Lecture 9
Lecture 9: Volcanoes
Game Plan
Topics to Cover:
Volcanoes
Volcanic eruptive products
Magma/lava types
Types of volcanoes
Other igneous rock formations
Volcanoes and plate tectonics
Reminders:
Lab 4 today & Tuesday (9/30)
Exam 1 on Friday, 10/10
Learning Outcomes
By the end of this lecture, you should be able to:
Identify and describe the 3 different types of lava, including their properties and tectonic expectations.
List and define various forms of igneous intrusions.
Understand the different types of volcanoes, their compositions, and tectonic affiliations.
Note: Specific eruptions or volcanoes mentioned are for exemplification only; detailed knowledge of individual volcanoes is not required.
Definition of Volcanoes
Volcano: A hill or mountain created from the accumulation of erupted lava and pyroclasts around a central vent.
Why Do Volcanoes Erupt?
Magma and Gas Importance:
Magma contains dissolved gases including H₂O, CO₂, and SO₂, which significantly influence eruption styles.
High confining pressure keeps gases dissolved deep underground. As magma rises, pressure drops, allowing gas bubbles to form (similar to opening a soda bottle).
If bubbles form rapidly, they enhance magma buoyancy, causing it to rise more easily and potentially erupt.
Volcanic Eruptive Products
Besides lava flows, volcanoes emit various products:
Pyroclasts: Fragments formed from magma shattering due to rapid gas release. Example includes cinders, which are gravel-sized basalt pieces.
Bombs: Large lava fragments ejected during eruptions; these can cool into rounded shapes.
Volcanic Ash: Tiny pulverized rock and minerals (less than 2 mm in diameter) that can cause significant hazards, such as the threat to aircraft engines demonstrated during the Eyjafjallajökull eruption in 2010.
Pyroclastic Deposits
When pyroclasts settle back to Earth, they create large deposits, which solidify over time. Key types are:
Welded Tuff: Formed from compacted volcanic ash.
Volcanic Breccia: Composed of fragmented minerals or rocks cemented together.
Types of Lava
Three main lava types produce unique landforms:
Basaltic Lava (1000-1200°C):
Composition: 45-55% SiO₂ (high in Fe, Mg, Ca; low in K, Na)
Characteristics: Low viscosity, flows readily.
Andesitic Lava (800-1000°C):
Composition: 55-65% SiO₂ (medium amounts of Fe, Mg, Ca, K, Na)
Characteristics: Intermediate viscosity and gas content; explosive potential.
Rhyolitic Lava (600-800°C):
Composition: 65-75% SiO₂ (low in Fe, Mg, Ca; high in K, Na)
Characteristics: High viscosity leads to explosive eruptions.
Volcano Types
Volcano classification is based on erupted materials and form:
Types:
Shield Volcano: Broad, gently sloping sides with mainly basaltic lava, characterized by peaceful eruptions.
Cinder Cone: Built from pyroclastic fragments, tends to be small with explosive spitting eruptions.
Stratovolcano (Composite Volcano): Steep-sided, formed from alternating layers of lava flows and pyroclasts, typically associated with explosive eruptions above subduction zones.
Volcanic Dome: Mound-like feature composed of high-viscosity lava, can cause explosive eruptions when they trap gas.
Caldera: A large basin formed after a violent eruption empties the magma chamber, leading to the collapse of the overlying rock.
Importance of Magma Composition in Eruptions
Differences Between Magma Types:
Felsic Magma:
High viscosity due to silica content.
Higher volatile content leads to more explosive eruptions.
Mafic Magma:
Low viscosity allows for gentle eruptions as gases can easily escape.
Eruption Mechanics:
Basaltic lava flows easily, while rhyolitic lava is sticky, leading to potential explosive activity when pressure builds.
Characteristics of Lava Flows
Basaltic Flow: Fluid and can travel long distances, forming thin sheets.
Andesitic Flow: More viscous and breaks as it flows.
Rhyolitic Lava: Very viscous; tends to form thick, bulbous deposits at the vent.
Summary of Magma Viscosity and Eruptions
Basaltic lava erupts gently, allowing gases to escape, whereas rhyolitic lava can cause explosive eruptions due to gas buildup.
Concept Questions
What role does gas play in eruptions?
Compare and contrast cinders, bombs, and ash.
Define the viscosity differences among lava types and their eruption styles.
How does the composition of lava influence the resultant rock type?
Identify which type of lava is associated with high viscosity and which is fluid-like.
Volcanoes and Igneous Rock Formations
Plutons:
Large igneous intrusions that cut across existing rock layers.
Formed from more viscous magma (intermediate to felsic).
Forms of Igneous Intrusions
Sills:
Sheet-like bodies injected between bedded rock layers; can vary in thickness and can extend over large areas.
Dikes:
Magma intrusions that cut across layered rock; important for magma transport within the crust.
Flood Basalts
Large, flat lava flows from fissures instead of central vents, creating vast regions of basaltic rock.
Plate Tectonics and Volcanism
Different tectonic settings lead to various magma compositions and resultant volcanic activity:
Divergent Boundaries: Primarily form basaltic lava while creating new oceanic crust.
Convergent Boundaries: Subduction leads to intermediate and felsic lava, often producing stratovolcanoes.
Hotspots: Can result in both mafic and felsic lavas, leading to different eruption styles depending on magma composition.
Summary of Plate Tectonics and Volcano Types
At ocean-ocean convergences, basaltic lavas dominate, while ocean-continent convergences yield a mix of magmas leading to andesitic volcanism.
Hotspots may produce varied volcanic forms depending on underlying crust.
Activity Discussion Questions
Identify volcano types based on rocks and expected eruptions at various tectonic settings.
Discuss how magma composition influences volcano types and resultant geological features.