Chapter 18 – Volcanic Activity
18.1 – Magma
Types of Magma
- the three main types of magma are andesitic, basaltic and rhyolitic
o rhyolitic is used to describe the magma that solidifies to from
granite
o basaltic has the same composition as basalt
 Hawaii and Iceland are examples of these (18.2)
o andesitic has the same composition of andesite
 Mount St. Helens is an example of this
Magma Composition
- there are a number of factors that determine type of magma (table
18.1)
- one is viscosity – the internal resistance to flow
Basaltic Magma
- this normally forms when rocks in the upper mantle melt and has low
viscosity – flows easier
Andesitic Magma
- this is found along continental margins, where oceanic crust is
subducted into Earth’s mantle
- has intermediate viscosity
Rhyolitic Magma
- forms when molten material rises and mixes with the overlying crust
- this has a high viscosity
Viscosity
- the viscosity of magma and lave depend upon both temperature and
composition
o the hotter the magma, the lower the viscosity
o the amount of silica increases the viscosity
Skipping 18.2, vocab is still needed
18.3 – Volcanoes
Anatomy of a Volcano
- lava erupts through an opening called a vent
o as the lave flows onto the surface it cools and hardens around
the vent
o over time the hardening lave can accumulate and form a
volcano
o at the top of the volcano a bowl shaped depression, called a
crater, forms
o large craters are called calderas
o fig 18-11 shows a lake that formed in a caldera
Types of Volcanoes
- the appearance of a volcano depends on two factors
o the type of material that forms the volcano
o type of eruptions that occur
Shield Volcanoes
- this has a mountain with a broad, gently sloping side and a nearly
circular base
- they form when layer upon layer of basaltic lava accumulate during
nonexplosive eruptions
o Hawaii
Cinder-Cone Volcanoes
- this forms when material ejected high into the air falls back to Earth
and piles up around the vent
- these have steep sides and are generally small
- these are more explosive then shield volcanoes
Composite Volcanoes
- these form when layers of volcanic fragments alternate with lava
- these are much larger then cinder-cone volcanoes and can be very
explosive
- Mount St. Helens
Size and Slope
- fig 18-12 through 18-14
Volcanic Material
- rock fragments thrown into the air during an eruption are called tephra
o they are classified by size
 dust < 0.25 mm
 ash < 2.0 mm
 lapilli > 2.0 mm - largest can be the size of a car or
small building
Pyroclastic Flow
- rapidly moving tephra and volcanic ash which rapidly moves down
the volcano
- can move at 200 km/h and temps can exceed 700 degrees Celsius
Where do Volcanoes Occur?
- most form at plate boundaries
o 80% at convergent, 15% at divergent
Convergent Volcanism
- fig 18-16
- these volcanoes occur in 2 major belts
o The Circum-Pacific Belt and Mediterranean Belt
Divergent Volcanism
- these form at rift zones
- most occur under water along ocean ridges
Hot Spots
- these are volcanoes found far away from plate boundaries
- Hawaii was formed by this