Plates and Plate Boundaries: Understanding Earth's Tectonic Processes, Lecture notes of Earth Sciences

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Chapter 9: Plates and Plate Boundaries

Fig. 9.

OBJECTIVES

• Identify the physical and chemical divisions in Earth’s outer layers.
• Understand that the lithospheric plates are buoyant and that this buoyancy controls the relationship between crustal elevation, crustal thickness, and crustal density.
• Compare and contrast the three types of plate boundaries and describe the three main ways boundaries interact: spreading apart, coming together, and sliding past one another.
• Describe the processes that occur at divergent boundaries and explain how new ocean floor is created.

• Plate motion creates and destroy ocean basins, forms mountain

belts, and moves continents.

• Plates interact along plate boundaries: splitting apart, colliding,

or sliding past each other.

Plates and Plate Boundaries: An Overview

Fig. 9.

Plates on Earth’s Surface

• Earth’s outer layers can be subdivided in terms of - Chemical properties (crust, mantle, core) - Physical properties (lithosphere, asthenosphere)
• Lithosphere
  • Rigid, outermost layer
  • Makes up plates
  • Crust plus uppermost mantle
• Asthenosphere
  • Weak, ductile layer
  • Moves slowly (convects)
  • Lower part of the upper mantle Fig. 9.

• The base of the crust has a shape that mirrors the overlying

shape of Earth’s surface.

• Buoyancy: lithosphere “floats” on the asthenosphere below.
  • High crust areas must be supported by deep crustal “roots”

that project into the mantle.

• Continental crust is lighter and so rides higher on Earth’s

surface; denser oceanic crust rides lower.

Plates and Isostasy

Fig. 9.

• Isostasy: balance reached by the

lithosphere as it floats upon the

asthenosphere.

• Depends on lithosphere volume

and density

• Balance = isostatic equilibrium
• Decrease in thickness or density

causes isostatic rebound.

Plates and Isostasy

Floating icebergs demonstrate the concept of isostasy. These two icebergs float in such a way that the same proportion of each iceberg is above the water line. Fig. 9.

Plates and Isostasy

• Erosion in mountainous regions causes uplift of the mountain root (isostatic rebound).
• Deposition of sediment causes subsidence of the crust and rest of the lithosphere. Fig. 9.

• Three Types of Motion: extension , compression , and shear
• Divergent Plate Boundaries
  • Plates move apart (extension)
  • New lithosphere is formed ( constructive plate margins)
• Convergent Plate Boundaries
  • Plates collide
  • Lithosphere can be destroyed ( destructive plate margins)
• Transform Plate Boundaries
  • Plates slide past each other
  • link other plate boundaries ( conservative plate margins)

Plate Boundaries

Fig. 9.

Mid-Ocean Ridges and Ocean Opening

Fig. 9.

Mid-Ocean Ridges and Ocean Opening

• Mid-ocean ridges are

produced by extension

and separation.

• Are found in all the world’s

major oceans.

• Form an interconnected system 65,000 kilometers long.
• With seafloor spreading, the crust moves away from mid-

ocean ridges to form the flat, abyssal plain of the deep

ocean.

Ocean-Ocean Subduction

Ocean-Continent Subduction

Fig. 9.

Continent Convergence to Continental Collision

Fig. 9.

Transform boundary in oceanic lithosphere between mid-ocean ridge segments

Transform boundary in continental lithosphere between mid-ocean ridge segments

Transform

Boundaries

Figs. 9.26, 9.

• Hotspots are small, isolated areas of higher-than-average

heat associated with volcanoes.

• Found on continents and in oceans
• Most located far from plate boundaries
• May be a result of mantle plumes , areas of upwelling of heat.
• Provide evidence of the movement of tectonic plates
• May play a role in continental rifting

Hotspots

Fig. 9.