1. Volcanoes & Other Igneous Activity
Chapter 10
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2. How & Where Volcanoes Form
What is a volcano?
1. opening in Earth’s crust through which molten rock (magma), gases, & ash erupt
2. land around opening in Earth’s crust
ES0901 Visualization
Strato-volcanoes
Where (in general) do volcanic eruptions occur?
where magma (molten rock) rises to the surface
at plate boundaries
mantle hot spots

3. Origin of Magma sec. 10.2 What is the asthenosphere?
partially melted (“plastic”) layer of the mantle that underlies the lithosphere
Why is most of the asthenosphere solid?
because of the pressure exerted on it by the lithosphere above
some materials do melt
causes convection currents
due to density differences
The lithosphere is made of the crust & uppermost mantle

4. Origin of Magma
What 3 conditions allow for magma formation in the asthenosphere?
decrease pressure
rifts
increase temperature
hot spot
adding water
subduction boundaries
1. A decrease in pressure (& therefore density) can lower the melting temperature of the materials.
along rift valley at mid-ocean ridge where the lithosphere is thinner & exerts less pressure
2. An increase in temperature can cause materials to melt.
at a hotspot
3. An increase in the amount of water in the asthenosphere can lower the melting temperature of the materials.
At subduction boundaries

5. Plate Tectonics & Igneous Activity sec. 10.3
Where are volcanoes found?
along mid-ocean ridges/rifts
at subduction boundaries
over hot spots
Volcanism at a Subduction Zone Animation
Volcanism Along a Rift Zone Animation
along mid-ocean ridges  plates moving apart (diverging)
Where lithospheric plates diverge ( ) at a rift… mantle material rises from deeper, hotter regions
the pressure is lower than the surrounding area, so melting temperatures are lower… Causes large amounts of magma to form
The magma is less dense than the materials around it, so it rises through the rift to the surface
at subduction boundaries  plates moving together & the denser plate is forced under the less dense plate
Oceanic-Continental & Oceanic-Oceanic… Volcanoes always form on the overriding (top) plate
On continental plate at oceanic-continental convergence  Ex. Cascade Mountains (Pacific coast of N. America)
On the overriding oceanic plate at oceanic-oceanic convergence  Ex. Mariana Islands in Pacific (volcanic islands)
Most magma that reaches the Earth’s surface is at divergent boundaries along mid-ocean ridges
over hot spots  an area of volcanic activity that results from a plume of hot solid material that has risen from deep within Earth’s mantle

6. Plate Tectonics & Igneous Activity
What is a hot spot?
areas of volcanic activity that result from plumes of hot solid material that have risen from deep in the mantle
as material rises, it melts due to decreasing pressure
Do hot spots move or stay in the same place?
stay in the same place
plate moves over it
how Hawaii formed
The Hawaiian Islands were formed as the Pacific plate moved NW over a hot spot
The largest & youngest island, Hawaii, is directly above the hot spot (volcanically active)
Older islands (extinct volcanoes) extend to the NW
Originally formed over the hotspot
plate moved them away from the hot spot
Underwater volcanoes (seamounts) are also part of this chain
Volcanic Islands Forming Over a Hot Spot Animation

7. The Nature of Volcanic Eruptions sec. 10.1
How do differences in magma affect volcanic activity?
Kilauea Mount St. Helens Yellowstone caldera

8. The Nature of Volcanic Eruptions
More on Viscosity
How does magma type/silica content affect a magma’s viscosity & how a volcano erupts?
felsic (rhyolitic) magmas
thick & gooey
difficult for gases to escape
explosive
mafic (basaltic) magmas
thin & runny
easy for gases to escape
gentle, flowing
Silica is the main ingredient in magma. Why? b/c most rock forming-minerals are silicates
What 2 elements do all silicates contain? Silicon & oxygen
What is viscosity? resistance to flow (…how thick or thin it is)
Greater amounts of silica = High viscosity/resist flow (thick, gooey… like syrup)
Type of eruption = explosive (b/c gases expand, build up due to “thickness” of magma & propel magma rapidly…)
Lower amounts of silica = low viscosity/flow easily (thin, runny.. Like water)
Type of eruption = gentle, flowing (“flood-like”, “fountain-like”.

