Ch. 20 Sec. 2
Orogeny
Section 20.2
Orogeny
Convergence causes the crust to
thicken and form mountain
belts.
Review Vocabulary
island arc: a line of islands that forms
over a subducting oceanic plate
Orogeny - all processes that form
mountain ranges
also known in geology as orogenic belts
I. Mountain Building at Convergent
Boundaries
Most of Earth’s mountain ranges formed along
plate boundaries.
Fig 20.7
Pg. 567
Compressive forces
• folding
• faulting
• metamorphism
• igneous intrusions
A. Oceanic-oceanic convergence
1. Subduction
a. Plate melts
b. Magma rises
Fig 20.8
Pg. 568
2. Island arc complex
a. Aleutian Islands
(Alaska)
b. Lesser Antilles
(Caribbean)
Section 20.2
Orogeny
Hot Spot Island Formation
3. Rock types
a. basaltic and andesitic magmas
b. sedimentary rock – uplifted or
folded against arc (Japan)
B. Oceanic-continental convergence
Convergence between oceanic and
continental plates produces
mountain belts that are much bigger
and more complicated than island
arc complexes.
1. Uplift of continental plate
(beginning of orogeny)
2. Compression causes continental
crust to fold and thicken
3. Igneous activity and metamorphism
are common along boundaries.
Fig. 20.9
Pg. 569
C. Continental-continental convergence
Intense folding and faulting along
continental-continental
boundaries produce some of the
highest mountain ranges on
Earth.
Fig 20.10
Pg. 570
Another common characteristic of
the mountains that form when two
continents collide is the presence of
marine sedimentary rock near the
mountains’ summits. The marine
sedimentary rocks were formed in
the ocean basin that existed between
the continents before their collision.
Orogeny
Convergence
II. The Appalachian Mountains—A
Case Study
Geologists have divided
the Appalachians into
several distinct regions.
Each region is
characterized by rocks
that show different
degrees of
deformation.
Fig. 20. 11
Page 571
Section 20.2
Orogeny
Folding Rocks
A. The early Appalachians
1. 800 to 700 mya separation from Africa
a. two oceans— the
ancestral Atlantic
Ocean and a shallow,
marginal sea
b. continental
fragment
2. 700 to 600 mya
a. Convergence
b. Island arc east of
North America
3. 500 – 400 mya
a. continental
fragment thrust
over younger
rocks
b. Blue Ridge
Mountains
B. The final stages of formation
1. 400 to 300 mya
a. Island arc attaches
b. Piedmont Province
c. Blue Ridge rocks
pushed farther
west
2. 300 and 260 mya
a. Pangaea forms
b. Extensive folding
and faulting
c. Valley and Ridge
Province
Section 20.2
Orogeny
Visualizing the Rise and Fall
of the Appalachians
The Appalachians formed hundreds of
millions of years ago as a result of
convergence.
Convergence causes the crust
to thicken and form mountain belts.
 Orogeny refers to all of the processes
that form mountain belts.
 Most mountain belts are associated
with plate boundaries.
 Island arc complexes, highly
deformed mountains, and very tall
mountains form as a result of the
convergence of tectonic plates.
 The Appalachian Mountains are
geologically ancient; they began to
form 700 to 800 mya.
Examine the illustration. What is
happening at this mountain belt?
Possible answer: An oceanic plate
is subducting beneath a
continental plate. As the
subducting plate sinks, it gets
warmer and water is released
from minerals that contain it.