In order to understand the origins of earthquakes, volcanoes, and tsunami, one needs to understand the role that the tectonic cycle plays in Earth's recycling of the outer layers. The Earth is composed of different density layers; a solid metallic inner core surrounded by a liquid outer core, and mantle core composed of heavy rock. This mantle core is composed of the asthenosphere layer (a weak solid), the lithosphere (a strong solid), oceanic crust (low-density rock), and a hydrosphere (liquid) layer. Above the crust, we have the atmosphere composed of gases. In order to visualize the way that plate tectonics works, and the different layers that make up the Earth, the metaphor of a hard-boiled egg is a useful one. The Earth's core is the yolk, the rest of the egg is the mantle, composed of the shell (lithosphere), the slippery inner lining (asthenosphere), and the egg white (Abbott, 2004).
In the tectonic cycle, melted asthenosphere flows upward as magma, and solidifies to make up the ocean floor or lithosphere. The tectonic cycle can be described as the recycling of the upper layers of the Earth (asthenosphere, lithosphere, and the oceanic or continental crust). Through seafloor spreading, this new lithosphere moves sideways from the oceanic crust on top of the asthenosphere. When two separate slabs of oceanic lithosphere collide (depending on the age, temperature, and density of the slabs), can either turn downward into the asthenosphere (subduction) where it is reabsorbed into the mantle, or if the layer is less dense the slab can override it.
Most everyone is familiar with the concept of plate tectonics, the study and movement and interactions of the lithosphere plates. But what is the role that these plates play in the formation of mountains, volcanoes, and earth movements (earthquakes)? Because of the seafloor spreading, these lithosphere plates can either pull apart at divergent zones, slide past each other at transform faults, or collide with each other at convergent zones, Each of these processes account for the different types of earthquakes, mountain formations, and volcanoes.
Most of the earthquakes and volcanoes can be explained in terms of plate tectonics. The lithosphere is broken down into different plates that depending on whether they collide, move away, or go past each other, generate different types of earthquakes. When the plates pull-apart, they cause the rock to fail and smaller earthquakes take place. These earthquakes do not tend to pose great threat to humans. When the plates move past each other in horizontal movements, also called transform faults, larger earthquakes are a consequence of the irregularity in the plate boundaries. It is where two plates converge that the most serious earthquakes originate. These usually occur at subduction zones where one of the plates is pushed back into the mantle releasing incredible amounts of energy that can produce catastrophic earthquakes.
Earthquakes
Most earthquakes originate along fault lines. A fault is referred to a fracture or a crack in the Earth's layers. There are three different types of faults: dip-slip, strike-slip, and transform. There are two separate kinds of dip-slip faults depending on whether the fracture in a vertical direction by the plates pulling apart (extensional or normal faults); or whether the fracture is in a vertical direction but the rock layers are pushed together (reverse fault). If instead the movement along the fault is horizontal, where the motion of the two plates is in opposite directions, these faults are referred as strike-slip faults. The most famous of these faults is in San Andreas, California. Finally, transform faults are a special type of horizontal fault movement that occurs along the sides of two plates. A fault can be described as strike-slip and as a transform fault.
In order to understand the earthquake history of a region, it is necessary to know what the plate-tectonic history of the regions is. Most often, when describing earthquakes as natural disasters, it is the conjunction of multiple factors such as the type of tectonic-fault, the resonance between seismic waves, the intensity, the sediment foundation, population density, and building design, that influence the intensity and damage they can produce.
Some of the most known earthquakes that have occurred throughout history include Santorini, 1628 B.C.E.; Vesuvius, 79 C.E.; Tambora, 1815; Krakatau, 1883; Mount St. Helens, 1980; and Pinatubo, 1991. Each of them have the same commonality in the destruction and devastation they left behind, reminding us that the forces of nature are in constant motion, and that have direct and indirect effects both in those locations and across the different regions of the Earth.
Volcanoes
The process by which a volcano is formed can simply be described as melted rock that erupts to the surface of the earth. The relation between plate tectonics and volcanoes is a direct one, in that 80 percent of volcanism takes place at the centers of two plates separating. These eruptions tend to be peaceful as compared to those volcanoes originating in subductions zones, which involve magma rich in crustal rock and gas-rich magma producing some of the most explosive and devastating volcanoes. There is little or no volcanism associated with transform faults.
The process by which rock melts and becomes magma due to increase temperature, decreased pressure and water content affect the way that a volcano will erupt. The most important volcanic materials include lava, pyroclastic debris, and glass. In order to understand the different eruption styles, as well as the volcanic landforms, one needs to take into account the three V's (viscosity, volatility, and volume). Depending on the levels of viscosity, volatiles, and volume (from low to high) the different volcanic landforms originating are shield volcanoes, flood basalts, scoria cones, stratovolcanoes, lava domes, and calderas. The size of a volcanic eruption is measured by the volcanic explosivity index (VEI) ranging on a scale from 0 to 8.
Although New England is not a prone area to earthquakes or volcanoes, students might be curious to find out that earthquakes do indeed occur in our area. Some of them significant in the intensity and damaged produced. The most severe earthquake in Connecticut took place in East Hadam on May 16, 1791. A great resource to find out the date, location, and intensity of the most recent earthquakes worldwide, including New England, can be located in the U.S. Geological Survey web page (see Teacher Resources).
Tsunami
Another type of natural disaster that is associated with plate tectonics is "tidal waves" or tsunami. In Japanese, the literal definition of "tsunami" is "harbor crossing wave". "Tsu" means either "ferry crossing" or "harbor crossing" and "nami" means wave. The origin of the meaning of the word tsunami is related to the fact that fishermen thought that these types of waves only occurred in the harbors. The rationale is that a tsunami at deep sea may cause little or not visible effect but when the fisherman arrived back to their harbors they would encounter the devastation of their villages and harbors.
A tsunami is a series of waves generated by the sudden rapid displacement of a body of water due to earthquakes, landslides, volcanic eruptions, or meteorite impacts. Like volcanoes and earthquakes, the most devastating tsunami originate in the subduction zones where one of the plates is pushed back into the mantle releasing incredible amounts of energy. The most common cause is an undersea earthquake. The most devastating tsunami to date has its origin in the December 26, 2004 Indian Ocean Earthquake. At a 9.15 magnitude, this earthquake started a series of tsunami that killed over 230,000 people ranging in distance from the proximity of the earthquake in Indonesia, Thailand, and the northwestern Malaysia, to as far as Somalia, Kenya, and Tanzania in eastern Africa.