Major features which a student of geology must know to understanding the formations of landscapes related to tectonics.

1. FEATURES RELATED
TO PLATE
BOUNDARIES
Mustafa Ahmed Wazeer
Department of Geology, University of Karachi.

2. TECTONICS
 Tectonics is the study of the origin and
arrangement of the broad structural features of
Earth’s surface, including not only folds and
faults but also mountain belts, continents, and
earth-quake belts.
 Plate tectonics has come to dominate geologic
thought today because it can explain so many
features.
 The basic idea of plate tectonics is that Earth’s
surface is divided into a few large, thick plates
that move slowly and change in size.

3.  Intense geologic activity occurs at plate
boundaries where plates move away from one
another, past one another, or toward one
another.
 The eight large lithosphere plates, plus a few
dozen smaller plates, make up the outer shell
of Earth (the crust and upper part of the
mantle).
 The concept of plate tectonics was born in the
late 1960s by combining two preexisting
ideas—continental drift and seafloor

4.  Continental drift is the idea that continents
move freely over Earth’s surface, changing
their positions relative to one another.
 Seafloor spreading is a hypothesis that the
sea floor forms at the crest of the mid-oceanic
ridge, then
 moves horizontally away from the ridge crest
toward an oceanic trench.

5. THE MAJOR PLATES OF THE
WORLD

6. WHAT ARE PLATES ?
 A plate is a large, mobile slab of rock that is
part of Earth’s surface.
 The plates are composed of the relatively rigid
outer shell of Earth called the lithosphere.
 The surface of a plate may be made up
entirely of sea floor (as is the Nazca plate), or
it may be made up of both continental and
oceanic rock (as is the North American plate).
 Some of the smaller plates are entirely
continental, but all the large plates contain
some sea floor.

8. HOW DO PLATES MOVE ?
 Below the rigid lithosphere is the
asthenosphere, a zone of low seismic-wave
velocity that behaves in a ductile manner
because of increased temperature and
pressure.
 The ductile asthenosphere acts as a
lubricating layer under the lithosphere,
allowing the plates to move.

10. HOW DO WE KNOW THAT
PLATES MOVES ?
 In the 1960s, two critical tests were made of the
idea of a moving sea floor.
 These tests involved marine magnetic anomalies
and the seismicity of fracture zones.
 These two, successful tests convinced most
geologists that plates do indeed move.
 Alternating positive and negative anomalies form
a stripe like pattern parallel to the ridge crest
confirm the plate movement.
 Earthquakes, occur only on the section of the
fracture zone, the transform fault.

11. TYPES OF PLATE
BOUNDARIES
 Plate boundaries are of three general types,
based on whether the plates move away from
each other, move toward each other, or move
past each other.
 A divergent plate boundary is a boundary
between plates that are moving apart.
 A convergent plate boundary lies between
plates that are moving toward each other.
 A transform plate boundary is one at which
two plates move horizontally past each other.

12. FEATURES
RELATED TO
TRANSFORM
PLATE
BOUNDARIES

13.  At transform boundaries, where one plate slides
horizontally past another plate, the plate motion
can occur on a single fault or on a group of
parallel faults.
 Transform boundaries are marked by shallow-
focus earthquakes in a narrow zone for a single
fault or in a broad zone for a group of parallel
faults.
 First-motion studies of the quakes indicate strike-
slip movement parallel to the faults.
 The name transform fault comes from the fact that
the displacement along the fault abruptly ends or
transforms into another kind of displacement.

14.  The most common type of transform fault occurs
along fracture zones and connects two divergent plate
boundaries at the crest of the mid-oceanic ridge
(4.18B).
 When two oceanic plates begin to diverge, the
boundary may be curved on a sphere.
 Mechanical constraints prevent divergence along a
curved boundary, so the original curves readjust into a
series of right-angle bends.
 The ridge crests align perpendicular to the spreading
direction, and the transform faults align parallel to the
spreading direction.
 The boundary will then readjust into a series of
transform faults parallel to the spreading direction.

16. THE SAN ANDREAS FAULT
 The San Andreas fault is a 1,100-kilometer-long
rupture marking the border between the Pacific
and North American plates in California.
 But only about one-third of the approximately
2,000 kilometers of total slip between the two
places, the biggest plates on Earth, has taken
place along the fault during its 25 to 30-million-
year history.
 Its motion is right-lateral strike-slip (horizontal).
 The slip rate along the fault ranges from 20 to 35
mm (0.79 to 1.38 in)/yr.

