the level of damage caused by seismic waves. The soil type, water content, thickness, etc. can increase or decrease the amplitude of the shaking. One particular hazard is liquefaction, which occurs when shaking of water-saturated soils changes the behavior and the material flows like a fluid. Small amounts of flow can cause "settling" and destroy the foundations of buildings and homes, or even collapse the structure. In some cases, if water-saturated soil is "capped" or covered by a dry, relatively brittle layer, the shaking can cause the water-saturated sand to erupt in a sand blow or mud volcano.
Other Secondary Hazards
From a geologic perspective we can predict where most earthquakes will occur: along the plate boundaries. We can also predict the average rate of occurrence (over millions of years). Such predictions are not terribly useful for society, although they are not without value since they indicate our understanding of the basic processes of earthquakes. However, although predicting the exact time of an earthquake would be valuable, it does not solve all of our problems. For example, if I told you an earthquake was to occur in the next week, what could you do? You could (perhaps, depending on traffic) move to a safe location, but what about your house and other belongings? The best preparation for earthquakes is adequate building construction. The information scientists try to provide is the level of shaking expected in a given region. We try to map the surficial geology, the potential size and location of earthquakes. In essence, long-term preparation guided by predictions of the potential level of shaking in a region is the best way to prepare for earthquakes.
Tsunamis are waves generated by earthquakes or any large, sudden offset of the ocean floor (caused by submarine landslides etc.). They are sometimes INCORRECTLY called tidal waves, they have nothing to do with the tides. Tsunamis can be as high as 60 feet and are very destructive. Obviously they affect coastal regions and are particularly dangerous in the immediate vicinity of an earthquake. Tsunamis can also travel across oceans, so a large earthquake along the coast of South America can produce a tsunami that eventually travels to and damages coastal regions of Hawaii and or Japan. As they travel across the ocean, they are very small in height, and unnoticeable. When they reach the shore, the shallowing of the water produces an increase in wave height. They travel about as fast as an airplane, so we have hours to warn distant localities. But they can strike within a few minutes close to the earthquake.
We can divide volcanic hazards into short term, and long term.
The short term hazards are generally associated with immediate affects of an eruption. The long term hazards can be more widespread and last for some time.
Lava flows can often be avoided personally, although often structures that we build are lost to the steady march of lava.
Pyroclastic flows are very dangerous since they travel upwards of 150 km/hr they can seldom be outrun. They are a mixture of superheated ash, gas, and rock and level or bury pretty whatever is in their path.
Lahars are probably the most dangerous. They are volcanic mudflows. They also travel fast and are generally mixtures of water, ash, pyroclastic material. The water may come from lakes rivers or melted ice.
Ash falls is another hazard associated with volcanic eruptions. Ash can choke people and collapse houses. They can also cause problems for airplane engines.
Landslides are common on steep-sided volcanoes and thus represent another secondary hazard for those that live near a volcano.
Climate changes - throughout Earth's history, large volcanic eruptions have impacted the climate. We can see evidence for incredible volcanic eruptions such as that which formed crater lake. These voluminous events can alter the amount of sunlight reaching Earth by ejecting ash into the high atmosphere.
With only a few exceptions, floods are natural processes associated with normal development of streams and rivers. Floods are caused by
Much of the hazard associated with flooding occurs because of our historical and understandable utilization of river and stream flood plains.
A flood plain is a relatively flat surface bordering a stream or river that occasionally floods during times of high water discharge. Past development of flood plains is easy to understand:
But these exist conveniences because the area floods!
Fortunately, like earthquakes and volcanoes, small floods are more frequent that large floods. You may have heard the terms 50-year or 100-year floods. These are only identifying the average recurrence interval between such floods. Natural floods are not periodic - they have a random component in their history.
The construction of communities on or near streams and rivers can greatly influence the character of the water system. Normally, precipitation is percolated through the soil until the soil saturates, then runoff occurs and the water flows across the surface to the stream. The water in soil is eventually added to the ground water, which also often drains into streams. Covering the soil with buildings, streets, parking lots, etc. can cause an enhanced runoff (since the water can't permeate the asphalt) and result is stream flooding. Over time, the lack of ground water recharge can result in a dry stream for much of the year. Flooding of populated regions can also wash pollutants into the water system resulting in substantial problems down stream.
