Chapter 3 Section 1 Volcanoes and Plate Tectonics
I.
What is a volcano?
a. Volcano is a weak spot in the crust
b. Magma is melted rock-forming substances PLUS gases
PLUS water
c. Lava is magma that is on the surface.
Location of Volcanoes
d. Most under water – 600 on land
e. Ring of Fire – belts where many volcanoes can be found
f. Find on divergent boundaries or in subduction zones
i. Also hot spots
At diverging boundaries, crust is thin or broken and magma
free to come out. Most of these stay under water
Converging boundaries
g. Density is important – remember, denser rock goes
down. It melts and the magma is now less dense than
the rock around it, so it starts to rise.
h. Island Arcs – where 2 oceanic plates collide. Many are
near deep ocean trenches
i. Alaska mountains result from oceanic and continental
plates colliding.
II.
Hot Spot Volcanoes
a. Magma melts through crust like a blowtorch – no
subduction involved.
b. Can form a series of islands as the plate drifts over
the hot spot, like Hawaiian Islands.
i. Or other activity like at Yellowstone.
Chapter 3 Section 2 Volcanic Activity
III. How magma reaches the surface
a. Magma rises – less dense than surrounding rock, so
rises like a balloon under water
b. Volcano erupts.
i. Gases are under tremendous pressure
ii. Rush out, like a shaken can of Coke
Inside a Volcano
c. Magma chamber is the pocket of magma below the
surface.
d. Pipe is narrow, vertical crack magma rises through.
e. Leaves ground through vents
i. Side vent is a branch through side of mountain
f. Crater is bowl-shaped area at top of many volcanoes,
around central vent
g. Lava flow is the area covered by the escaping lava.
Characteristics of magma
h. Temperature – hotter naturally flows faster
i. Silica content is other main variable – the more silica
present, the thicker lava is. High silica lava is light
colored and doesn’t flow very far.
i. Basalt (oceanic crust) is low silica
Types of eruptions
j. Quiet eruptions.
i. If magma flows easily, lava oozes out
k. Explosive eruptions.
i. If thick and sticky, pressure builds up until it
blows up. Forms discrete particles
1. Ash – small as grains of sand
2. Cinders – pebble-sized pieces
3. Bombs – as big as a car.
ii. Pyroclastic flow throws out these particles as
well as gases.
1. Mt. St. Helens is good example.
IV. Stages of a Volcano
a. Active – erupting or showing signs that may erupt soon
b. Dormant – sleeping, but may become active again
c. Extinct – unlikely to erupt again
V.
Other types of volcanic activity
a. Hot springs – groundwater heated by magma.
b. geysers – when a hot spring is trapped until pressure
builds up & water and steam burst out.
c. geothermal energy
i. Direct – pipe hot water into houses to heat
ii. Make electricity using steam turbines
VI. Monitoring Volcanoes
a. Can predict better than earthquakes – surface
movement, magnetic field, water level/temp, gases
escaping, earthquakes
VII. Volcano Hazards
a. Lava, fires, buried in ash, mud slides, avalanches,
poisonous gases
Chapter 3 Section 3 Volcanic Landforms
VIII. Landforms from lava and ash
a. Shield volcanoes
i. Wide, sloping mountains from lots of thin layers
ii. What kind of lava?
iii. What do Hawaii mountains look like under water?
b. Cinder cone volcanoes
i. Steep sided, made of cinders and bombs
ii. Thick, sticky lava
c. Composite volcanoes
i. Alternating lava flows and explosions
d. Lava plateaus
i. Thin lava spreading out from cracks, repeatedly
e. Calderas
i. Mountain hollows out, then collapses inward.
Often forms lake
ii. Why might you have an island in a caldera?
Soils from lava and ash
f. Very rich (fertile) because it has lots of minerals
Landforms from magma – cools under surface, then
surrounding rock wears away
g. Volcanic necks, dikes, & sills
i. Neck was the pipe; dike is a vertical layer of
magma, & sill is a horizontal layer (think “door
sill”)
h. Batholiths are hardened magma chambers that can be
big enough to be mountains
i. Dome mountains are like batholiths, but smaller.
Magma is stopped by a horizontal layer of rock, which
is bent upwards.