12. Earth's Layers

Image source: http://mediatheek.thinkquest.nl/~ll125/en/fullstruct.htm

The crust that is not below the ocean is called continental crust and the crust formed by magma at the bottom of the ocean (more on this later) is called oceanic crust. The most important part of the distinction is that the oceanic plate is much denser than continental crust and their chemical compositions are different. (Continental crust has more silica, SiO2 while oceanic crust has more iron and magnesium with little silica.) When geologists and seismologists talk about the earth, they characterize the layers not based on their chemical properties (metal, rock etc.) but rather use their physical properties, which are more important when discussing their effects on the Earth.

Image source: http://images.yourdictionary.com/lithosphere

The uppermost layer of the Earth is classified as the lithosphere (rock sphere), which is both the “crust” and the uppermost solid region of the mantle. The asthenosphere (weak sphere) comes next, a semisolid layer that allows the lithosphere to “slide” slowly on top of it. The rest of the mantle below it is considered the lower mantle. The inner and outer core nomenclature remains the same. In the rest of this information sheet, the layers will be referred to as the core, lower mantle, asthenosphere and lithosphere.

Volcanoes and earthquakes are both directly related to the seismic activity of Earth. The lithosphere is composed of over 60 pieces, called tectonic plates, which are constantly in motion over the asthenosphere. There are seven primary plates, seven secondary plates and some fifty tertiary plates. There is some debate over certain tertiary plates, as they might be part of primary plates that are breaking apart. These tectonic plates move very slowly, at a rate of millimeters to centimeters per year.

There are two major theories about what directs the movement of tectonic plates. The theory that is taught most often is that of convection current. Radioactive decay in the core of the Earth heats up material in the lower mantle, causing it to become less dense. The material slowly rises, and cools as it goes through the upper mantle and reaches the asthenosphere. As the material cools, it falls back down again towards the core, creating a cycle like that of a lava lamp. This cycle happens all around the core, creating convection currents that move the asthenosphere like a conveyer belt, slowly moving the lithosphere above it.

Image source: http://www.montgomerycollege.edu/Departments/planet/AstroLink2.0_html/earth/index.htm

The second and newer theory claims that the convection current theory is flawed, because the asthenosphere is “weakly coupled to the lithosphere, and has little effect on its motions” (Stern, 2007). The direction of motion of the asthenosphere has also been shown to not always correspond to the lithosphere, which raises questions about how the lithosphere is moving.

Rather than forces within the Earth powering the lithosphere, this new theory proposes a top-down process, instead of a bottom-up process. When oceanic crust is formed at oceanic ridges, it is less dense and cooler than the asthenosphere below it. The mantle part of the lithosphere slowly cools over time, causing the entire lithosphere to become denser. It is important to remember that the Earth has been cooling since the moment it was formed and that heat is rising from the core and dissipating through the lithosphere and the atmosphere. Eventually the lithosphere will become denser than the asthenosphere, and become “negatively buoyant” (more dense than the material below it). As this happens, the lithosphere submerges under the asthenosphere.

Convection currents are the accepted theory of how tectonic plates move, although newer findings may someday prove otherwise. While the cause of tectonic plate motion may still be under study, oceanic trenches and subduction zones are proof that the plates are moving. There would not be earthquakes if the plates were not in motion.

The boundaries of tectonic plates meet in different ways. The first kind that we will discuss is not actually a “meeting” of boundaries, but rather the situation where two or more plates are moving away from each other. These diverging, constructive plate margins are where new plates are formed in the rifts between the plates. Magma rises up and pushes the plates apart, which creates new oceanic plate. When tectonic theory was first being developed, scientists did not understand the diverging oceanic ridges, because they suggested that the Earth was growing bigger from the inside. The answer to this conundrum was the discovery of another boundary: ocean-continent plate convergences, where the oceanic plate subducts under the continental plate. The plate that is subducted heats up while in the asthenosphere, rising to form volcanoes on the continental crust. The oceanic crust subducts beneath continental crust, causing the volcanoes that build mountain ridges and forming oceanic trenches. An example of subduction is the west coast of South America, where the Nazca plate is subducting under the South American plate at a rate of approximately 7 cm per year. The Andes Mountains are the result of this action.

