GEOL 1001 : Exam 2 Geol 1
Chapter 5:
Volcano: vent where molten rock surfaces, mountain built from magmatic eruptions
Caused by tectonic activity, eruptions transfer inside earth material to the surface
Forms of eruptions:
Lava flows: molten rock moves over the ground
Pyroclastic debris: fragments blown out of a volcano
Volcanic gases: vapor and aerosols that exit a volcano
Lava flow: Style depends on viscosity
Composition: Silica Fe and Mg content, Temperature, Gas content, and crystal content
Types of flows:
Basaltic (mafic) lava flow: hot, low silica, low viscosity, thin and fluid
Travels far due to rapid cooling crates cooled crust, this crust serves as insulation, allowing hot interior
to flow and remain liquid for a distance
Lava Tubes prevent cooling, causing flows for miles will eventually become empty
Lava becomes a solid blanket of rock when freezes
Pahoehoe: Forms when extremely hot basalt forms a skin (glassy)
A’a: forms when hot flowing basalt cools and thickens (jagged)
Columnar jointing: flows that contact with vertical fractures that are hexagonal in cross section
(develops in dikes and sills)
Pillow basalt: cooled underwater, doesn’t flow, lava pressure ruptures a pillow to form (common in mid-
ocean ridges)
Andesitic Lava Flow: moderate silica content, more resistant to flow
Rhyolite Lava Flow: highest silica, most resistance to flow(pile into mounds at the vent)
Volcaniclastic deposits: Pyroclastic debris, pre-existing rock fragments, landslide debris, and mudflows
Types:
Basaltic pyroclastic debris: bubbles as it surfaces, bubbles burst and eject clots and drops of molten
magma upward forming dramatic fountains.
Explosive pyroclastic debris: More viscous then basaltic magma(Si02), more gas, larger quantities of
volcanic ash (intermediate and felsic magmas erupt)
Pyroclastic flows: avalanche of hot ash pulled down side of volcano by gravity
Pyroclastic Deposits: from ash shards to large blocks(Tephra: unconsolidated of pyroclastic grains)(Tuff:
lithified ash)
Others: volcanic debris flows result from rain mixing melting snow and tephra(Lahars: wet slurry mixture
of ash, coarser debris, and water)(deadly)
Volcanic Gas: Water(most abundant) Carbon dioxide(second abundant) Sulfur dioxide(rotten egg smell)
Magma with more silica contain greater portion of gas
Mafic Magma consist of a smaller % of volatiles
As magma rises gases are expelled(due to concomitant P drop) SO2 reacts with H2O to form aerosol
sulfuric acid H2S
Low viscosity (basalt): Easy escape; mellow eruption
High viscosity (rhyolite): difficult escape; violent eruptions
Characteristics:
Magma Chamber: upper crust, some magma cools here and some rises forming volcano
Fissures and vents: surfaces through cracks
Craters: bowl shaped, summit eruptions(within summer crater) Flank eruptions(along side volcano)
Calderas:big circular depression collapse center
Distinctive profiles:(shapes)Shield volcanos(broad domed shaped lateral flow of low viscosity basaltic
lave low slope cover large geographic area) Cinder cones(piles of tephra smallest volcano built by
ejected lapilli-sized fragments pilled at vent symmetrical with deep summit center slope at the angle of
repose) Stratovolcanoes( large cone shaped composed of altering layers of lava and tephra)
Eruption styles:
Effusive eruptions: produce lava flow
Explosive eruptions: blow up, produce pyroclastic flow
Controls:
Viscosity: ease of lava flow Basalt: flows away from vent Felsic: builds up at vent
Gas pressure: greater pressure favors explosive style Basalt: allows gas to release Felsic: prevents gas to
release
Environment: Sub aerial lava flowing on land cools slower then sub marine lava which is quickly
quenched
Remember:
Mt. St. Helens: may 18 1980 at 8:32 am earthquake triggered landslide releasing pressure, violent
expansion of gases blasted out sides of volcano
Tectonic boundaries where volcanos occur:
Mid ocean ridges: spreading axes, most lava erupts here
Convergent boundaries: subduction zones(island arc and continental arc)
Continental rifts:
Oceanic and continental hot spots: mantle plumes
Fool-basalt eruptions: diverting plate boundaries, voluminous lava eruptions above plume due to thin
lithosphere erupting lava through long fissures
Volcanic hazard:
Lava flows, pyroclastic debris, explosions, mudflows/lahars, landslides, earthquakes, Tsunamis, gas
clouds
Volcanos are active then dormant then extinct, can be large enough to alter climate
Chapter 6:
Sedimentary rocks: Form at or near earth surface by cementing
Loose grains are produced from: physical or chemical weathering of pre existing rocks, precipitation of
minerals from water solution, shells and shell fragments, and growth of masses of shell-producing
organisms
Earth is covered ‘veneer’ of sediment, the upper portion of the crust behave as a cover capping igneous
and metamorphic(basement) rocks
Thinner: igneous and metamorphic rocks outcrop
Thicker: sediments can accumulate
The four classes of sediment:
Clastic: made from weathered fragments
Biochemical: cemented shells of organisms
Organic: the carbon-rich remains of plants
Chemical: minerals that crystallizen(precipitate) directly from water
Clastic sedimentary rocks reflect purposes:
Weathering: generation of detritus
Erosion: removal of sediment grains
Transportation: dispersal by wind water and ice
Deposition: settling out of transporting fluid
Lithification: transformation into solid rock
Burial: more sediment added to previous layer
Compaction: pressure caused be weight of overlying material, squeezes out trapped water and air
Document Summary
Volcano: vent where molten rock surfaces, mountain built from magmatic eruptions. Caused by tectonic activity, eruptions transfer inside earth material to the surface. Lava flows: molten rock moves over the ground. Pyroclastic debris: fragments blown out of a volcano. Volcanic gases: vapor and aerosols that exit a volcano. Composition: silica fe and mg content, temperature, gas content, and crystal content. Basaltic (mafic) lava flow: hot, low silica, low viscosity, thin and fluid. Travels far due to rapid cooling crates cooled crust, this crust serves as insulation, allowing hot interior to flow and remain liquid for a distance. Lava tubes prevent cooling, causing flows for miles will eventually become empty. Lava becomes a solid blanket of rock when freezes. Pahoehoe: forms when extremely hot basalt forms a skin (glassy) A"a: forms when hot flowing basalt cools and thickens (jagged) Columnar jointing: flows that contact with vertical fractures that are hexagonal in cross section (develops in dikes and sills)
