Volcanoes and Plate Tectonics
This map shows the margins of the Pacific tectonic plate and surrounding region. The
red dots show the location of active volcanism. Notice how the majority of the
volcanism is focused in lines along the plate boundaries? For this region is the area
known as the “Pacific Ring of Fire”.
Volcanism is
mostly focused at
plate margins
Pacific Ring of Fire
Remember that there are
three types of plate boundaries
• Divergent
• Convergent
• Transform
Divergent Boundaries
• Spreading ridges
– As plates move apart new material is erupted
to fill the gap
Island Arc
• Many volcanoes occur near boundaries where
two oceanic plates collide.
• Through subduction, the older, denser plate
sinks beneath a deep-ocean trench into the
mantle.
• Eventually, the magma breaks through the
ocean floor, creating volcanoes.
• These volcanoes create a string of islands
called an ISLAND ARC.
Island arc of Aleutian trench
What are Hotspot Volcanoes?
Firstly, what are hotspot volcanoes and how do they form?
They are hot mantle plumes breaching the surface in the middle of a tectonic plate.
• A hotspot is a location on the Earth's
surface that has experienced active
volcanism for a long period of time.
•The source of this volcanism is a mantle
plume of hot mantle material rising up
from near the core-mantle boundary
through the crust to the surface (see left
diagram).
•A mantle plume may rise at any location
in the mantle, and this is why hotspot
volcanoes are independent from tectonic
plate boundaries.
•The Hawaiian island chain are an
example of hotspot volcanoes (see right
photograph).
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.
The volcanoes get younger from one end to the other.
Hotspot’s commonly form volcanic island chains (like the
Hawaiian islands). These result from the slow movement
of a tectonic plate over a FIXED hotspot.
Persistent volcanic activity at a hotspot will create new
islands as the plate moves the position of the “old”
volcanic island from over the hotspot.
Therefore at one end of the island chain you see the
youngest, most active volcanic islands (directly over the
hotspot) and along the island chain the extinct
volcanoes become older and more eroded (see
diagram).
This way geologists can use hotspot volcano chains to
track the movement of the tectonic plate through time.
Properties of Magma
• Element: a substance that cannot be broken
down into other substances.
• Compound: a substance made of two or more
elements that have been chemically
combined.
• Each substance has a particular set of
physical and chemical properties. These
properties can be used to identify a
substance or to predict how it will behave.
Physical Properties
Any characteristic of a
substance that can be
observed or measured
without changing the
composition of the
substance. It always
has the same physical
properties under
particular conditions.
EX: density, hardness,
melting point, boiling
point.
Chemical Properties
Any property that
produces a change in
the composition of
matter.
EX: ability to burn
Reaction with other
substances that cause
one to change color,
produce a gas or form
a new, solid substance.
Viscosity
• Viscosity: a physical property of liquids. It is
the liquid’s resistance to flow.
• The greater the viscosity of a liquid, the slower
it flows: honey.
• The lower the viscosity of a liquid, the more
easily it flows: water.
Viscosity of Magma
• The viscosity of magma depends upon its silica
content and temperature.
• Silica is a compound and one of the most
abundant materials in the Earth’s crust.
• More silica in the magma, the higher the
viscosity of the magma.
Types of Lava
Pahoehoe
fast-moving hot lava,
low viscosity, low silica
Aa
slow-moving cooler
lava, high viscosity, high
silica
Life Cycle of a Volcano
• Active, or live, volcano is one that is erupting
or has shown signs that it may erupt in the
near future.
• Dormant, or sleeping, volcano is one that is
not doing anything but could become active
again.
• Extinct, or dead, volcano is unliely to erupt
again.
Three types of volcanoes
Lava Plateau/Calderas
A lava plateau is made up of many
layers of thin, runny lava that erupt
from long cracks in the ground.
Crater Lake, above, fills an almost
circular caldera. A caldera forms
when a volcano’s magma
chamber empties and the roof of
the chamber collapses.