Dynamic Earth
Volcanoes
Deadly, but life-giving. Hot, but cooling. These are just a couple of the paradoxes brought to us by volcanoes. We've all heard about volcanoes spewing molten, hot lava and suffocating ash. But did you know that without volcanoes, we would not have our atmosphere? Or that some of the gases and dust erupted by volcanoes actually cool the Earth by reflecting solar radiation back into space? It's true. Volcanoes are technically any location where molten material flows from the interior onto the surface of Earth; either from a crack in the ground, a split at the bottom of the ocean, or a hole at the top of a mountain. As far as mountain shaped volcanoes go, there are three types: shield, composite, and cinder cone.
The type of volcano is determined by the type of magma that erupts from it and creates it. Felsic magma has high amounts of silica. Silica melts at a low temperature and is very viscous (viscosity is a measure of a substance's resistance to flow). Felsic magma tends to hold on to gas. The gas builds up and, when it releases, explodes in a violent eruption. The eruption results in a lot of ash, dust, and pyroclastic flows which is a super-heated cloud of gas and ash that speeds down a mountain, burning and choking everything in it. Mafic magma, on the other hand, has low amounts of silica. This magma is hot and runny. The runny nature is caused by the high temperature. Even though it erupts in fountains of fire, we consider them to be gentle compared to the felsic eruptions. We name the lava types that come from mafic magma with Hawaiian names: a'a' (pronounced "ah-ah") and pahoehoe (pronounced "pah-ho-way-ho-way"). A'a' lava is very hot and runny. When it cools, it forms a brittle crust on top. Pahohoe is more ropy and gooey. When it cools, it looks like ropes wrapped together.
Deadly, but life-giving. Hot, but cooling. These are just a couple of the paradoxes brought to us by volcanoes. We've all heard about volcanoes spewing molten, hot lava and suffocating ash. But did you know that without volcanoes, we would not have our atmosphere? Or that some of the gases and dust erupted by volcanoes actually cool the Earth by reflecting solar radiation back into space? It's true. Volcanoes are technically any location where molten material flows from the interior onto the surface of Earth; either from a crack in the ground, a split at the bottom of the ocean, or a hole at the top of a mountain. As far as mountain shaped volcanoes go, there are three types: shield, composite, and cinder cone.
The type of volcano is determined by the type of magma that erupts from it and creates it. Felsic magma has high amounts of silica. Silica melts at a low temperature and is very viscous (viscosity is a measure of a substance's resistance to flow). Felsic magma tends to hold on to gas. The gas builds up and, when it releases, explodes in a violent eruption. The eruption results in a lot of ash, dust, and pyroclastic flows which is a super-heated cloud of gas and ash that speeds down a mountain, burning and choking everything in it. Mafic magma, on the other hand, has low amounts of silica. This magma is hot and runny. The runny nature is caused by the high temperature. Even though it erupts in fountains of fire, we consider them to be gentle compared to the felsic eruptions. We name the lava types that come from mafic magma with Hawaiian names: a'a' (pronounced "ah-ah") and pahoehoe (pronounced "pah-ho-way-ho-way"). A'a' lava is very hot and runny. When it cools, it forms a brittle crust on top. Pahohoe is more ropy and gooey. When it cools, it looks like ropes wrapped together.
Cinder Cone - Cinder cone volcanoes are made entirely out of felsic magma. These volcanoes erupt violently and form quickly on a geologic scale. Made almost entirely out of ash and dust, cinder cone volcanoes form in the interior of continents where the felsic continental crust has melted to give these volcanoes their fuel. They are steep-sided, conical mountains when fully formed. Example: Paricutin, Mexico.
Shield - Looking like a domed shield from edge on, shield volcanoes rise gently above the waves in the middle of the ocean. Shield volcanoes are formed from mafic magma (which comes from melted ocean crust) that cools on contact with the water. Eventually, these piles of lava rock poke up above the water and form islands. Due to the runny nature of the pahohoe lava, shield volcanoes have broad bases with gently sloping sides, giving it its domed appearance. Examples: any of the Hawaiian islands/volcanoes.