Humans have invested much time and effort into managing streams and rivers. At times, streams have been straightened, levees have been constructed along rivers and streams, and dams have been constructed to manage river discharge. Often these "fixes" have created other problems. For example, straightening a stream causes an increase in the rate of flow which causes increased erosion at the up-slope side and increases deposition down-slope of the altered channel.
Levees offer good protection against small floods, but also provide a false sense of security for those behind them. Floods such as the 1993 Mississippi and Missouri drainage basin can severely test the strength of levees, cause some to fail, and simply transfer the flood hazard to another part of the river.
Finally, changes in river and stream flow can reduce the amount of sediment reaching coastal regions which may result in beach erosion.
Landslides are second only to earthquakes in terms of financial loss. Landslides are a part of natural erosion processes, but can also be influenced by human design and construction. The steepness of a slope is the most significant factor that contribute to land-sliding. Other factors such as soil type, water content, rock type, and bedding orientation (sedimentary rocks) can influence the tendency to slide. Human landscaping can also upset the stability of a slope. We must be careful when altering the slope of hills (where we like to build) and insure that the region is well drained and well supported.
In some regions, the extraction of groundwater (or petroleum) has produced tens of feet of subsidence. If you recall, much of the available fresh water is stored beneath Earth's surface, in the pore-spaces of rocks. We often tap those resources and drain the water for drinking, irrigation, etc. If we pull something out of the ground, and don't replace it the weight of the overlying material will cause subsidence.
Another cause of surface subsidence is the natural dissolution of limestone. Limestone (which is very common in this area) slowly dissolves in water. The result is the formation of caves. At times, the collapse of the cave roof may cause near-surface subsidence called sink holes. Sink holes can be large and form suddenly, swallowing human structures in the process. Most often the process is slow and the features are more stable (on our time scales).
The basic principle behind the operation of a green house is that short-wavelength heat passes through the glass windows, but long-wavelength heat is trapped inside by the glass. Sunlight is rich in short-wavelength heat - that heat is absorbed by materials in the greenhouse that then radiate infrared heat, which can't escape through the glass. For visible radiation the glass acts like a window; for infrared radiation, it acts like a mirror. For a planet, the atmosphere takes the place of the glass building and the role of glass is mimicked by chemicals in the atmosphere. Without a greenhouse effect, Earth would be about 75deg. colder. Venus has a "runaway" greenhouse effect, and its surface is hot enough to melt lead (400deg.C).
On both Earth and Venus, the primary greenhouse gas is carbon dioxide (CO2). Since the industrial revolution and the burning of fossil fuels we have dramatically increased the amount of CO2 into the atmosphere. We have also seen an increase in the average temperature of Earth.
This is a very difficult question to answer and is the subject of current research. Other factors could be at work, such as increased output of energy from the Sun. Over the last few thousand years, the correlation of CO2 and temperature have correlated well - but the exact relationship is unknown.
Other factors in the Earth system can be sources and sinks for the excess carbon in the atmosphere. And other chemicals that we pump into the atmosphere actually result in lower temperatures. The system is very complex and the interactions of many different components of Earth interact - which makes it a a very interesting scientific problem.
Another hazard to which we may be contributing is the depletion of atmospheric ozone (O3).
Ozone is a natural sunscreen that blocks some (about 30%) of the ultraviolet radiation traveling from the Sun to Earth. Ultraviolet radiation is a high-energy wave that can cause cell damage in plants and animals. Natural ozone is created by the ultra-violet light interacting with oxygen molecules. Problems occur when chlorine is present - then ultra violet light breaks down ozone, depleting the atmosphere of this shielding molecule. A 1% decrease in the ozone concentration can result in a 6% increase in skin cancers.
Again, we have observed a trend of decreasing ozone since the industrial revolution. The main chemical culprit are chloroflurocarbons which are unreactive in the lower atmosphere, but catalyze the destruction of ozone in the upper atmosphere.
Recently, an "ozone hole" (a region of ozone depletion) has formed in the south-pole region and efforts are under way to study the atmospheric chemistry involved and construct ways of maintaining safe ozone levels in Earth's atmosphere.