Volcanism. Refers to the rise of magma which makes its way to the earth’s surface as LAVA cooling above the earth’s surface.
Magma - a mixture of liquid rock, crystals, and dissolved gases beneath the earth’s surface (within the crust).
Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses from a constricted vent onto the earth’s surface. Magma rises in a narrow, pipe like conduit from a magma reservoir beneath and flows at the earth’s surface as LAVA.
Volcanoes form from sources of magma inside the earth. The main sources of magma are:
- Subduction at convergent plate boundaries.
- Sea floor spreading at divergent plate boundaries.
- Hot spots .
Locations of Volcanic Activity and Volcanic Hazards Volcanic activity is controlled by plate tectonics , because plate movements relate to where sources of magma originate inside the earth. Nearly all active volcanoes are located in one of three plate tectonic settings:
Subduction zones at convergent plate boundaries
Example: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific “Ring of Fire”.
Rifting and sea floor spreading at divergent plate boundaries ,
Example: Volcanic eruptions at mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley.
Example: The Hawaiian Islands and the Galapagos Islands.
Map of the world’s active volcanoes, showing that the majority of active volcanoes (about 66%) occur in the Pacific Ring of Fire .
Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red, and those with the lowest risk are shown in blue. Areas that are uncolored are considered “risk free”.
Zooming in for a closer look at the western U.S., we see in this figure that the volcanoes of the Cascade Range , which extends from northern California north into Oregon and Washington, are related to the presence of the Cascadia Subduction Zone offshore. The most recent major eruption in the Cascade Range was that of Mount Saint Helens in 1980. The Cascade Range is part of the Pacific Ring of Fire.
Viscosity is a measure of a fluid’s resistance to flow . The main factor that determines the viscosity of magma is its silica (SiO2) content . The more silica in the magma, the more viscous it is. The more viscous (silica-rich) the magma, the more violent the eruption .
We are concerned, then, with three types determined by the chemical composition of the magma.
Mafic or Basaltic magma – low silicon and oxygen , high iron and magnesium. Simple silicate minerals. Dark magmas. Low Viscosity.
Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and magnesium. High Viscosity.
Felsic or rhyolitic magma - high silicon and oxygen , low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity.
Left: The basic building block of all silicate minerals - the silica tetrahedron. Four oxygen atoms surrounding a single atom of silicon.
The gas content of a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting. The higher the gas content, the more violent the eruption .
The composition of the gases in magma are:
Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases
But it is the amount of dissolved water that typically inspires a volcano to violence
Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock.
As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (makes bubbles).
When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution. In other words, the gas bubbles begin to expand.
Steps to a Volcanic Eruption: Two Possibilities
If the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the earth’s surface and simply burst, and a non-violent eruption will occur, usually as a lava flow.
If the magma has a high viscosity the gas will not be able to expand very easily creating a high pressure inside which will cause them to burst explosively on reaching atmospheric pressure. This will cause and explosive volcanic eruption.
It turns out that viscosity and gas content of magma both relate to plate tectonic setting: Divergent boundaries (mid-ocean ridges) and hot spots both draw their magmas from the upper mantle . This magma is called MAFIC MAGMA , and is characterized by relatively low silica and low gas content . In contrast, the magma at convergent boundaries comes from melting of subducted oceanic plates . This magma is called FELSIC MAGMA , and is characterized by relatively high silica and high gas content .
Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries!
The main HAZARDS associated with volcanic eruptions are:
Lava flows that burn and destroy what they overrun.
Ash falls that cover vast areas of landscape, creating respiratory problems, messy conditions, and potential lahars.
Pyroclastic flows : hot, fluid mixtures of rock particles and gas that travel at great speed down the flanks of a volcano; have caused thousands of fatalities.
Lahars : fast-moving mud flows caused by mixing volcanic ash with water (from rain or from eruptions melting snow and ice on the volcano); responsible for more death and destruction than any other volcanic hazard.
Gases emitted during eruptions that may be toxic and/or corrosive; the most common is CO2 gas - when present in large enough quantities it causes suffocation.
Lava is molten magma that flows out and onto the Earth’s surface.
Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots .
Fluid basalt lava flows are called Hawaiian type lava and can come in two varieties. Smooth, runny pahoehoe (top) and chunky aa (bottom).
The lower silica content (and therefore low viscosity) of mafic magma allows the lava to run down slopes easily. Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but rarely fatalities.
Ash falls form when an eruption column of tephra and gas is blown into the air by an explosive eruption. The eruption column can rise up more than 20 km into the atmosphere.
Tephra is a general term for any size of fragmental material blown out of a volcano.
Shown right: Volcanic Bombs form from the rapid cooling of lava thrown in the air. When the lava cools in mid-air, it forms the characteristic ellipsoidal shape. Shown left: Pele is the goddess of the volcano. Teeny tiny volcanic bombs are known as Pele’s tears. Large-sized tephra typically falls back to the ground on or close to the volcano.
Volcanic ash , the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano. 1980 Eruption of Mt. St. Helen and map of ash fall distribution and thickness.
Ash spreads in upper atmosphere around the globe. The suspended ash can decrease the insolation from the sun and lower global temperatures!
