#EAPS#EAPS106
Generating Magma §
- Magma = melted rock = lava
- Magma is below the surface, lava is above the surface
- Volcanism is the process by which melted rock beneath the surface is transported to the surface where it either flows out as lava (an effusive eruption) or explodes as ash and pyroclastic flows (an explosive eruption)
- Tephra is all the ash and stuff that comes from an eruption
- Most of the mantle is below it’s melting temperature. Magma only occurs in very specific places and comprises only a small percentage of the mantle
- Magma is really only located where we have volcanoes, there is no “great ocean of magma” under the surface
- There are three ways to get hot rocks to melt
- Increase temperature
- This works, but there aren’t many ways that it can happen, so it is not a common way to induce volcanism
- Decrease pressure (called depressurized melting)
- Add water to the mineralogy (called hydrogen-induced melting)
- This reduces the rocks melting temperature and can cause already hot rocks to melt
- There are three (main) places volcanism occurs
- Mid-ocean ridges
- Solid (but fluid) mantle is drawn upwards towards the ridge where it experiences depressurized melting and collects into magma chambers. It then erupts or cools to form new oceanic lithosphere as it moves away from the ridge
- Subduction zones
- Water is pushed from the subducting plate down into the mantle, which lowers the melting temperature of the hot mantle above causing it to melt
- Subduction zone volcanism occurs everywhere plates are subducted, causing a “Ring of Fire” around the pacific ocean
- All subduction zone volcanoes from inboard of the trench, above where the subducting plate reaches ~100km depth
- Because there is no active subduction zone in the southwestern US, there are fewer active volcanoes compared to the northwestern US
- Hot spots
- These can be found far from plate boundaries, but the rest are on boundaries
- Hot spot volcanism begins when the base of the mantle is heated by the outer core causing buoyant plumes of solid mantle to rise
- When the hot spot plume reaches the lithosphere, it begins to melt by depressurized melting causing massive volcanism, called flood basalts
- When a tectonic plate moves over a hots spot, volcanoes initiate and then go extinct as the volcano moves away form it and a new volcano develops
- The Hawaiian islands are a hot spot track where the islands progressively get older to the northwest as the pacific plate continues to move over the mantle plume
- The Hawaiian islands are at the end of a long chain of hot spot volcanoes that show the movement of the pacific plate over the past 70 million years
- These hot spot tracks like the Hawaiian islands are found all over the world
Explosive vs Effusive Eruptions §
- Explosive is obvious, effusive is gentle lava flows
- Two factors that determine the type of eruption
- Viscosity of magma (how easily it flows) determines the type of volcano that forms
- High viscosity = less flow, low viscosity = more flow
- Low viscosity magmas lead to broad shallow sloped volcanoes, while high viscosity magma leads to narrow highly sloped volcanoes
- The broad ones are called “shield volcanoes”
- The steep ones are called “stratovolcanoes” or “composite volcanoes”
- The Hawaiian islands are shield volcanoes; their lava is very low viscosity that flows far leading to broad topography
- Stratovolcanoes are midsized volcanoes that form due to alternating layers of pyroclastic flows (debris form explosive eruptions) and high viscosity effusive lava flows
- Very high viscosities lead to cinder cone volcanoes, small volcanoes built when tephra (air cooled lava fragments) are thrown out of a volcanic vent
- Whether you get an explosive or effusive eruption comes down to pressure
- If magma can flow to the surface through a vent or crack without building pressure, the result is an effusive lava flow
- If rising magma is trapped beneath the surface and pressure builds up, the result is an explosive eruption
- Dissolved gas is the other factor that can affect the type of eruption
- At depth, magma contains dissolved gas. As magma moves up to the surface the pressure on it decreases, causing dissolved gas to expand to form gas bubbles
- Gas bubbles take up a lot more volume, causing pressure to build within the magma
- Ash are shards of the volcanic bubbles, broken glass!
- A magmas viscosity and chemistry determines whether the volcano is likely to erupt effusively or explosively
- Opening a can of soda is analogous to an eruption
- Shaking the soda and then opening it causes it to explode
- Volcanic eruptions are categorized based on how explosive they are
- Hawaiian eruptions are the calmest type due to very fluid lavas with low gas content
- Strombolian eruptions are driven by the bursting of large gas bubbles that form from the collection of many small ones
- Volcanian eruptions are large enough to generate shock waves
- Plinian eruptions are the largest and most violent, sending columns of pulverized rock and ash 10s of miles up into the atmosphere
- What type of volcano is Mr. Doom?
- Stratovolcano!
Volcanic Hazards §
- More than 250,000 people have been killed by volcanic hazards in the past 500 years
- Lava is the least dangerous near a volcano and pyroclastic flow is the most dangerous
- Lava is over 1200 degrees Celsius!
- Lava flows can sometimes be stopped by spraying water on them!
- It all depends on the size and the volume of the lava flow
- Lava flows can sometimes be redirected!
- Explosive eruptions throw a variety of rocks (collectively called tephra) that comes in different shapes, sizes, and densities
- Volcanic Ash
- Not fluffy like the ash from your grill
- Shards of volcanic glass left over from magma gas bubbles
- Smaller than sand grains
- Hard, abrasive, corrosive, electrical conducting, and does not dissolve in water
- 15 cm of wet ash can collapse a roof
- 5 cm can kill crops
- 1 mm will close an airport, you don’t want ash in airplane engines
- Ash is a good nucleating site for water droplets to come together making pseudo rain clouds, which often leads to rain and thunderstorms during eruptions
- Explosive eruptions can throw out pumice, a volcanic rock with so many trapped gas bubbles that it floats
- Volcanic bombs form when an eruption ejects previously cooled lava chunks from the side of the volcano
Pyroclastic Flows and Lahars §
- Pyroclastic flows are the heavier parts of an explosive eruption that run down the flank of a volcano at up to 200 mph powered by expanding hot gases at over 1000 degrees Celsius
- The eruption of Mt. St. Helens in 1980 initiated with an avalanche that became a large pyroclastic flow
- Living with a pyroclastic hazard: The people that live beneath Mount Unzen, Japan, have built channels that direct pyrotechnic flows away from residential areas
- Lahars (mudflows) are avalanches of ash, soil, rock, and water that can occur days or even months after an eruption
- The mudflow following the eruption of Mt. St. Helens extended 10s of kilometers and reached over 10 meters high
Volcanic Gases and Predicting Eruptions §
- Volcanic gases can lead to lakes of sulfuric acid, but it takes weeks or months
- Volcanoes can create lakes dense with sulfuric acid (from mixing sulfur, oxygen, and water), but the process takes many years to build up a strong acid content
- Sulfur gas combusts on contact with the air to create blue fire?
- In 1986, a carbon dioxide gas cloud descended from the Cameroon volcanic Lake Nyos, suffocating 1,746 people and 3,500 livestock in the surrounding region
- Explosive volcanic eruptions are generally preceded by several precursors associated with a build up of pressure beneath the surface that enables eruptions to sometimes be predicted within a few days
- Gas release increases
- Rapid ground deformation (uplift or tilt)
- Earthquake frequency increases
- Harmonic tremors
- Small earthquakes in terms of shaking that last a very long time. Happens when magma moves underground