VOLCANISM
Parts of a Volcano
Where do volcanoes occur?
Convergent plate boundaries
Divergent plate boundaries
Hot spots
Why do volcanoes erupt?
Types of eruptions
What determines eruption style?
Composition
Viscosity
How easily the magma flows
Determined by composition and temperature.
Higher viscosity magmas generally produce explosive eruptions
Lower viscosity magmas generally produce effusive eruptions.
Higher temperature = lower viscosity
The hotter the magma the easier it will flow.
Think of cold syrup versus hot syrup.
Lower temperature = higher viscosity
Colder magmas congeal faster.
Temperature
Dissolved Gases (volatiles)
Volcanic Materials
Variety of material can come out of one eruption.
Lava Flows
Aa flow
Pahoehoe flow
San Juan Parangaricutrio, Mexico. In 1943 buried by
aa flow from Parícutin a cinder cone
Pyroclasts
Cinders
Pyroclastic Flows
Fig. 09.01a
Stephen Marshak
Pompeii & Mount Vesuvius
Pompeii victims of Mt. Vesuvius
Types of volcanoes
Broad, slightly domed.
Generally large sized.
Relatively quiet eruptions.
Low viscosity
Low silica content (basaltic)
High temperature
Low gas content
Associated with Divergent plate boundaries and hot spots.
Ex: Mauna Loa in Hawaii
Shield Volcanoes
MAUNA LOA
Fujiyama, Japan
Interbedded layers of lava and pyroclasts (ejected lava fragments).
Large size.
Most violent eruption type.
Lots of pyroclasts
Pyroclastic flows
Some lava flows
High viscosity (cold honey)
High silica content (Andesitic to rhyolitic)
Low temperature
High gas content
Associated with subduction zones.
Mt. Vesuvius, Italy & Mount St. Helens, Washington.
Composite Cones/ Stratovolcanoes
Built from pyroclasts (ejected lava fragments)Small size.
Commonly found in groups. Usually small eruptions.
Pyroclasts
Lava flows
Low viscosity.
Low silica content (basaltic).
High temperature.
Spotty occurrence.
High gas content.
Ex: Sunset Crater
Cinder Cones
Sunset Crater, AZ
SP Crater, Arizona
CINDER CONE
Crater vs. Caldera
Created by collapse.
Large depression with roughly circular form.
Size = > 1 km in diameter.
3 types of calderas
Crater Lake-Type
Hawaiian-Type
Yellowstone-Type
Formed by collapse of summit of
composite volcano following large
explosive eruption of pumice and ash.
Crater Lake formed by collapse of
Mount Mazama ~ 7,000 years ago.
Erupted 50-70 cubic kilometers of
pyroclastic material. Wizard Island
small cinder cone created by
later volcanic activity.
Kilauea caldera & other volcanic features
Formed by collapse of the top of a shield volcano from draining of central magma chamber.
Kilauea & Mauna Loa both have large calderas to the top.
Walls of Kilauea’s caldera are nearly vertical so they look like flat bottomed pits.
Caldera is 3.3 km x 4.4 km (2 x 3 miles) and 150 m deep (500 ft).
Hawaiian-Type Calderas
Yellowstone caldera from low-earth orbit
Formed from collapse of large area after colossal amounts of ash and pumice discharged along ring fractures.
~ 630,000 years ago 1,000 cubic km of pyroclastic material erupted.
Ash reached all the way to the Gulf of Mexico
Caldera is 70 km (46 miles)across.
Volcanic material extruded from a fissure (crack) in the crust. Very low-viscosity basaltic material. Greatest volume of volcanic material on earth’s surface.
Can cover very large areas 100-1,000s km and can be several 100s of m deep. Columbia River basalts cover 200,000 square km and are nearly 1 mile thick.
Fissure Eruptions/Basalt Plateaus
Magma travels from magma chamber to surface through pipes
Weathering resistant vents and pipes left standing after volcanic cone eroded away are called volcanic necks.
Cinder cones erode fastest, but eventually all volcanoes will erode away, leaving behind a volcanic neck.
Volcanic Pipes/
Necks
SHIP ROCK, NM
Evolution of igneous structures over time
Igneous rocks that cooled below the surface.
Tabular
Sills
Dikes
Massive
Batholiths
Stocks
Laccoliths
Sill – intrusive body that is nearly horizontal or parallel to surrounding sedimentary rocks. Magma exploits weakness & flowed between layers of sedimentary rocks.
Dikes&Sills
Giant’s Causeway in Northern Ireland.
Dike – intrusive body that cuts across rock layers or other structures in host rock. Normally found in groups called dike swarms.
Both can exhibit columnar jointing, when magma cools it shrinks & fractures generally producing tall think six-sided pillar-like forms
Sills & Dikes
Batholiths: Largest intrusive bodies (plutons).
Greater than 100 square km. Tend to be hundreds of km long by 100 km wide. Generally made up of granitic and andesitic composition rocks.
Often called granite batholiths
Consist of hundreds of separate plutons crowded next to or penetrating each other.
Generally emplaced over millions of years.
Stocks: Any pluton that is less than 100 square km.
Most stock appear to be portions of batholiths that are not yet exposed
Batholiths & Stocks
Igneous intrusion that lifts the sedimentary rocks that it penetrates.
Molten rock forcibly injected between sedimentary layers arching
the layers above & leaving the ones below relatively flat.
Stocks can be misidentified as laccoliths.
Laccoliths