19. FEATURES
RELATED TO
DIVERGENT
PLATE
BOUNDARIES

20.  Divergent plate boundaries, where plates move
away from each other, can occur in the middle of
the ocean or in the middle of a continent.
 The result of divergent plate boundaries is to
create, or open, new ocean basins.
 Oceanic divergent boundaries create what are
known as mid-ocean ridges, such as the Mid-
Atlantic Ridge.
 Rising convection currents in the athenosphere
press upward on the thin oceanic plates, causes
the plates to bulge upwards.

21.  As these currents reach the plates, they also
spread outward, pulling the plates apart.
 As the plates are stretched thin by the upward
and outward forces, they fracture.
 These fractures are quickly filled by solidifying
magma and the process begins again.
 This process produces sub-surface mountain
ranges, fissure eruptions, shallow earthquakes,
new seafloor and a widening of the ocean basin.
 This divergent process is characterized by slow
and steady expansion approximately 2.5cm a
year.

23. MID-OCEAN RIDGES
 A mid-ocean ridge (MOR) is an underwater
mountain system formed by divergence of
plates.
 It consists of various mountains linked in
chains, typically having a valley known as a rift
running along its spine.
 This type of oceanic mountain ridge is
characteristic of 'oceanic spreading center',
which is responsible for seafloor spreading.
 Mid-ocean ridges are geologically active, with
continuing volcanism and seismicity.

24.  Two of the most well-known mid-ocean ridges are
the Mid-Atlantic Ridge and the East Pacific Rise.
 As you might guess, most of the Mid-Atlantic
Ridge is in the Atlantic, and most of the East
Pacific Rise is in the Pacific.
 The Mid-Atlantic Ridge is spreading one to two
inches a year, and the East Pacific Rise is
spreading at two to six inches per year.
 The mid-Atlantic Ridge begins in the Arctic Ocean
extends across Iceland all the way to the bottom
of the Atlantic near Africa before dividing into two.

26. CONTINENTAL DIVERGENT
ZONES
 Continental plates are much thicker than oceanic plates.
 The force produced by upward currents in these divergent
boundaries is not strong enough to create a single break
through the entire plate.
 Instead, the plate bulges upward as it is stretched and fault
lines develop on each side of the crest.
 When these faults fracture, intense earthquakes are
produced and the center block drops, forming a rift-like
structure.
 This continental divergent process is much choppier than the
smoother oceanic divergence, and is characterized by more
sudden, irregular and intense shifts in the rift structure.
 The East Africa Rift Valley is a classic example of this type
of plate boundary.

27. EAST AFRICA RIFT VALLEY

30. FEATURES
RELATED TO
CONVERGENT
PLATE
BOUNDARIES

31.  At convergent plate boundaries, two plates move
toward each other (often obliquely).
 The character of the boundary depends partly on the
type of plates that converge.
 A plate capped by oceanic crust can move toward
another plate capped by oceanic crust, in which case
one plate dives (subducts) under the other.
 If an oceanic plate converges with a plate capped by a
continent, the dense oceanic plate subducts under the
continental plate.
 If the two approaching plates are both carrying
continents, the continents collide and crumple, but
neither is subducted.

32. OCEAN-OCEAN CONVERGENCE
 Where two plates capped by sea floor
converge, one plate subducts under the other
(the Pacific plate sliding under the western
Aleutian Islands is an example).
 Ocean-ocean convergence forms a
subduction zones, trench, and volcanic
island arc.

33. SUBDUCTION ZONES
 Subduction is a geological process that takes
place at convergent boundaries of tectonic plates
where one plate moves under another and is
forced or sinks due to gravity into the mantle.
Regions where this process occurs are known as
subduction zones.
 Subduction zones are sites that usually have a
high rate of volcanism and earthquakes.
 Rates of subduction are typically in centimeters
per year, with the average rate of convergence
being approximately two to eight centimeters per
year along most plate boundaries.

34. MAKRAN SUBDUCTION
ZONE
 The 1000-km long Makran subduction zone
extends from southern Pakistan to Iran, with
the Himalayas to the east and the Zagros
orogenic belt to the west (Fig.1a).
 This subduction zone is bounded by the left-
lateral Ornach Nal Fault to the east and the
right-lateral Minab–Zendan Fault to the west.
 Along this plate boundary, the Arabia and
Ormara Plates subduct northward underneath
the Eurasia Plate with a 3 to 4cm/yr
convergence.

