Chapter 6 Objectives:
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Geography 12
Worksheet 6.5
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Collision and Subduction Zones: Converging Plate
Boundaries
Read pages 103 – 110 of Planet Earth: A Physical Geography. Answer
the following questions:
Vocabulary (1 mark each)
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Subduction:
The downward movement and eventual melting of an oceanic plate as it sinks into the
asthenosphere along converging plate boundaries

Andesitic Magmas:
Highly viscous, silica-rich, gaseous, molten rock formed by the melting of subducting
oceanic plates in the asthenosphere that produces violent volcanic activity

Continental Volcanic Arcs:
An arc-like chain of volcanic mountains formed on a continent bordering a subduction
zone at a converging plate boundary
Island Arcs:
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Appreciate the power and
scope of tectonic processes
and their effects
Appreciate the slowness of
tectonic processes based on
a human time scale
Understand that the
lithosphere is an everchanging part of a dynamic
planet
Understand the general
pattern of tectonic activity
over geologic time and
explain the location pattern
of tectonic activity over the
earth’s surface
Explain the tectonic
processes that shape the
earth’s surface, including
folding, faulting, and
volcanic activity
Describe and explain the
pattern of major surface
features created by
tectonic processes
Predict the nature and
general patterns of
occurrence of tectonic
activities and processes,
especially earthquakes and
volcanic eruptions
Describe the positive and
negative aspects of tectonic
activities
An arc-shaped chain of volcanic mountains, often rising above sea level as islands,
formed on the ocean floor at a subduction zone at a converging plate boundary
Composite Volcanoes:
A smooth-sloped volcanic peak with a summit crater made up of alternating layers of ash and lava,
formed from andesitic magma at subduction zones
Volcanic Necks:
A block of hard rock left standing above the landscape when an extinct composite volcano is eroded
away over millions of years.
Cinder Cones:
A steep-sided volcanic peak, with a large summit crater, composed mainly of volcanic ash and rock
spewed out of the vent during explosive eruptions
Dykes:
An intrusion of magma that cuts across the original rock beds and cools beneath the earth’s surface to
form igneous intrusive rock
Sills:
An intrusion of magma that follows the layers of the original rock beds and cools beneath the earth’s
surface to form igneous intrusive rock
Laccoliths:
An intrusion that forces apart the local rock beds to form an enlarged, dome-shaped chamber of magma
that cools to form igneous intrusive rock
Batholiths:
A massive, often bottomless intrusion of magma that cools beneath the earth[s surface to form igneous
intrusive rock
Anticlines:
Rock beds that have been folded or bent upwards ro form a hill or mountain
Synclines:
Rock beds that have been folded or bent downwards to form a valley
Recumbent Folds:
A fold that has been compressed so severely that it falls over or is overturned
Short Answer (2 marks each)
1. What are the two types of converging plate boundaries?
Subduction zones (where an oceanic plate is slipping below a continental plate)
Collision zones (where two continental plates are meeting)
2. Briefly describe the process of subduction?
-thin ocean plates with heavy basaltic rock descent into the plastic asthenosphere beneath lighter, thicker
granitic rocks of continental plates
-Descending plates are cooler so they sink down. That is subduction.
3. Briefly describe the process of collision zones?
-two continental plates collide
-rocks of the continental plates are lighter and less dense than those of the asthenosphere…don’t sink
-rocks collide to create massive mountain ranges of twisted rocks (Himalayas, Appalachian, Alps)
4. Explain why some volcanoes are more spectacular and destructive than others.
Use the term andesitic magma in your explanation.
Cooler materials of the andesitic magmas, with silica and gases, tend to clog up the vents of volcanies
leading to the build up of pressure. When the pressure becomes great enough, a major explosion hurls
magma onto the earth’s surface and into the atmosphere.
5. What is the difference between continental volcanic arcs and island arcs?
Same thing, but the only difference is where the arcs are found. Continental arcs are located in
continents while island arcs are formed from volcanoes build on the ocean floor.