Download Volcanoes and Igneous Activity Earth

Origin of magma
There is no permanently molten layer within
the Earth’s mantle or crust.
• Melting within the upper mantle/crust occurs due
to local conditions
• Increasing temperature
• Decreasing pressure
• Change in chemistry (addition of water)
Factors that lead to melting: Pressure
Increase pressure = increase melting temperature
Decrease in pressure = decompression melting
Occurs at mid-ocean ridges
Mantle
peridotite
Mid-ocean ridge basalt
MORB
Origin of magma
Factors that lead to melting
• Role of volatiles
• Primarily water
• H2O -- 70-85%
• CO2 -- 10-15%
• Wet rock = lower melting temperature
• Subduction zones
Partial melting of basalt and mixing with subducted
ocean sediments, produces andesitic/rhyolitic magma
Origin of magma
Factors that lead to melting
• Heat
• Normal geothermal gradient = 25°C/kilometer
• Mantle plumes (hot spots)
Hawaii Hotspot Track
Origin of magma
There is no permanently molten layer within
the Earth’s mantle or crust.
• Partial melting/Fractional crystallization
• Igneous rocks are mixtures of minerals
• Melting occurs over a range of temperatures
• Produces a magma with a higher silica content than the
original rock
Peridotite
Bulk composition of Earth’s mantle
Basalt
Andesite
Rhyolite
Formation of a
batholith
Granitic batholiths in N America
Magma/country rock interactions
A sill in the Salt River
Canyon, Arizona
Basalt dike, Grand Canyon AZ
Quartz dike, Lone Pine CA
Shiprock NM, a volcanic neck
Volcanic eruptions
Eruptive violence is due to:
• Magma temperature
• Magma composition
• Dissolved gases in the magma
All of these affect magma’s viscosity
• Viscosity is a measure of resistance to flow
Basaltic Lava Flows
Pillow basalts
Nicasio reservoir
Marin County CA
Basaltic Lava Flows
• Pahoehoe (pa-hoy-hoy) is a Hawaiian word describing
basalt that solidifies with a glassy, ropy texture.
– Pahoehoe forms when extremely hot basalt forms a skin.
– With flow, the skin is rolled into ropy ridges and furrows.
Basaltic Lava Flows
A’a flows
Basaltic Lava Flows
Columnar jointing at
Devil’s Postpile
Basaltic Lava Flows
Columnar jointing at
Giant’s Causeway
Andesitic Lava Flows
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•
•
•
Higher silica content makes andesitic lavas very viscous.
Andesitic lava flows slowly (1-5 m/d).
The outer crust fractures during flow, creating rubble.
Andesitic lava flows remain close to the vent.
Rhyolitic Lava Flows
• Most viscous lava because of very high silica content.
• Rhyolitic lava rarely flows.
• Rather, lava piles up as a lava dome, plugging the vent.
Explosive Volcanic Eruptions
• The products of volcanic eruption take three forms:
– Lava flows – sheets of lava that move over the ground before
solidifying.
– Pyroclastic debris – fragments blown out of a volcano that fall
and accumulate.
– Volcanic gases – vapor and aerosols that exit a volcano.
Pyroclastic Debris
• Fragmental material ejected from a volcano.
• Consists of glass shards, fragmented lava and rock.
• Wide size range:
– Ash – powdery glass shards.
– Lapilli – pea to plum-sized material.
– Blocks and bombs – apple to refrigerator-sized.
• Blocks – Pre-existing rock torn from the volcano.
• Bombs – Streamlined fragments of ejected lava.
Pyroclastic Debris
• Fragmental material ejected from a volcano.
• Consists of glass shards, fragmented lava and rock.
• Wide size range:
– Ash – powdery glass shards.
– Lapilli – pea to plum-sized material.
– Blocks and bombs – apple to refrigerator-sized.
• Blocks – Pre-existing rock torn from the volcano.
• Bombs – Streamlined fragments of ejected lava.
Pyroclastic Debris
• Fragmental material ejected from a volcano.
• Consists of glass shards, fragmented lava and rock.
• Wide size range:
– Ash – powdery glass shards.
– Lapilli – pea to plum-sized material.
– Blocks and bombs – apple to refrigerator-sized.
• Blocks – Pre-existing rock torn from the volcano.
• Bombs – Streamlined fragments of ejected lava.
Pyroclastic Debris
• Fragmental material ejected from a volcano.
• Consists of glass shards, fragmented lava and rock.
• Wide size range:
– Ash – powdery glass shards.
– Lapilli – pea to plum-sized material.
– Blocks and bombs – apple to refrigerator-sized.
• Blocks – Pre-existing rock torn from the volcano.
• Bombs – Streamlined fragments of ejected lava.
Pyroclastic Debris
• Fragmental material ejected from a volcano.
• Consists of glass shards, fragmented lava and rock.
• Wide size range:
– Ash – powdery glass shards.
– Lapilli – pea to plum-sized material.
– Blocks and bombs – apple to refrigerator-sized.
• Blocks – Pre-existing rock torn from the volcano.
• Bombs – Streamlined fragments of ejected lava.
Volcanic glass
Pumice
Pumice
Obsidian
Obsidian
Ash
A size comparison of three types of volcanoes
Shield Volcano: Mauna Loa
Composite Volcano: Mt St Helens
Cinder Cone: SP crater, N. Arizona
Paracutin (central
Mexico) in eruption
Plinean eruption: Explosive eruption in
which ejecta is lifted vertically by hot gases
and distributed over a wide area
A pyroclastic flow on Mt. St. Helens
Mt Unzen, Japan (1991)
Soufrier Hills volcano (West
Indies (1996)
The eruption of Mt. Vesuvius
in 79 A.D. buried the Roman
city of Pompeii beneath
several meters of ash
Archaeologists began
excavating the city in the
18th century
Crater Lake, Oregon (Mt Mazama;
eruption ~6000 years ago)
Crater Lake in Oregon
Fissure flows occur when
low viscosity lava extrudes
along a linear fracture.
Flows are often high
volume
The Columbia River basalts
Columbia Plateau flood basalts -- flows cover 200,000 km2
in Oregon, Washington to an average thickness of 1000
meters
Deccan Traps, India -- eruption ~ 65 million years ago,
covers 5-10 million km2 to an average thickness of 2-4 km
Long Valley eruption
~730,000 years ago
Cross-section of Long Valley today
Yellowstone Hotspot Track
Stage 1:
Massive
Basalt
National
Parklands
Stage 2:
Rhyolite
Blue #’s = Age of Volcanism (Million Years Ago)
Phreatic (steam)
eruptions occur when
groundwater comes in
proximity with magma