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Earthquakes: An earthquake is the sudden and violent shaking of the Earth's surface in localized regions. Though the crust is always moving in extreme slow motion along plate boundaries, the movement that causes an earthquake is much faster and much more unpredictable. The plate boundaries responsible for quakes are Convergent and Transform - these boundaries put pressure on the plates, friction builds up and then releases suddenly.
The study of earthquakes has given scientists a vast amount of information about Earth's interior. The seismic waves created by an earthquake travel through the entire planet as vibrations. Using seismographs to track the movement of the seismic waves created, scientists can record the magnitude and the distance the waves travel. Earthquakes produce 3 basic types of seismic wave: P-waves, S-waves, and Surface waves. P-waves are Primary waves and are the 1st type of wave produced. Primary waves are the fastest but not very strong; they move the ground in a compression motion like a slinky or inch-worm. P-waves can move through both solid and liquid matter, making them easy to track with a seismograph anywhere on the planet. S-waves are also called Secondary waves and are the 2nd type of wave produced by an earthquake. S-waves are slower and cannot move through liquids; they move in an up-and-down motion. Tracking Secondary waves have led scientists to the conclusion that parts of the Earth's interior is in the liquid state because the seismograph data shows the waves being stopped by something, unable to travel across the globe like P-waves. These waves are usually the first to be felt during an earthquake. The last wave is called a Surface wave. These waves only travel through the crust, but they are the most noticeable. Surface waves caused the ground to move in rolling motions and are responsible for the damage and destruction for which earthquakes are feared.
The largest quakes are caused by Convergent Boundaries. Earthquakes are measured using a scale of magnitude. Magnitude is based on the maximum motion of the seismograph readings, and it increases by powers of 10 for each level. The Richter Scale is used to express the magnitude of quakes; it ranks them from 0 to 10, with rankings of 0-2 being so small they are hardly noticed by people. Though records do not span our entire history, the strongest recorded quake is a 9.5 in Valdivia, in southern Chile in 1960. Other locations that have experienced large earthquakes are Alaska, Japan, and Indonesia. The number of deaths related to quakes are directly connected to the population of the area struck. Chile didn't have as many casualties because it was not a very populated area. Indonesia had nearly 250,000 deaths because it hit at the nation's capital city.
Volcanoes: A volcano is a vent or fissure in Earth's surface through which lava, ash, rock fragments, and a variety of gases exit. Volcanoes on land usually form mountains, growing each time they erupt in slow flows or being destroyed if they erupt violently. Volcanoes underwater can form ridges, such as along Divergent Plate Boundaries, or seamounts that can grow into volcanic islands and island chains. The source of a volcano's lava is the layer of Earth called the Mantle. The Mantle's upper sections, especially the Asthenosphere, is influenced by Convection Currents in the Mantle that move the semi-liquid, molten rock around in cycles. In areas of upward flow, the magma of the Mantle weakens the crust to create pressure-release vents. Magma does not tend to travel directly to the Crust, but builds up in magma chambers. Sometimes the magma never makes it through the Crust; it cools and hardens into Igneous rock. Magma that does reach the surface is renamed as Lava. Magma/lava can contain a variety of minerals and gases, making each lava flow capable of different eruptions. There are 2 main types of eruption: Effusive and Explosive. The more viscous the magma/lava, the less easily it flows and the more likely it is to trap gases. Magma high rich in Silica is very viscous and sticky, while magma rich in iron or magnesium is less viscous. Effusive eruptions have a low viscosity, making them runny. These runny flows allow gases to easily escape and the lava flows smoothly over the land before solidifying into Extrusive Igneous rock like Obsidian. Explosive eruptions have a high viscosity, making them thick. This thick magma/lava traps gases and may block the exit to the surface causing pressure to build up in the volcano. Once the pressure is too great, the volcano explodes violently and spews ash, gases, and rock fragments into the air. Especially violent eruptions can include a Pyroclastic Flow that contains extremely hot and poisonous gases that travel down slope at high speeds, destroying everything in their path. The ancient city of Pompeii was hit by a pyroclastic flow that covered the city in hardening ash and killed hundreds of people.
Volcanic Landforms are made by the materials ejected from the volcano during eruptions. There are several:
- Shield volcanoes have gentle slopes that extend away from the summit, resulting from effusive eruptions.
- Stratovolcanoes, also called Composite Volcanoes, have steeper slopes and high cone summits, shaped more like a mountain, from alternating effusive and explosive eruptions.
- Cinder Cone volcanoes have conical shapes that can reach high altitudes due to the build up of explosive eruptions, often pyroclastic.
- Lava Domes are formed by highly viscous effusive eruptions with little trapped gases. These flow slowly and solidify not far from the summit. The dome may reform in a crater, in more of a traditional thinking manner.
- Calderas are large, circular depressions that are very wide. It forms after a magma chamber is completely emptied by a explosive eruption and the ground collapses.
- Magmatic Dikes look like chimney stacks. They form when magma cuts through older, horizontal rock and hardens within it in a vertical pattern.
- Sills are very much like dikes. They look like cliffs and form when magma intrudes into cracks in rock formations, but they are parallel to the rock they harden in.
- Volcanic Plugs or Volcanic Necks are formed when magma hardens within the interior pipe of a volcano. These become more apparent as surrounding rock erodes away over time, exposing the neck to resemble a butte.
Volcanology is the study of volcanoes. The prediction of when a volcano will erupt is not possible at this time. However, studying lava viscosity, gases, tremors, variations of the surrounding surface, and temperatures of nearby bodies of water can help scientists forecast when a volcano may erupt. Studying the materials ejected from a volcano also provide information about Earth's interior.
Tsunamis are huge waves or a series of very large waves caused by underwater earthquakes of a magnitude of 7.5 or higher or violent underwater volcanoes (though sometimes especially violent island, coastal eruptions, or landslides can create them, too). In the deep ocean, waves do not grow to great heights due to the distance from the ocean floor and the way energy travels in waves through liquids. When the waves get closer to the shore, the distance to the ocean floor decreases and the waves begin to grow in height. Tsunamis can travel around 800 miles per hour and only begin to slow as they get closer to shore and friction with the ocean floor increases. Once they hit land, they cause a surge of ocean water to pour inland for miles, destroying everything and creating dangerous flood currents.
Japan requires its citizens to practice tsunami drills, just like we practice fire drills, because their location places them very close to an underwater Convergent Boundary that generates strong earthquakes as well as several volcanic areas.
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