9. The Nature of Volcanic Eruptions
What is lava?
magma that has reached Earth’s surface
What factors affect lava flows:
on land?
temperature & speed
hotter, basaltic lava flows quickly
forms pahoehoe (smooth, ropelike surfaces)
cooler, basaltic lava flows slowly & cools quickly
forms aa (rough, jagged surfaces)
underwater?
contact w/ water cools outside faster than inside
forms rounded, pillow-like crust  pressure builds  crust cracks  lava pours out… cycle repeats
forms pillow lava
felsic (rhyolitic) magmas  explosive
mafic (basaltic) magmas  gentle, flowing

10. Volcanic Material What are pyroclastic materials?
solid fragments ejected when trapped gases are released from lava
What is a pyroclastic flow?
when pyroclastic materials combine w/ hot gases
dense, superheated cloud  travels rapidly downhill
Explosive eruptions usually involve magmas which contain trapped gases
When gases are released, solid pyroclastic material may be ejected
Classified by size
Smallest  ash
Intermediate  lapilli
Largest  blocks & bombs

11. Types of Volcanoes What determines the shape & structure of a volcano?
nature of its eruptions & materials it ejects
Types of volcanic landforms:
shield volcanoes
cinder cones
composite volcanoes (stratovolcanoes)
calderas
lava plateaus
The term “volcano” can refer to the volcanic vent and the landform that develops as the materials from an eruption harden.

12. Shield Volcanoes runny, thin, basaltic lava
builds up in layers
broad base, gently sloping sides
form over (oceanic) hot spots
Shield volcanoes
Basaltic (mafic) lava  Low viscosity (runny, thin)  Tends to flow long distances before hardening  Builds up in layers = Broad bases & gently sloping sides
Tend to be less explosive than other types of volcanoes  However, flows may be frequent & large in volume
Tend to form over hotspots & in oceans (b/c oceanic crust is made of basalt)
Ex. Mauna Loa in Hawaii is 9170 m tall (4170 m above sea level & 5000 m below sea level)

13. Cinder Cones molten lava thrown into air forming fragments
fragments accumulate
small, cone-shaped mound, oval base
often form in groups on side of larger volcanoes
Cinder cones
Forms when molten lava is thrown into the air from a vent  As it falls, lava breaks into fragments that harden before hitting the ground  Fragments accumulate, forming a cone-shaped mound with an oval base
Tend to be smaller than other types of volcanoes
Typically form in groups & on the sides of larger volcanoes

14. Composite Volcanoes (Stratovolcanoes)
gas-rich magma
explosive
cause lahars (fast flowing mudflow)
alternating layers of hardened lava flows & pyroclastic material
large, steep, symmetrical cones
form at subduction zones
Composite volcanoes (also known as stratovolcanoes)
Develop when layers of materials from successive explosive eruptions accumulate around a vent
steep-sided, symmetrical cones of large dimension built of alternating layers of hardened lava flows & other pyroclastic materials
Can be quiet for a long time between eruptions
Gas-rich (usually andesitic) magma builds up pressure, bulge may form
Earthquakes burst the bulge & explosive eruptions release magma, water, & gases in a massive cloud of superheated ash & stones
Can cause a “lahar” (fast-moving mudflow)
Ex. Mount St. Helens (1980)
Mt. Shasta
Mount St. Helens

15. Other Volcanic Landforms
What is a caldera?
large crater-shaped depression
How do calderas form?
formed when magma beneath a volcano is released & top of volcano collapses
can fill w/ water  lake
Calderas
Large crater-shaped basin (depression) formed when magma beneath a volcano is released after the top of the volcano collapses
Seen in may active volcanoes
Sometimes fills with water to become a lake  Ex. Crater Lake in Oregon
Some are still active  Like the one in Yellowstone National Park which feeds hot springs & geysers

16. Other Volcanic Landforms
What is a lava plateau?
flat, wide surface formed from basaltic lava
How do lava plateaus form?
poured from a fissure in Earth’s surface & spread across the land