EOSC 114 Study Guide - Final Guide: Yellowstone Hotspot, Shield Volcano, Rhyolite

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Published on 31 Jan 2013
EOSC 114: Volcanoes Learning Goals
1. Explain what magma density and magma
viscosity are.
Density: less mass/volume so magma (melted
rock) rises above solid rock.
Viscosity: how runny, fluid magma is; low
viscosity (runny, flowing) will rise through
crust; dependant on SiO2 content and
temperature more SiO2, higher viscosity
(stronger bond = more solid)
- For magma to rise through crust, it must
have lower density than crust, low viscosity,
and hot enough to stay liquid
2. List the different categories of volcanic
rocks and explain the differences between the
magmas they came from.
- Intrusive rocksmagma that cools slowly
underground, crystals visible
- Extrusive rocks magma cools quickly on
surface, crystals too small to see
-Mafic magma: low SiO2 content, low viscosity,
less explosive (ex. form basalt rocks)
-Felsic magma: high SiO2 content, high
viscosity, more explosive (ex. form rhyolite
3. Explain why some magmas erupt
explosively (as pyroclastic material) and some
magmas erupt effusively (as lava).
Depends on gas content and viscosity
Lava: low gas content, low viscosity, flows as
pahoehoe or a’a (see 4)
Volcanic glass: magma cools quickly before
crystallizationobsidian if solid or pumice if
Pyroclastic material: high gas content, high
viscosity more explosive
4. Explain the differences between pahoehoe
and a'a lavas
Pahoehoe: smooth, rope-like texture
A’a: rough, painful surface
5. Describe the different types of volcanic
eruptions and how they are related to magma
6. Describe the morphology, dominant rock
type and typical eruption style of the different
types of volcanoes.
Mafic volcanoes:
-Shield volcano flat, broad (100s of km wide)
basalt lava flows; low explosivity
-Scoria/Cinder conessmall, volcanic vent
made of pyroclastic material, found around
other volcanoes (ie. shield) or on own;
medium-high volatility
Intermediate to Felsic volcanoes:
-Composite/Stratovolcanoesconical shape
with steep slope, made of layers of lava and
pyroclastic material; medium-high volatility
(bigger than cinder cones so more dangerous)
-Lava domelava flow collection over vent,
keeps expanding with explosions; medium
volatility when erupts
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