Composite (Stratovolcano) - Seemingly a combination of both shield and cinder cone volcanoes, composite volcanoes really do exhibit characteristics of both. They form on the edge of continents and oceans where the ocean crust subducts beneath the continent. The melted ocean crust rises through the continental crust and melts it, too. This creates a mix of both felsic and mafic magma from which the volcano is created. Felsic magma result in explosive pyroclastic eruptions, as with cinder cone volcanoes, and mafic magma results in gentle lava eruptions, as with shield volcanoes. The mountain that forms from these alternating eruptions is one of the most picturesque mountains (in my opinion) on the planet. Examples: Mt. St. Helens, Mt. Vesuvius, Mt. Fuji, and many others.
Hot Spot - Hawaii and Yellowstone National Park represent a unique type of volcano that is not associated with plate boundaries or crustal interactions. Somehow, superheated plumes of mantle material burn through the crust in certain locations. One of these locations is in the middle of the Pacific plate (Hawaii). Another is close to the middle of the North American plate (Yellowstone). The plate moves, but the hot spot stays put. Seeing the apparent movement of the hot spot over time allows us to track the movement of the plate.
Supervolcano
Yellowstone National Park sits on a hot spot (see above). The park is famous for its geysers and hot springs, but it was only recently that the cause of these wonders was discovered...a supervolcano. The entire park is in the caldera (collapsed crater) of a supervolcano. A supervolcano is a really, really big volcano. So, when it erupts, it's likely to be a global catastrophe.
Yellowstone National Park sits on a hot spot (see above). The park is famous for its geysers and hot springs, but it was only recently that the cause of these wonders was discovered...a supervolcano. The entire park is in the caldera (collapsed crater) of a supervolcano. A supervolcano is a really, really big volcano. So, when it erupts, it's likely to be a global catastrophe.
Earthquakes
Rocks move. The ground shakes. Californians aren't the only ones who experience earthquakes. In August of 2011, rocks in Central Virginia shifted and caused a minor (magnitude 5.8 earthquake) that was felt as far away as Pittsburgh and Michigan (I have reliable sources...my family members who live in these areas). Everybody was caught off-guard because we can go our whole lives in this area and never experience an earthquake, but there it was. It really happened. So, what exactly did happen? Well, all earthquakes are caused by rocks moving along cracks in the Earth called faults. Faults typically come from plate boundaries (as rocks slide past, move toward, or move away from each other, they crack). However, the cracks formed during at these boundaries can stay in the rocks for millions of years, occasionally slipping as the rocks continue to settle. This is what happened to Virginia. 300 million years ago, the Appalachian Mountains were formed when what is now Africa slammed into what is now North America. These mountains are riddled with cracks. Just as old houses continue to "settle," so too do rocks. Thus, we had an earthquake.
Earthquakes happen all over the world practically all the time. Most earthquakes are associated with volcanic activity at plate boundaries, but this is not an absolute.
Rocks move. The ground shakes. Californians aren't the only ones who experience earthquakes. In August of 2011, rocks in Central Virginia shifted and caused a minor (magnitude 5.8 earthquake) that was felt as far away as Pittsburgh and Michigan (I have reliable sources...my family members who live in these areas). Everybody was caught off-guard because we can go our whole lives in this area and never experience an earthquake, but there it was. It really happened. So, what exactly did happen? Well, all earthquakes are caused by rocks moving along cracks in the Earth called faults. Faults typically come from plate boundaries (as rocks slide past, move toward, or move away from each other, they crack). However, the cracks formed during at these boundaries can stay in the rocks for millions of years, occasionally slipping as the rocks continue to settle. This is what happened to Virginia. 300 million years ago, the Appalachian Mountains were formed when what is now Africa slammed into what is now North America. These mountains are riddled with cracks. Just as old houses continue to "settle," so too do rocks. Thus, we had an earthquake.
Earthquakes happen all over the world practically all the time. Most earthquakes are associated with volcanic activity at plate boundaries, but this is not an absolute.
Seismic Waves - When an earthquake happens, the energy released by the movement of rocks moves away from the place of movement on the fault which is called the focus. But we can't see underground, so we need to find a place on the surface that will help us locate the fault and then focus. This point is an imaginary point directly above the focus on the surface and is called the epicenter. So, how do we find the epicenter? We need to look at the records of 3 seismograph (a device that records seismic waves) stations. Seismograph stations record the arrival of P waves and S waves and Surface waves.
P waves - also called Primary waves; arrive 1st; fastest waves; move through solids and liquids; move similar to an accordion by stretching and compressing the rock as it moves.