36.  Map of the Makran subduction zone, where
the Arabia and Ormara plates subduct under
Eurasia.

37. TRENCHES
 Oceanic trenches are topographic depressions of the sea floor,
relatively narrow in width, but very long.
 These oceanographic features are the deepest parts of the ocean
floor.
 Oceanic trenches are a distinctive morphological feature of
convergent plate boundaries, along which lithospheric plates move
towards each other at rates that vary from a few millimeters to over
ten centimeters per year.
 A trench marks the position at which the flexed, subducting slab
begins to descend beneath another lithospheric slab.
 Trenches are generally parallel to a volcanic island arc, and about
200 km (120 mi) from a volcanic arc.
 Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below
the level of the surrounding oceanic floor.
 The greatest ocean depth measured is in the Challenger Deep of
the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea
level.

38. THE MARIANA TRENCH
 The Mariana Trench or Marianas Trench located
in the western Pacific Ocean approximately 200
kilometers (124 mi) east of the Mariana Islands,
and is the deepest known point in the world's
oceans.
 The Mariana trench contains the deepest part of
the world's oceans, and runs along an oceanic-
oceanic convergent boundary. It is the result of
the oceanic Pacific plate subducting beneath the
oceanic Mariana plate.
 It is a crescent-shaped trough in the Earth's crust
averaging about 2,550 km (1,580 mi) long and 69
km (43 mi) wide.

39. VOLCANIC ISLAND ARC
 An island arc is a curved line of volcanoes that
form a string of islands parallel to the oceanic
trench (figure 4.25).
 The distance between the island arc and the
trench can vary, depending upon where the
subducting plate reaches the 100-kilometer depth.
 If the subduction angle is steep, the plate reaches
this magma-generating depth at a location close
to the trench, so the horizontal distance between
the arc and trench is short.
 If the subduction angle is gentle, the arc-trench
distance is greater.

41. OCEAN-CONTINENT
CONVERGENCE
 If an oceanic plate converges with a plate
capped by a continent, the dense oceanic
plate subducts under the continental plate.
 Ocean-continent convergence forms a trench,
a magmatic arc, and a young mountain belt
on the edge of the continent.
 Examples of this type of boundary are the
subduction of the Nazca plate under western
South America and the Juan de Fuca plate
under North America.

42. MAGMATIC ARC
 The magma that is created by ocean-continent
convergence forms a magmatic arc, a broad term
used both for island arcs at sea and for belts of
igneous activity on the edges of continents.
 The surface expression of a magmatic arc is
either a line of andesitic islands (such as the
Aleutian Islands) or a line of andesitic continental
volcanoes (such as the Cascade volcanoes of the
Pacific Northwest).
 Beneath the volcanoes are large plutons in
thickened crust.

43. YOUNG MOUNTAIN BELT
 The hot magma rising from the subduction
zone thickens the continental crust and makes
it weaker and more mobile than cold crust.
 Crustal thickening causes uplift, so a young
mountain belt forms here as the thickened
crust rises iso-statically.
 Another reason for the growth of the mountain
belt is the stacking up of thrust sheets on the
continental side of the magmatic arc.

45. CONTINENT-CONTINENT
CONVERGENCE
 If the two approaching plates are both carrying
continents, the continents collide and crumple,
but neither is subducted.
 The collision of two continents forms a young
mountain belt in the interior of a new, larger
continent.
 The most famous example of continent-
continent collision is the collision of India with
Asia (formation of Himalayas).

46. Mountain Belt (Himalayas)
 The Himalayas , or Himalaya form a mountain
range in Asia , separating the plains of the Indian
subcontinent from the Tibetan Plateau.
 The Himalayan range has many of the Earth's
highest peaks, including the highest, Mount
Everest .
 The Himalayas include over fifty mountains
exceeding 7,200 meters (23,600 ft) in elevation,
including ten of the fourteen 8,000-metre peaks.
 By contrast, the highest peak outside Asia (
Aconcagua, in the Andes) is 6,961 meters (22,838
ft) tall.

47.  The Himalayas are spread across five
countries : Nepal , India , Bhutan , China and
Pakistan .
 The 6,000-kilometre-plus journey of the India
landmass (Indian Plate) before its collision
with Asia (Eurasian Plate) about 40 to 50
million years ago.
 Its formation is a result of a continental
collision or orogeny along the convergent
boundary between the Indo-Australian Plate
and the Eurasian Plate.
 The Himalayas rising by about 5 mm per year,

49. CONCLUSION
 Now u can know about the importance of plate
boundaries that it controls many geologic
features.
 Plate tectonics has come to dominate geologic
thought today because it can explain so many
features.

50. THANK YOU