Volcanism and Plutonism

Extrusive Igneous Activity: VolcanismSummary of Important Concepts

• 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 HazardsVolcanic 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.

• Hot spots• Example: The Hawaiian Islands and the Galapagos Islands.

The majority of active volcanoes (about 66%) occur in the Pacific Ring of Fire.

Magma and the Driving Force Behind Eruptions• Magma may be ejected onto the earth’s surface as:– Lava– Pyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses.• The violence of a volcanic eruption depends on the magma’s viscosity and gas content. The more viscous (thick) and more gaseous the magma, the more explosive the eruption.

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.

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:

• Water vapor • Carbon Dioxide • 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

Steps to a Volcanic Eruption• 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.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!

Hazard SummaryThe 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 Flows• 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.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• 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. • Large-sized tephra typically falls back to the ground on or close to the volcano. 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.• Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano. Ash spreads in upper atmosphere around the globe. The suspended ash can decrease the insolation from the sun and lower global temperatures!

Pyroclastic FlowsPyroclastic 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.

Mudflows or Lahars Lahars have caused more fatalities than any other volcanic hazard because they

are more common, and they can occur at any time. Volcanic Mudflows or Lahars form by mixing water with loose volcanic ash and

debris on the flanks of a volcano. As the mud moves downslope, it gathers rocks of all sizes accelerating as it goes.

The water can come in several ways including:o A major rainstorm.o An eruption melts large amounts of snow and ice on the flanks of the

volcano. When moving, a lahar looks like a mass of wet concrete that carries rock debris

ranging in size from clay to boulders. Most lahars travel much too fast for people to outrun.

Poisonous Gases 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. 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.

Types of VolcanoesShieldCompositeCinder ConeThey differ in igneous rock chemistry, eruption style, physical features and

geographic location.

Shield Volcano Stats • Found along hotspots and divergent boundaries. • Mafic (basaltic) magma• Low viscosity• Low gas content (no water because no subduction). • GENTLE eruptions of lava flows. • 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.

Stratovolcano Stats • Found along convergent boundaries paralleling subduction zones.• Andesitic (intermediate) to rhyolitic (felsic) magma.• High viscosity• High gas content (subduction drags down water)• VIOLENT eruptions. Dangerous and Explosive. The main hazards are:

• pyroclastic flows• lahars• ash falls• Poisonous gas emissions

• Landform composed of alternating layers of tephra and lava.• Steeply sloping due to the piling up of tephra around the central vent and the high viscosity of the lava that glues it together.• Can reach heights of roughly 3500 meters.• Andesite and rhyolite are the most common rock type.• The majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by subduction in the Pacific Ring of Fire.

Cinder Cone• 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.

Intrusive Igneous Activity: PlutonismCrystallize below the earth’s surface so what igneous texture would we see? Fast or

slow cooling?Intrusive bodies are classified based on shape or size.Magma travels through (intrudes) into another rock body (commonly called country

rock).Common intrusive rocks are peridotite, granite and gabbro.

Shallow Intrusive Bodies occur <2km depth. 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.

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.

Deep Intrusive BodiesBATHOLITHS 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.