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Earth Systems 3209
Unit: 4
The Forces Within Earth
Reference:
Chapters 4, 15, 16, 19; Appendix A & B
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Unit 4:
Topic 2.5
Plate Boundaries - Divergent
Focus on . . .
 describing a rift valley and explaining how it evolves
into a divergent boundary.
 demonstrating through a drawing the processes
associated with plate divergence.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Plate Boundaries
 According to the Plate tectonic theory, three
boundaries exist at the edges of tectonic plates.
1) Divergent Boundary (Ridge)
2) Convergent Boundary (Trench)
3) Transform Boundary
Text Reference:
Pages 526-539
Figure 19.17, pg. 528
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Divergent Plate Boundary
 A divergent plate boundary is characterized by two
plates moving away from one another as a result of
tensional forces exerted on them by convection
currents in the upper mantle (Asthenosphere).
 As the plates move apart, hot molten material upwells
at the boundary forcing the edges of both plates
upwards, creating a Ridge.
 New ocean floor is created along the length of ridges.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Divergent Plate Boundary
 Divergent plate boundaries are referred to as
constructive margins since new oceanic crust is
being formed there.
 Examples include;
Mid-Atlantic ridge and Mid-Indian ridge.
Ocean
Moho
Ocean Crust
Magma
Continental Crust
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evolution of Rift Valley into a Divergent Plate Boundary
A) If tensional forces, due to the motion of
convection currents in the upper mantle, begin
beneath continents, it can cause continents to
split into two or more smaller segments.
Text Reference:
Pages 532 - 33
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evolution of Rift Valley into a Divergent Plate Boundary
B) A spreading center is formed and a rift valley
is formed as a result of the associated faulting
(normal) and thinning of the crust. Eventually
the thinning becomes significant enough for
ocean water to move into the rift valley.
Rift
Valley
Text Reference:
Pages 532 - 33
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evolution of Rift Valley into a Divergent Plate Boundary
C) Ridge (Rift) volcanism and seafloor
spreading begin and a divergent plate
boundary is produced.
Text Reference:
Pages 532 - 33
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evolution of Rift Valley into a Divergent Plate Boundary
D) As both continue, the landmasses on both sides of
the ridge continue to move apart from one another.
 Examples: East African Rift Valley
Text Reference:
Pages 532 - 33
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Sample Problem
 Using a labelled diagram, briefly describe what
happens at a mid-ocean ridge.
Diagram:
Answer:
Tensional forces cause the plates to move apart resulting in upwelling
of molten material from the mantle to create new ocean floor.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Plate Boundaries - SUMMARY
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Example 1:
At which of the following is crust being created?
(A) ridge
(C) trench
(B) transform fault
(D) subduction zone
What rock type is generated at oceanic ridges?
(A) andesite
(C) dunite
(B) basalt
(D) granite
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Example 2:
Which plate boundary does tensional forces dominate
resulting in tectonic plates moving apart?
(A) convergent
(C) transform
(B) divergent
(D) subduction
The rocks of the Avalon Peninsula and North Africa are
similar. Which tectonic process was responsible for the
separation of the Avalon from Africa?
(A) Convergence
(B) elastic rebound
(C) Rifting
(D) subduction
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Your Turn . . .
Take the time and complete the following questions . . .
(Solutions to follow)
Question:
Which sequence correctly illustrates the
evolution of a divergent plate boundary?
(A) 1, 2, 3, 4
(B) 1, 3, 2, 4
(C) 4, 2, 1, 3
(D) 4, 2, 3, 1
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Solutions . . .
Question:
Which sequence correctly illustrates the
evolution of a divergent plate boundary?
(A) 1, 2, 3, 4
(B) 1, 3, 2, 4
(C) 4, 2, 1, 3
(D) 4, 2, 3, 1
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Summary . . .
Overview of Points covered:
At Divergent Boundaries;
 Plates move apart, resulting in upwelling of molten material
from the mantle to create new ocean floor.

Features called Ridges are formed.
 Tensional forces cause the plates to move apart.
 New oceanic crust is being formed, and these boundaries are
referred to as constructive margins.
 Examples include; Mid-Atlantic ridge and Mid-Indian ridge.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador