When the Earth Burps

Energy is everywhere and can be observed in various forms at all levels: from chemical reactions in atoms, to thermal radiation entering the Earth’s atmosphere from the sun, to the sound waves that travel through the air; energy is constantly moving all around us.  Scientifically speaking, energy is defined as the ability to do work; work in turn, is defined as the exertion of force over a distance (Trefil, 2010).  Therefore, every form of energy must be capable of causing movement by applying a strong enough force to its surrounding molecules to cause them to shift from their original positions.  There are two main forms of energy: potential energy and kinetic energy.  Potential energy is energy that is stored within an object, waiting to be released.  Potential energy does not move on its own, but it has the potential to convert to another form of energy and travel from its original location.  Kinetic energy is energy in motion.  Earthquakes provide dramatic examples of the ability of energy to do work and clearly demonstrate the necessary conversions energy must pass through in order to perform that work.

The geological energy of earthquakes begins as built up pressure in the rocks deep beneath Earth’s crust.  This pressure creates elastic potential energy within the rocks.  When the rocks eventually give out and break, the elastic potential energy is instantly converted into kinetic energy and sent up to Earth’s surface by way of seismic waves (Trefil, 2010). These seismic waves cause the tremors felt on Earth’s surface, known as earthquakes.

The Earth’s crust, along with the top layer of the mantle, is broken up into more than a dozen tectonic plates.  These plates are responsible for the movement of the continents and the changes to the Earth’s surface features, as well as volcanoes and earthquakes.  The movement within Earth’s hot liquid core causes these plates to move around on the surface of the planet, dramatically altering the positioning of the continents while creating new oceans, mountains, and islands.  If satellite footage of the Earth from 240 million years ago could have been taken, the picture would show a planet almost unrecognizable as Earth.  That is because over the past 200 million years the tectonic plates lying under Earth’s surface have been in constant motion, sliding along each other, crashing into each other and pushing one another away.  All of this movement over the millennia took what was once a single continent, known as Pangea, and slowly pushed and pulled the land apart creating the seven continents now in existence (Trefil, 2010). This tectonic movement is a constant and ongoing process that continues to change our planet every day.

English: Pangea animation
English: Pangea animation (Photo credit: Wikipedia)

The Great East African Rift Valley is an excellent example of the effects of plate movement.  The East African Rift system is the result of one tectonic plate breaking into two separate plates and pulling away from each other, causing the continent of Africa to slowly split in half from north to south.  Millions of years from now Africa will be two individual continents, East and West Africa, separated by an ocean.  Evidence of this divergence can already be seen in the formation of the Red and Dead Seas: as the plates began to pull apart, these bodies of water replaced the area formerly covered by land and as the continents continue to separate, the size of the ocean will continue to increase, much like the Mid-Atlantic Ridge in the middle of the Atlantic Ocean, which has been growing in size since the Americas began separating from Pangea 240 million years ago (Trefil, 2010).  The San Andreas Fault in California offers another active example of plate tectonics.  Due to the sliding movement of the plates at this fault line, scientists have predicted that about 15 million years from now, Los Angeles and San Francisco will be neighboring cities (USGS, 2011).

The borders of tectonic plates are called plate boundaries and are composed of many faults.  The faults at these plate boundaries are responsible for the majority of earthquakes.  There are three main types of plate boundaries beneath Earth’s surface: Divergent, Convergent, and Transform Plate Boundaries; and each have different causes and effects associated with their movement.

Divergent plate boundaries are plates that are being separated or pulled apart from one another by volcanic activity at Earth’s surface.  When the magma that has been released from the hot core cools on the surface it forms new plate material and forces the plates farther apart; old divergent boundaries are all found beneath oceans, like the Mid-Atlantic Ridge, whereas new divergent boundaries can be found in the middle of continents, such as the Great East African Rift.  The knowledge we have gained by studying the effects of the divergent boundary at the Mid-Atlantic Ridge gives great insight into what will eventually happen to the continent of Africa as the rift grows bigger each year.  Divergent boundaries are accountable for creating new bodies of water and for the current distribution of our oceans.  Low energy earthquakes at divergent boundaries occur when the original plate material breaks and spreads the plates apart forming new plate material (Trefil, 2010).

Divergent plate boundary: Mid-Atlantic Ridge
Divergent plate boundary: Mid-Atlantic Ridge (Photo credit: Wikipedia)

The movement at convergent plate boundaries is directly linked to the effects of the spreading of Earth’s crust at divergent boundaries.  Convergent boundaries are formed when two tectonic plates are forced together.  The spreading of tectonic plates at divergent plate boundaries forces other tectonic plates to collide, causing subduction of one of the plates and compression of the other plate.  When a convergent plate is subducted, the plate material is forced back down into the Earth’s core, where it heats up and returns to its former state as magma, ready to begin the process of forming new crust material over again.  The compression on the other plate causes the land to fold together, forming new mountain ranges along the boundary.  Earthquakes at convergent boundaries are an effect of the collisions of the two tectonic plates and are responsible for the creation of the majestic mountain ranges, mysterious deep seas, and volcanic islands that populate the planet.  The Northwest coastline of the United States, running from Washington down through Oregon, is a convergent plate boundary.  The large North American continental plate and the much smaller Juan de Fuca oceanic plate collided, causing the oceanic plate to subduct beneath the continental plate, forming the volcanic Cascade Mountain Range from the materials of the compressed continental plate (Geology.com, 2011).

Both divergent and convergent plate boundaries involve volcanic activity and the formation of new plate materials, mountains, and oceans.  Transform plate boundaries on the other hand are not associated with volcanic activity and do not create new plate materials.  Transform boundaries are faults where one tectonic plate is trying to slide past another, in an opposite direction.  Friction usually holds these plates in place and prevents them from moving; however the build-up of stress and pressure over time can become too powerful for the rock to contain, causing it to break and release a sudden burst of energy.  At transform boundaries, an earthquake is capable of causing the rock to move several meters in one powerful burst (Trefil, 2010).  Transform boundaries are characterized by the oppositional direction of plate movement, which can be seen in the San Andreas Fault in California.  The fracture zone of this fault clearly shows the boundary where the two plates are trying to move past each other.  The Pacific plate, which includes the city of Los Angeles near its eastern border, is sliding northwest of the North American plate, where San Francisco rests on the western border (Geology.com, 2011).  Assuming the pattern of movement observed at this fault zone continues for the next 15 million years, the Pacific plate will slide up along the North American plate, until, at least for a time, Los Angeles will be just a few kilometers west of San Francisco, about 600 kilometers north of its current location (USGS, 2011).

The distribution of these plate boundaries and the forces they exert play a key role in assessing the risk of an earthquake for a particular region.  Most of the continental United States, for example, sits on top of the North American plate, which is a large, relatively stable tectonic plate that does not interact with many other plate boundaries, making most of the United States a fairly stable environment, at least as far as earthquakes are concerned.  The greatest risk of earthquakes in the United States lies along the western coastline of the continent.  There are a few different plate boundaries in that region that increase the risk of seismic activity.  The previously discussed San Andreas Fault transform boundary is only one of the seismic dangers on the west coast; convergent boundaries between the Pacific plate and the North American plate are constantly pushing on each other, subducting and compressing with every geologic movement; volcanic activity on the Pacific plate, west of the continental United States, continues to make new crust materials, as the Pacific plate grows larger from the production of these new materials its convergent boundaries with the North American plate are stressed even further.  All of these factors raise the risk to the west coast of the continent.  While the west coast holds the highest risk of earthquakes in the United States, earthquakes have been known to happen in the middle of the North American plate.  While these events are not yet fully understood, the expansion and contraction of heated and then cooled rock could play a role in these seemingly random events (Trefil, 2010).

The distribution of earthquakes throughout the rest of the world follows the same patterns.  The closer to an active plate boundary an area is the higher the risk of an earthquake.  The majority of the world’s earthquakes, about 90% of all recorded seismic activity, occur in the “Ring of Fire,’ located in the Pacific Ocean along boundaries between the North American plate and the Pacific plate as well as consuming the upper half of the Pacific plate (Geology.com, 2011).  This “Ring of Fire” got its name from the tremendous amount of volcanic activity that occurs there, which is directly linked to the high number of earthquakes in this region.  As the volcanoes form new crust materials, the old ones must be subducted back into the earth to repeat the cycle.  This cycle illustrates the close connection between divergent boundary changes and convergent boundary occurrences.

Earthquakes prove to be a complex problem for scientists as they don’t yet have the ability to predict seismic activity before it happens, let alone control it.  While the scientific community has gained much knowledge and insight about earthquakes, the ability to forecast an exact event still eludes them.  A recent earthquake in Taiwan on November 8, 2011 had a magnitude of 6.9.  While this was the highest magnitude earthquake that week, it was far from the only one.  In fact, there were 266 earthquakes recorded in the 7 days prior to November 12, 2011.  The magnitudes of these seismic events ranged from as low as 2.5 up to the 6.9 event in Taiwan (USGS, 2011).  The abundant number of seismic activities in just one week alone makes the prediction of an exact location of an earthquake nearly impossible and the ability to determine the magnitude of any upcoming activity is even more elusive.

The deadliest and most devastating earthquake ever recorded occurred in China on January 23, 1556, killing approximately 830,000 people.  The second deadliest earthquake in history killed approximately 316,000 people.  This earthquake occurred near Haiti on January 12, 2010.  Before the 2010 event in Haiti, the most recent earthquake with a significant number of deaths was again in China, on May 12, 2008 (USGS, 2011).  With over 450 years and almost 14,000 kilometers spanning the world’s two most deadly earthquakes, comparisons made would never hold up to testing through the scientific method.  While the frequency of earthquakes combined with the differing magnitudes of each event make the prediction of specific events near impossible, the advancements made through research and technological developments have brought us much closer to understanding the seismic energy held deep within the planet.  At the present time our knowledge of plate boundaries and risk distribution has already helped save lives by developing the best buildings and road structures to reduce damage from these uncontrollable natural events and using this new technology in the highest risk areas.  As with all scientific research, victories come in small developments and patience is needed to find the answers to the bigger questions.

This slow, but steady pace in scientific development can be seen in the study of seismic events and the tools that were developed to help with the testing.  The first observed earthquake led early thinkers to question what was happening beneath their feet.  With limited knowledge about this natural phenomena and a lack of technology when it came to scientific tools, scientists of the time were only able to measure the ground shaking in a single place during an earthquake.  Hundreds of years later, as science gained a new understanding of energy, and specifically wave energy, better seismographs were developed that were able to measure the individual S- and P- waves radiated from an earthquakes hypocenter, giving a radius of how far away, in any direction, the earthquake may have come from.  For a while this radius was the best guess scientists had when it came to determining the origin of an earthquake.  Once again, technology drove science forward even further.  As better methods of communication became available, the use of triangulation was adapted to help locate the exact origin of an earthquake.  With seismographs set up in multiple locations, scientists were able to pinpoint the earthquakes hypocenter by finding where the radiuses of three seismographs, in three separate locations, intersected on a map.

Energy can be found throughout nature in many different forms; there is chemical energy, mechanical energy, thermal energy, biological energy and as we have seen throughout this paper, geological energy.  Each type of energy performs different functions to get different kinds of work done, but all energy does work and therefore all energy is the same at some level.  When comparing geological energy to biological energy we can see some differences in its intent, but there are several similarities as well.  Geological energy comes from deep inside Earth’s core.  This energy has been working its way out of the core, venturing towards space, since the creation of the planet.  Biological energy, such as the calories we get from the food we consume, is found on Earth’s surface, regenerating in all living things.  Although these two forms of energy are found far away from each other and perform different kinds of work, they have one very big thing in common: both sources of energy originated in the sun.  Geologic energy deep within the Earth’s core was originally solar energy absorbed by algae on the Earth’s surface.  Over millions of years this algae settled to the Earth’s core where it now provides geological, mechanical energy in the movement of the tectonic plates (Trefil, 2010).  Chemical and biological energy also originate from the sun, where calories are taken in by plants through the process of photosynthesis and carried up the food chain to the products we consume every day.  Another important similarity in all energy forms is its ability to convert from its original form into any other form of energy.  Geological energy starts as elastic potential energy and is converted into kinetic energy as it sends seismic waves (yet another form of energy) through the crust to the surface.  Biological energy starts as solar energy and converts to chemical energy that our bodies then convert to physical energy, used to make our bodies do work.

No matter what its original purpose or form, energy is everywhere and without energy there would be no life.  Earthquakes represent an extreme and dangerous form of natural energy but not all energy is dangerous, in fact most energy is beneficial.

References

Geology.com.  (2011).  Convergent Plate Boundaries.  Retrieved from http://geology.com/nsta/convergent-plate-boundaries.shtml

Geology.com.  (2011).  Plate Tectonics Map.  Retrieved from http://geology.com/plate-tectonics.shtml

Geology.com  (2011).  Transform Plate Boundaries. Retrieved from http://geology.com/nsta/transform-plate-boundaries.shtml

Trefil, J.  (2010).  The Sciences: an Integrated Approach.   John Wiley & Sons Inc.

United States Geological Survey.  (2011).  Earthquake Topics for Education.  Retrieved from http://earthquake.usgs.gov/learn/topics/

United States Geological Survey.  (2011).  National Seismic Hazard Mapping Project.  Retrieved from http://earthquake.usgs.gov/hazards/

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