1)
2)
3)
If erosion stripped off the top of a dome, what would
be found?
a. The oldest rocks are exposed in the center.
b. The oldest rocks are exposed along the edges of the
dome.
c. The youngest rocks are exposed in the center.
d. The rocks in the center are the same age as those
along the edges.
The convergence of two continental plates would
produce
a. trenches.
c. folded mountains.
b. rift valleys. d. island arcs.
What type of mountains are produced by ocean-ocean
convergence?
a. a continental volcanic arc
b. mainly volcanic mountains
c. parallel belts of uplifted mountains and volcanic
mountains
d. parallel belts of folded mountains and volcanic
mountains
Mountain Formation
Chapter 11, Section 3
Mountain Building at Convergent
Boundaries
 With
the original development of the
theory of plate tectonics, an
accepted model for orogenesis
emerged
 Most mountain building occurs at
convergent plate boundaries
 Colliding plates provide the
compressional forces that fold, fault,
and metamorphose the thick layers
of sediments deposited at the edges
of landmasses
Ocean-Ocean Convergence
 Ocean-ocean
convergence mainly
produces volcanic mountains
 These features tend to be long-lived
 The accumulation of sediment
scraped off of the subducting plate
increases the volume of the crust
 Examples: Aleutian Islands (Alaska)
and Japan
Development of Mature Island Arc
The Japanese Islands
Ocean-Continental Convergence
The convergence of the continental block and
the subduction of the oceanic plate leads to
deformation and metamorphism of the
continental margin
 Partial melting of mantle rock above the
subducting plate generates magma that
migrates upward
 Accretionary wedge - the accumulation of
different sedimentary and metamorphic rocks
with some scraps of ocean crust that gathers
on the landward side of a trench
 The types of mountains formed by oceancontinental convergence are volcanic
mountains and folded mountains
 Examples: Andes (South America) and
Cascades

Ocean-Continental Convergence 1
Ocean-Continental Convergence 2
Ocean-Continental Convergence 3
Andes Mountains
Continent-Continent Convergence
Continental crust floats too much to be
subducted
 At a convergent boundary between two plates
carrying continental crust, a collision between
the continental fragments will result and form
folded mountains
 Most of the oceanic crust that separated
landmasses, before a collision, is subducted,
but some gets caught up in a collision zone and
raised above sea level
 Examples: Himalayas (Asia), Urals (Russia),
and Appalachians

Continent-Continent Convergence
Concept Check
 Why
can’t continental crust be
subducted?
Mountain Building at Divergent
Boundaries
 Most
mountains are formed at
convergent boundaries, but some are
formed at divergent boundaries
 The mountains that form along ocean
ridges at divergent plate boundaries
are fault-block type mountains
 Example: the 65,000 kilometer long
mid-ocean ridge system
Mid-Atlantic Ridge
Concept Check
 Where
range?
is the longest mountain
Major
Mountainous
Landforms of
the Western
U.S.
Continental Accretion
Accretion – the process by which fragments
collide with a continental plate and become
stuck or embedded into the continent
 Terrane – any crustal fragment that has a
geologic history distinct from the adjoining
terranes
 Terranes can contain anything from sediment
off the ocean floor to volcanic island arcs
 The newly added material increases the width
and thickness of the continental crust
 The accretion of larger crustal fragments, such
as a mature island arc (Hawaii), may result in a
mountain range
 Because of its buoyancy, an island arc will not
subduct beneath the continental plate

Submerged Crustal Fragments and
Oceanic Plateaus
Collision of Volcanic Island Arc with
a Continent
Accreted Terranes
Isostasy
Isostasy – concept of a floating crust in
gravitational balance
 Many mountain belts stand high above the
surface because they have buoyant (less
dense) crustal “roots” that extend deep into the
mantle
 Isostatic adjustment – the processes of
establishing a new level of gravitational
equilibrium
 As weight is added to the top of a mountain,
the crust subsides, and as weight is lifted, the
crust will rebound
 Because of isostasy, deformed and thickened
crust will undergo regional uplift both during
mountain building and for a long period
afterward

Isostasy
Erosion and
Isostatic
Adjustment
Isostasy in Action – Iceberg
Assignment
Read Chapter 11, Section 3 (pg. 317-324)
 Do Section 11.3 Assessment #1-7 (pg. 324)
 Start Studying for the Unit 2 Test (Chapters
8-11)
