Transcript Unit 2.3: Volcanoes and Earthquakes

Unit 2.3:
Volcanoes
I. Location of Volcano Formation
A. @ Convergent Boundaries:
1. If oceanic crust subducts
underneath continental crust,
inland volcanoes form, as
part of a mountain range.
a. Oceanic crust melts as it
subducts  becomes less
dense  rises through
continental crust  melts
granitic rock  forms a
volcano.
2. If two oceanic crusts collide,
a volcanic island arc forms at
that boundary.
2
B. Intraplate volcanoes (hot-spot
volcanoes): form within a plate,
not at a boundary
1. Occurs over a “mantle plume”: an
area at the core-mantle boundary
with hotter-than-normal magma
a. Like a lava lamp, hot mantle material
rises
b. Causes “decompression melting” at
crust
1) Decrease in pressure means lower
melting temp. for rocks
c. As plate moves, new “hot-spot”
volcanoes form, creates a chain
• Ex: Hawaiian islands, older islands to
northwest
3
II. Magma: Composition and Eruption
A. Three factors that determine viscosity:
1. Temperature: (think of syrup)
a. hotter = more fluidity, “thinner”, less viscous
b. cooler = less fluidity, “thicker”, more viscous
2. Amount of Silicon (Si)
a. more Si = thicker: felsic rocks ~70% Si
(granite)
b. less Si = thinner: mafic rocks ~50% Si (basalt)
3. Amount of Water (and other gases):
a. more H2O = thinner magma
b. Prevents Si chains from forming
c. Closer to surface, H2O evaporates, so thicker
magma
4
5
B. Types of eruptions:
1. Pahoehoe: “on which one can walk,”
• ribbon-like
• hotter, more fluid
• As it cools, it can become aa flow
2. Aa: rocky, jagged blocks
3. Nuée Ardente: (Fr. “fiery cloud”)
• Glowing avalanche
• Made up of hot gases, glowing ash, and
pyroclastic materials
• Fast: 120+ mph
4. Lahar: mudflow
• Volcanic debris saturated with water
6
C. Progress of a volcanic eruption:
1. Partial melting: “melt” is less dense
than rock
a. Rises until reaches crustal rock w/less
density
2. Forms magma chamber, can cause
“bulging”, fracturing
3. Iron-rich minerals sink, so volatiles
become more concentrated…
a. If volatiles released, then fountain of lava
1) Like open soda, gases escape easily
2) Showy, but mostly harmless (quiescent)
b. But if volatiles restricted, then thick
magma expands until fracture, violent
release
1) Like shaking a closed soda, restricts volatiles
7
III. Structure of a Volcano:
A. Magma Chamber: underground pool of molten rock
B. Conduit/Pipe: passageway from magma chamber to vent
C. Vent: at the summit, can be classified according to size
1. Crater: smaller diameter
a. Forms when summit area collapses
2. Caldera: larger diameter (1 km+)
D. Secondary cone:
1. aka: Parasitic cone
2. On side of volcano
E. Fumaroles: (“fumus-” L. smoke)
1. Emit only smoke
8
9
IV. Three Main Types of Volcanoes:
A. Shield: broad base, slightly domed, like a warrior’s
shield
1. Shallow slopes because they are formed from thinner,
basaltic lavas that flow easily
2. Form at ocean floor b/c oceanic crust is mostly basalt
3. Can tell its age by its slope
a. If gentle slopes, then younger
b. If steeper slopes, then older
4. Ex: Hawaii, Iceland, Galapagos
10
B. Cinder Cone: also known as a “scoria” cone
1. Formed from pyroclastic fragments:
•
a.
b.
c.
“pyro-” = fire; “clast” = broken pieces
Ash (smallest)  dust  cinders  blocks/bombs (largest)
Scoria: red/black b/c more iron (basaltic)
Pumice: grey b/c more silica (granitic)
2. Steep sides
3. Ex: Cinder Cone in
Mt. Lassen Nat’l Park
11
C. Composite Cone: also called a stratovolcano
• ‘strato-’ = layered
1. Built of alternating layers of lava flows and pyroclast
2. Steep sides because thicker magma = violent eruptions
a. nuée ardente and lahar
3. Ex: Pacific Ring of Fire: Mt. Fujiyama, Mt. St. Helens
12
V. Other Volcanic Landforms
A. Extrusive landforms: forms
outside Earth’s interior (‘ex-’ = out)
1. Caldera: (“caldaria-” = L. cooking pot)
forms through collapse of:
a. Stratovolcano summit after eruption
• Ex: Crater Lake
b. Shield volcano summit b/c magma
chamber drains
• Ex: Hawaiian calderas
c. Large area b/c of ring-fractures
• Ex: Yellowstone
13
B. Intrusive landforms: form inside Earth,
under the surface, called plutons
• “pluto-” – Greek god of the underworld
1. Plutons classified by:
a. Shape: tabular (tabletop) or massive
b. Orientation to host rock:
1) Discordant: (disagree) if cuts across
2) Concordant: (agree) if parallel
2. Dike: tabular, discordant pluton
a. Magma injected into fractures
14
3. Sill: tabular, concordant pluton
a. Magma injected into sedimentary beds,
form flat “shelf”, (like a windowsill)
b. Form at shallow depths
c. Very fluid (basaltic) magmas
4. Laccolith: massive, concordant pluton
a. “lakko-” = Gr., lake, pond
b. Spreads laterally (like a lake) beneath the
surface, so it forces a dome-shaped bulge
at surface
c. Thicker (viscous) magma
5. Batholith: (“bathos” = Gr., depth)
a. Granitic or intermediate composition,
formed deep in the Earth
b. Large structures, make up core of many
mountains; Ex: Sierra Nevada Batholith
15