Pyroclastic flows (Ash Flows ) are avalanches of a very hot (1300-1800F) mixture of hot rock particles and hot gas that are blown out of the vent of the volcano as an eruption column which subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to over 200 km per hour and can travel for 10’s of kms burning, burying and suffocating everything in their path.
Above: 1997 eruption on island of Montserrat in the West Indies. Right: 1986 eruption of Augustine Volcano, Alaska.
...destroy by direct impact. Pyroclastic flows….. ...bury sites with hot rock debris. ...burn forests, crops, and buildings.
One of the greatest volcanic disasters in history was the destruction of the city of Saint Pierre, Martinique, by a pyroclastic flow from Mount Pelee in 1902. Before the eruption right…... … .and after the eruption left. An estimated 30,000 people died gruesomely by burning, burial, and suffocation.
Gases emitted during volcanic eruptions may be toxic and/or corrosive.
The most common hazardous gas is CO2. Carbon Dioxide is deadly to people, animals, and trees in high concentrations.
Above and Below: Stands of dead trees killed by excess CO2 seeping from the ground from magma close to the surface.
These cattle, and about 1700 people, were victims of asphyxiation from a massive CO2 discharge from Lake Nyos (Cameroon, West Africa) in 1986. Lake Nyos lies within a volcanic crater. Being heavier than oxygen, it can pour downslope and displace oxygen at the surface. Because it is colorless and odorless, it can suffocate without warning.
Landform composed almost entirely of relatively thin lava flows.
Gently sloping due to the low viscosity of the magma which allows lava to flow great distances before it cools.
The largest structures on earth! 9 km from seafloor to summit!
Basalt is the most common rock type.
The big island of Hawaii has three major volcanoes: Mauna Loa, Mauna Kea, and Kilauea. Mauna Loa is Hawaii’s largest volcano but its activity is infrequent. Kilauea is in the main growth stage having erupted almost continuously since 1983.
Because they form from relatively quiet eruptions of low viscosity, fluid magma, shield volcanoes are very large, with gently sloping sides and a convex shape. Left: a shield volcano in the Galapagos Islands created over a hot spot. Right: Iceland created where a mid-ocean ridge sticks up above sea level. Left: Mt. Etna, Sicily. The largest continental volcano on earth and the most active volcano in Europe, Etna erupts almost continuously, with an occasional violent episode.
This figure shows a composite of the main hazards associated with a typical stratovolcano .
The main hazards are:
Poisonous gas emissions
Map of the world’s active volcanoes, showing that the majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by subduction in the Pacific Ring of Fire .
Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red. *Yellowstone Caldera The volcanoes of the Cascade Range , which extends from northern California north into Oregon and Washington, are related to the presence of the Cascadia Subduction Zone offshore.
Cinder cones form when fluid basaltic magma rises along a fracture and encounters groundwater. The steam generated blows fragments of lava violently into the air which solidify and fall as cinders around the vent forming a circular or oval cone.
Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes.
Most cinder cones erupt for only a few months to a few years and rarely rise more than 300-500 m above their surroundings. Being unconsolidated they tend to erode rapidly.
Above: The photos the eruption of Paricutin Volcano, Mexico, a classic example of a cinder cone. Above: View of cinder cones near the summit of Mauna Kea Above: This cone is one of two cinder cones called the Red Cones located south of Long Valley Caldera in California. These basaltic cones were erupted about 5,000 years ago. Above: This cinder cone on the flank of Mount Etna is surrounded by a younger basaltic lava flow.
Cool quicker than deep LARGE intrusive bodies so crystals are on the smaller side.
Dikes are vertical structures that cut across sedimentary rock layers while Sills are horizontal running between sedimentary layers. It can bulge up between layers and form a Laccolith.
If the country rock has numerous fractures magma will fill in the cracks called veins.
Erosion of overlying country rock will expose the stable felsic intrusions.
Sill (left) Dike (right): Surrounding rock may be weaker than the igneous intrusion and therefore the dike remains vertical while the surrounding rocks are weathered away. Recall felsic igneous rocks are high in silica so they are composed of more stable minerals.
BATHOLITHS are large (>100 square kilometers) bodies made primarily of felsic granite.
Because they are so large in size, rocks in the center of the batholith cool SLOWER than those at the periphery. How will the igneous textures vary?
Felsic intrusions move slowly through the crust due to their high viscosity.
Stone Mountain in Georgia is an example of a batholith (large igneous pluton 100’s of square miles) that formed below the earth’s surface (within the crust). Here, the felsic magma cooled (slowly).
Lithospheric plates are essentially ‘floating’ on top of the asthenosphere. Where plates are thicker (continental crust), it weighs more and is sunk into the asthenosphere in these places (like floating a block of wood in water). When we remove overlying rock, the weight burden becomes less and continental crust is uplifted. Vertical movement of the crust is called isostacy.
Over time, weathering and erosion of the overlying rocks exposed Stone Mountain. What is actually seen of Stone Mountain is the ‘tip of the iceburg’. Most of it remains buried. As surrounding country rock (weaker than stmt) weathers away, it ‘lightens’ the weight on top of stone mountain. This causes stone mountain to uplift where it can become more susceptible to erosion, and too will weather into sediments….one day.