Volcanism and Extrusive Rocks

Physical Geology 12/e, Chapter 4

Volcanism and Earth’s Systems

• Atmosphere originally created from gases released from volcanic eruption

• Hydrosphere produced by condensation of volcanic water vapor

• Biosphere both positively and negatively influenced by volcanism– Lava flows and ash weather to produce fertile soils– Violent eruptions can destroy nearly all life in their paths– Large amounts of ash and volcanic gases in atmosphere can trigger rapid

climate changes and contribute to mass extinctions

Title + 10

• Pages 84-91– Titles/heading– 10 or fewer words per title

– SHARE

Volcanic Eruptions

• Lava is produced when magma reaches Earth’s surface

• Explosive eruptions can produce rapidly cooled rock fragments called pyroclasts– Size range from dust (ash) to boulders (blocks

and volcanic bombs)

• Calm oozing of magma out of the ground produces lava flows

• Pyroclastics and lava flows form extrusive igneous rocks

• Lava flows and pyroclasts pile up to form volcanoes

Eruptive Violence and Physical Characteristics of Lava

• Violence of eruptions controlled by:1. Dissolved gases in the magma

• Water vapor, carbon dioxide, sulfur dioxide, etc.

2. Ease/difficulty of gases escaping to atmosphere

• Viscosity- a fluid’s resistance to flow1. Silica content

• Higher silica contents produce higher viscosities

2. Lava temperature• Cooler lavas have higher viscosities

3. Amount of dissolved gases• The more dissolved gases, the more fluid the lava

Eruptive Violence and Physical Characteristics of Lava cont.page 90-91

Igneous Family

Silica Content

Viscosity Violence of Eruption

Silicic

(Felsic)

Exit Ticket

1) Give an example of a real life substance that has high viscosity.

2) Give an example of a real life substance that has low viscosity.

Eruptive Violence and Physical Characteristics of Lava cont.page 90-91

Igneous Family

Silica Content

Viscosity

& Flow

Violence of Eruption

Silicic

(Felsic)

65%

(Silica Rich)

High &

Sluggish

Violent

Intermediate 35-50% Both Violent and Lava Flows

Mafic Less than 50%

Deficient

Low &

Easily

Minimal

More flows

Extrusive Rocks and Gases

• Scientific Investigation of Volcanism– Rocks, gases and events from observed

eruptions compared to similar lavas elsewhere to infer the nature of past activity

• Rock Composition– Rhyolite - high silica; light color

– Basalt - low silica; dark color

– Andesite - intermediate silica and color

Extrusive Textures• Texture - appearance of a rock with respect

to size, shape and arrangement of its grains

• Glassy - glass without mineral crystals

– Obsidian

• Fine-grained - most crystals < 1 mm

– e.g., Basalt

• Porphyritic - larger crystals in a matrix of much finer crystals (groundmass) or glass

– e.g., Andesite

Extrusive Textures

• Vesicular/Frothy - trapped gas bubbles

– Vesicular basalt

– Pumice

• Fragmental - particles blasted apart by explosive eruptions

– Dust and ash (<2 mm)

– Cinders (2-64 mm)

– Blocks and bombs (>64 mm)

Volcanic Landforms

• Vent - opening through which lava erupts

• Crater - basin-like depression over the vent at the summit of the volcano

• Caldera - volcanic depression much larger than the original crater, having a diameter of at least 1 km

Types of Volcanoes pages 96-103

Type Diagram Define & any charac.

Lifespan

Eruptive Material

Rock Type

Shield

Composite(Stratovolcano)

Cinder

Dome

Virtual Volcano

• Build your own volcano– Write down the characteristics

– http://www.cosmeo.com/braingames/virutal_volcano/index.cfm?title=Virtual%20Volcano

What am I???

Types of Volcanoes

• Shield volcanoes– Broad

– Gently sloping

– Composed of solidified lava flows

– Flows often contain lava tubes

• Cinder cones – Small

– Steeply sloping

– Composed of a pile of loose cinders

Types of Volcanoes• Composite volcanoes

– AKA stratovolcanoes– Moderately to steeply sloping– Constructed of alternating layers of pyroclastic

debris and solidified lava flows– Composed primarily of intermediate

composition volcanic rocks (i.e., andesite)– Most common type of volcano at convergent

plate boundaries (e.g., Pacific Ring of Fire)

• Volcanic domes– Extremely high viscosity, degassed, felsic lavas

(often glassy, e.g., obsidian)

Other Eruption Types• Flood eruptions

– Very fluid (basalts)

– Extremely large in volume

– Create extensive lava plateaus

– Eruption times correspond with largest mass extinction events

• Submarine eruptions– Nearly always basaltic

– Mid-ocean ridge eruptions

– Pillow basalts

Columbia river flood basalts

Pillow basalts

Volcanism on Other Planets

• Shield volcanoes– Venus, Mars, Io

• Lava domes– Venus, Moon

• Flood eruptions– Very fluid (basalts)– Extremely large in volume– Extensive flat lava plains

(Moon, Mars, Venus, Mercury?)

Insert Fig. 22.18 here

Insert Fig. 22.27A here

Insert Fig. 22.14A here

Olympus Mons on Mars

Lava domes on Venus

Lava channel in volcanic plains on Moon

Volcanic eruption on Io

End of Chapter 4