Week 6 Flashcards
What are shield volcanoes
- Low aspect ratio due to the fluidity of lava
- non-explosive eruptions are common
- broad and gentle slopes
e.g Mauna Loa
What is the magma type for shield volcanoes
- lavas that erupt in Hawaii are basaltic and contain about 50% silica
- They erupt at about 1200 degrees c
- very fluid and pasty
- effusive
- low silica
- low viscosity
What is the eruption type for shield volcanoes
Hawaiian type eruption
Hawaiian type eruption
- Lava is very fluid and hot
-Bubbles can expand and escape easily - relatively gentle
- low energy eruption
What are stratovolcanoes
- Higher aspect ratio because of lava viscosity
- Explosive and non explosive eruptions
- e.g Mt rainier: subduction zone volcano
What is the magma type for stratovolcanoes
- most lavas that erupt on volcanoes like mt st helens are closer to rhyolitic and are about 65 to 75% silica
- they erupt at about 900 degrees and are pretty stiff
- high viscosity
- high silica magma
What is the eruption type for stratovolcanoes
Plinian type eruption
Plinian type eruption
- Lava is sticky and cooler
- Bubbles can’t expand or escape easily
- Explosive
- starts with glowing pyroclastic flows
- forms lava domes with steep sides
Characteristics of Plinian type eruptions
- Ash and smoke column that can extend to stratosphere
- large amounts of pumice
- powerful gas blasts
- large amounts of magma erupted
- caldera formation possible
What are the three parts of a plinian column
- lower kilometer or two is the gas thrust region
- the next few kilometers is the convective region
- the upper part is the umbrella region
What is calderas (ultra-plinian)
- formed by very voluminous and explosive eruption
What are the key characteristics of calderas
- caldera formation
- large ejecta volumes
- high energy release
- massive plumes
- magma is rhyolitic
What are cinder cones
- smallest in size
- e.g sunset crater, flagstaff , Arizona
Comparison of shield and stratovolcanoes
Shield volcanoes are far larger than stratovolcanoes
What are igneous rocks
-formed from the crystallization of magma
-extrusive: fine grained and quickly cooled
-intrusive: coarse grained and slowly cooled
Classification of Igneous rocks
- mineral composition
- grain size
- texture
Chemistry of silica
- magmas range from about 50 to 75% silica
- the more silica the stiffer the magma
- the less silica, the more fluid the magma
-“basalt” is lava that is about 50% silica
-“rhyolite” is lava that is about 75% silica
Chemistry of water
- the more water, the more explosive especially for rhyolites
- the effect of water on the stiffness of magma
- high water content and high silica content makes magma erupt more explosively than a low water and low silica magma
What are the three kinds of pyroclastic deposits
- Pyroclastic fall/ airfall
- Pyroclastic surges
- Pyroclastic flows
Pyroclastic fall/airfall
fallout from the plinian column blankets the ground with a nearly uniform mantle of fine particles
Pyroclastic surges
clouds of debris-laden gas jetting out of the volcano and racing over the countryside form layers of irregularly bedded ash and blocks
Pyroclastic flow
dense slurries of debris racing down the slops of the volcano deposit, which concentrate in valleys
