Download Earth Science Chapter 18: Volcanic Activity Chapter Overview

Earth Science
Chapter 18: Volcanic Activity
Chapter Overview
Section 1: Volcanoes
1. Zones of Volcanism
Volcanism describes all the processes associated with the discharge of magma, hot fluids,
ash, and gases
• Convergent volcanism – at oceanic-continental convergent boundaries volcanoes occur
as the oceanic plate descends and the crust melts. The melting crust forms a magma that
is less dense than the surrounding rock, as a result the magma rises through the
overlying rock and when it reaches Earth’s surface, forms a volcano. Volcanoes
associated with convergent plate boundaries form two major “belts”, the Circum-Pacific
belt and the Mediterranean Belt.
• Divergent volcanism – at divergent boundaries tectonic plates move apart and new ocean
floor is produced as magma rises to fill the gap. At these ridges, the lava takes the form
of giant pillows. Volcanism at divergent boundaries tends to be non-explosive.
• Hot spots – some volcanoes form far from plate boundaries over hot spots which
scientists hypothesize are unusually hot regions of Earth’s mantle where hightemperature plumes of magma rise to the surface. A primary example of hot spot
volcanism is the chain of islands that compose the state of Hawaii.
• Flood basalts – flood basalts form when lava flows out of long cracks in Earth’s crust
called fissures.
2. Anatomy of a Volcano
Magma chambers deep within Earth fuel the volcanoes that erupt at the planet’s surface.
Magma that reaches Earth’s surface is called lava. Lava flows upward through the crust in
tubes call conduits, and erupts through an opening in the crust called a vent. As lava flows
out onto the surface, it cools and solidifies around the vent. Over time, the lava can
accumulate to form a mountain known as a volcano. At the top of the volcano, around the
vent, is a bowl-shaped depression called a crater. Depressions larger than craters, called
calderas, can form when the summit or the side of a volcano collapses.
3. Types of Volcanoes
The appearance of a volcano depends on two factors: the type of material that forms the
volcano and the type of eruption that occur. Based on these two criteria, three major types of
volcanoes have been identified: shield volcanoes, cinder-cone volcanoes, and composite
volcanoes
• Shield Volcanoes
A shield volcano is a mountain with broad, gently sloping sides and a nearly circular
base. Shield volcanoes form when layer upon layer of basaltic lava accumulates during
nonexplosive eruptions
• Cinder-cone Volcanoes
A cinder-cone volcano forms when material ejected high into the air falls back to Earth
and piles up around the vent. Cinder-cone volcanoes have steep sides and are generally
small. The lava ejected from these volcanoes more water and silica than magma, making
them very explosive
• Composite Volcanoes
Composite volcanoes form when layers of volcanic fragments alternate with lava. As
with cinder-cone volcanoes, composite volcanoes contain large amounts of silica and
water. However, composite volcanoes are larger than cinder-cone volcanoes.
• Size and Shape
The smallest volcanoes are the cinder-cone volcanoes, which also have the steepest
sides. The shield volcanoes have the gentlest slope. Finally, the slopes of cinder-cone
and composite volcanoes are concave while the slopes of shield volcanoes are straight.
These differences in size and shape are the result of many factors, including the different
kinds of materials that make up each volcano, the vegetation that grows on the volcano,
local climate, and the history of the volcano
Section 2: Eruptions
1. Making Magma
All volcanoes are fueled by magma deep beneath Earth’s surface. A volcano’s explosivity
depends on the composition of the magma. An important factor in magma’s composition is
how rocks melt to make magma
• Temperature
Depending on their composition, most rocks begin to melt at temperatures between
800OC and 1200OC. These temperatures are found in the crust and upper mantle.
• Pressure
Like temperature, pressure increases with depth because of the weight of the overlying
rocks. As pressure increases the melting point of rock increases, Because of this, most of
the rocks in Earth’s lower crust and upper mantle do not melt to form magma
• Water
The presence of water also influences whether a rock will melt. At any given pressure, a
wet mineral or rock will melt at a lower temperature than the same mineral or rock under
dry conditions
2. Composition of Magma
The composition of magma determines a volcano’s explosivity. Factors that determine
composition include: the magma’s interaction with the overlying crust, its temperature,
pressure, amounts of dissolved gas, and the amount of silica contained in the magma.
• Dissolved gases
In general, as the amount of gases in magma increases, the magma’s explosivity
increases. Important gases in magma include water vapor, carbon dioxide, sulfur dioxide,
and hydrogen sulfide.
• Viscosity
Viscosity is a material’s resistance to flow. Temperature and silica content affect he
viscosity of a magma. Generally, cooler magma has a higher viscosity. Magma with high
silica content tends to be thick and traps gases.
3. Types of Magma
The silica content of magma determines not only its explosivity and viscosity, but also which
type of volcanic rock it forms as lava cools.
• Basaltic Magma
Basaltic magma typically forms when rocks in the upper mantle melt. Because basaltic
magma contains small amounts of dissolved gases and silica, the volcanoes it fuels erupt
relatively quietly
• Andesitic Magma
Andesitic magma is found along continental margins, where oceanic crust is subducted
into Earth’s mantle. The source material for this magma can be either oceanic crust or
oceanic sediment
• Rhyolitic Magma
Rhyolitic magma forms when molten material rises and mixes with the overlying silicaand water-rich continental crust. This magma has high viscosity and large quantities of
trapped gases, so the eruptions of volcanoes fueled by rhyolitic magma tend to be very
explosive
Section 3: Intrusive Activity
1. Plutons
When magma cools, minerals form. Over a very long period of time, these minerals will
combine to form intrusive igneous rock bodies. These rock bodies, called plutons, can be
exposed at Earth’s surface because of uplift and erosion and are classified based on their
size, shape, and relationship to surrounding rocks.
•
Batholiths and Stocks
The largest plutons are called batholiths, and are irregularly shaped masses of coarse2
grained igneous rocks that cover at least 100km and take millions of years to form.
Batholiths are common in the interiors of major mountain chains. Irregularly shaped
plutons that are similar to batholiths but smaller in size are called stocks. Both batholiths
and stocks cut across older rocks and generally form 10 – 30 km beneath Earth’s surface
• Laccoliths
Sometimes, when magma intrudes into parallel rock layers close to Earth’s surface, some
of the rocks bow upward as a result of the intense heat and pressure of the magma body.
When the magma solidifies, a laccolith forms. A laccolith is a mushroom-shaped pluton
with a round top and flat bottom. Laccoliths are relatively small, up to 16 kilometers wide.
• Sills and Dikes
A sill is a pluton that forms when magma intrudes parallel to layers of rock. Sills can
range from only a few centimeters to hundreds of meters thick. A dike is a pluton that
cuts across preexisting rocks, and often form when magma invades cracks in
surrounding rock bodies. Most dikes are a few centimeters to several meters wide and up
to tens of kilometers long
2. Plutons and Tectonics
Many plutons form as a result of mountain-building activities. In fact, batholiths are found at
the cores of many of Earth’s mountain ranges. Recall that many major mountain ranges form
along continental-continental convergent plate boundaries and also where two oceanic plates
converge.