Image source: http://www.tulane.edu/~sanelson/geol111/pltect.htm

Ocean-ocean converging plates work similarly to ocean-continent convergences, with one plate subducting under another and forming volcanoes on the other side. The older of the two meeting oceanic plates subducts under the newer plate because it is denser and thus heavier. This sort of convergence usually results in the formation of island arcs, such as Japan. Continent-continent convergences are the major source of the biggest mountain ranges, such as the Himalayas. Continental crust does not subduct under other continental crust because it is too buoyant. The force of the moving plates in collision folds the crust and builds mountains.

Image source: http://www.tulane.edu/~sanelson/geol111/pltect.htm

Most of the tectonic plates are massive and are named after the continents that sit on them, such as the North American plate, the Antarctic plate, or the African plate. Several plates are much smaller and account for just as much seismic activity, such as the Nazca plate off the west coast of South America (Andes Mountains) and the Indian plate (Himalaya Mountains). The “Ring of Fire” encompasses all the coastal edges of the Pacific Ocean. It includes the Californian coast and the Alaskan, Japanese, South East Asian, and Oceania regions. Stresses build up at fault lines when plates grind or push against each other and new plate is “grown” when plates move away from each other. Eventually, these stresses are relieved when the crust suddenly breaks and creates an earthquake.

Image source: http://www.extremescience.com/graphics/plate-tectonics-map-usgs.jpg

Earthquakes

During an earthquake, the energy released travels radially out from the hypocenter, which is the location of the rupture under the surface. The epicenter is defined as the location directly above the hypocenter, on the surface of the ground. Much like the ripples on a pond, or sound waves in air, the energy propagates through matter (the Earth) and causes the ground to move, which in turn causes us to feel the earth shake. The energy travels in four different seismic waves: two body waves and two surface waves.

The Primary, or P, waves are high-frequency compressional waves that travel much like sound waves. With particle motion in the same direction as the wave propagation, P-waves can travel through both solids and liquid. Secondary, or S, waves are shear waves. The particle motion is perpendicular to the direction of wave propagation. S-waves travel at about 60% the speed of P-waves and are only able to travel through solids.

Image source: http://www.geo.mtu.edu/UPSeis/waves.html

Surface waves are lower-frequency waves that travel along the surface of the Earth. Of the surface waves, the Love wave is faster and can cause the most damage, as the ground motion is perpendicular to the direction of wave propagation. In Rayleigh waves, the particle motion is vertical and rolling as the wave propagates.

Image source: http://www.geo.mtu.edu/UPSeis/waves.html

If the same earthquake is recorded at three different stations, it is possible to locate the epicenter of the earthquake. Because the P- and S-waves arrive at each station at different times due to their different speeds, it is possible to calculate the distance between the station and the epicenter. Each of these distances can be drawn in a circle around the recording site and the epicenter is located where the three circles intersect.

Image source: http://myweb.cwpost.liu.edu/vdivener/notes/earthquakes.htm

Body waves may be faster and reach a location first, but the surface waves have large ground displacements and often cause more damage than the body waves. The magnitude of a single earthquake is related to the length of the fault rupture. Earthquake magnitudes are measured on a logarithmic scale; one magnitude larger is a factor of 10 times more ground shaking and 30 times more energy released. This explains why small earthquakes do not prevent a much larger one from happening. In order to release the same amount of energy of a magnitude M9 earthquake, it would take 729,000,000 (729 billion) magnitude M3 earthquakes.

California is seismically active because it is the intersection of the North American and the Pacific plates. Many long faults, such as the San Andreas Fault, have the potential to generate large-magnitude earthquakes. In California, the integrity of infrastructure is of the utmost importance. Taller buildings made out of brick, concrete or adobe are brittle and more prone to being damaged, while shorter buildings made of wood are more flexible and less likely to be affected. The damage to the Santa Barbara Mission during the 1925 earthquake is a perfect example of this, as the adobe bell towers crumbled and huge cracks appeared in the walls. Now, many larger buildings in California are built on frictionless rollers that do not transmit the motion of the earth to the building. Modern buildings are also constructed on flexible pads or seismic dampers, which both minimize the energy transmitted. The seismic retrofit of older buildings and transportation structures, bringing them up to building code standards, is an ongoing project in California.