S waves - also called Secondary waves; arrive 2nd; slightly slower than P waves; cannot move through liquids; move similar to a ripple in a pond which is a rolling motion.
Surface waves - do the most damage; arrive last; much slower than the other two; move in a side-to-side motion which shakes structures.
Scale - We can use two different scales to measure earthquakes: the Richter Scale and the Modified Mercalli Scale.
Richter Scale - The Richter Scale measures the magnitude (the energy released) of the earthquake. Using a logarithmic scale from 1-10, the Richter Scale is the most commonly used. It's also the most accurate because it is a measurement of the earthquake regardless of where it happened. Because it's a logarithmic scale, each increase in number is 10 times more powerful than the number before it (2 is 10x more powerful than 1; 4 is 1,000x more powerful than 1; 7 is 1,000,000x more powerful than 1).
Modified Mercalli Scale - The Modified Mercalli Scale is slightly older than the Richter Scale, having been developed in the late 1800s. It measures the intensity (the amount of damage done) of the earthquake using a Roman numeral scale of I-XII (1-12). Because the damage done is subjective and dependent on the construction of buildings, moderate earthquakes can cause a lot of damage in very poor areas and very large earthquakes can cause no damage if they happen in places with no people. This makes the Modified Mercalli Scale unreliable for today's earthquakes. But it's easier to determine for historic earthquakes when all we have to go by is the damage done.
P waves - also called Primary waves; arrive 1st; fastest waves; move through solids and liquids; move similar to an accordion by stretching and compressing the rock as it moves.
S waves - also called Secondary waves; arrive 2nd; slightly slower than P waves; cannot move through liquids; move similar to a ripple in a pond which is a rolling motion.
Surface waves - do the most damage; arrive last; much slower than the other two; move in a side-to-side motion which shakes structures.
Scale - We can use two different scales to measure earthquakes: the Richter Scale and the Modified Mercalli Scale.
Richter Scale - The Richter Scale measures the magnitude (the energy released) of the earthquake. Using a logarithmic scale from 1-10, the Richter Scale is the most commonly used. It's also the most accurate because it is a measurement of the earthquake regardless of where it happened. Because it's a logarithmic scale, each increase in number is 10 times more powerful than the number before it (2 is 10x more powerful than 1; 4 is 1,000x more powerful than 1; 7 is 1,000,000x more powerful than 1).
Modified Mercalli Scale - The Modified Mercalli Scale is slightly older than the Richter Scale, having been developed in the late 1800s. It measures the intensity (the amount of damage done) of the earthquake using a Roman numeral scale of I-XII (1-12). Because the damage done is subjective and dependent on the construction of buildings, moderate earthquakes can cause a lot of damage in very poor areas and very large earthquakes can cause no damage if they happen in places with no people. This makes the Modified Mercalli Scale unreliable for today's earthquakes. But it's easier to determine for historic earthquakes when all we have to go by is the damage done.
Faults
There are three types of faults, with one being further divided into two types: normal, reverse and thrust, and strike-slip faults. The normal, reverse, and thrust faults describe movements of the hanging wall in relation to the foot wall. The hanging wall is the block above the fault and the foot wall is the block below the fault.
There are three types of faults, with one being further divided into two types: normal, reverse and thrust, and strike-slip faults. The normal, reverse, and thrust faults describe movements of the hanging wall in relation to the foot wall. The hanging wall is the block above the fault and the foot wall is the block below the fault.
Tsunami
When a seismic event (earthquake/volcanic eruption/plate shift) occurs under the ocean, it can transfer a lot of energy to the water above. This energy takes the form of a giant wave, which in the deep ocean is barely noticeable. But as it approaches shore, the energy does not diminish and turns the water into what is generally called a tidal wave. More accurately, this is a tsunami. As the tsunami approaches land, it pulls the water from the shore to be a part of the massive wave.
When a seismic event (earthquake/volcanic eruption/plate shift) occurs under the ocean, it can transfer a lot of energy to the water above. This energy takes the form of a giant wave, which in the deep ocean is barely noticeable. But as it approaches shore, the energy does not diminish and turns the water into what is generally called a tidal wave. More accurately, this is a tsunami. As the tsunami approaches land, it pulls the water from the shore to be a part of the massive wave.
Plate Tectonics Concept Map
This concept map brings together volcanoes, earthquakes, and the tectonics that create them. Learn more about plate tectonics on the previous page by using the link below. Below, you can also download the map for printing.
|