CHAPTER 4 CHAPTER 4

Plate MovementsPlate Movements

Chapter 3: Plate TectonicsChapter 3: Plate Tectonics

Earth: Our Home Full Geography

You will learn:

- to explain the formation, characteristics and distribution of landforms

- to describe types of volcanoes

- to explain the formation, characteristics and distribution of earthquakes

Plate Movements

• Compression Process of forcing something into a smaller

structure by reducing it in volume by pressing it together.

• Tension A pulling force, tending to stretch, to cause

an extension of a body or to restore the shape of an extended elastic object.

Landforms

• Fold Mountains

• Block Mountains

• Rift Valleys

• Volcanoes

Fold Mountains

Process:

• Formed via compressional forces.

• 2 tectonic plates collide.

• Rock layers buckle and fold.

• Result in the uplifting and compression of Earth’s crust.

Fold Mountains• Parts of a fold:

Hinge / AxisLimbs

Fold Mountains

Characteristics:

• AgeYoung – > 100 million years, 8000 mOld – > 270-400 million years, lower in height

than young fold mountains.

Why?

- due to weathering and erosion

Fold Mountains

Characteristics• Types of folds:

AnticlineSynclineSymmetricalAsymmetricalOverfoldRecumbentOverthrust

Anticline Fold

• Layers of rocks pushed upwards to form an inverted-V shape.

Syncline Fold

• Layers of rocks pushed downwards to ‘sink’ and form a V shape.

Symmetrical Fold

• Layers of rocks are pushed towards each other due to equal strength of compressional forces on both sides.

Types of folds

• Increasing strength of compressional forces applied unevenly on both sides eventually results in the following:

- Symmetrical fold

- Asymmetrical fold

- Overfold

- Recumbent fold

- Overthrust fold

Where are Fold Mountains Found?

Block Mountains

Process:

• 3 movements – TensionalCompressional Sliding

A highland formed when the crust between

2 normal faults is raised.

Tensional and Compressional Forces

When tensional forces pull two blocks apart, a normal fault is formed.

When compressional forces push two blocks together, a reverse fault is formed.

When either tensional or compressional forces cause two blocks to slide past each other, a tear fault is formed.

Block Mountains

Process:

A highland formed when the crust on both sides of 2 normal faults sink due to tensional forces.

Tensional forces from opposite ends of the crust pull the rock within the area away from each other.

The tensional forces cause normal faults or cracks to appear.

When the crusts on either side of the normal faults move apart, they sink and leave the central block standing higher than the rest, thus forming a block mountain.

Formation of block mountains(due to tensional forces)

Block Mountains

Process:

A highland formed when the crust between 2 reverse faults is uplifted due to compressional forces.

Compressional forces from opposite ends of the crust push the rock within the area towards each other.

The compressional forces cause reverse faults or cracks to appear.

When compressional forces on either side of the reverse faults push two crusts together, the central block is raised to form a block mountain.

Formation of block mountains(due to compressional forces)

Rift Valley

• A lowland formed by the sinking of crust between normal faults as a resulting of tensional forces.

Tensional forces from opposite ends of the crust pull the rocks within the area away from each other.

The tensional forced cause normal faults or cracks to appear.

As the crusts on either side of the normal faults pull apart, the crust in between sinks to form a rift valley.

Rift valleys

Where are Block Mountains and Rift Valleys found?

Volcanoes

• A conical or dome-shaped landform formed by the accumulation of lava emitted onto the Earth’s surface.

Volcanoes

Process:• Vulcanicity

Plate movements along boundaries and high temperatures increase pressure in the mantle.

Magma forces its way upwards through vents to the Earth’s crust.

Magma pours onto the Earth’s surface as lava.Lava cools and solidifies around the vent.Lava accumulation results in the formation of a

volcano.

Nature & Composition of Lava

Basic Lava• 1,100 – 1,200 C• Highly fluid with low viscosity• Solidifies less quickly

Acid lava• 800 – 1,000C• Thick, sticky and resistant to flow; high viscosity• Solidifies more quickly

Types of Volcanoes

• Shield volcano

• Acid volcano

• Composite volcano

Shield Volcano

• A gentle sloping volcano that is flat near the top.

• Usually formed from basic lava, low in viscosity.

• E.g. Mauna Loa, Hawaii

Acid Volcano

• A volcano formed from acid lava.• Steep slopes are a result of its viscous

lava which solidifies near the vent.• Typified by violent eruptions:

Quick solidification blocks vents and pipes.Pressure builds up in the magma chamber.Violent eruption ensues.

Composite Volcano

• A highly common volcano which consists of alternating layers of ash and cinder, and lava.

• Process:Viscous lava forces its way up the vent.Magma is pushed up via other pipes and cracks in the

Earth’s surface.Quick solidification of lava results in the escape of

magma and gas.Results in mounting pressure in magma chamber.Violent eruption ensues.

• Appearance:Steeper at the top, with a gentler base.

Why? Violent eruption of ash and cinder is followed by lava. Wind sorting of volcanic material by size accounts for

appearance. Smaller particles are blown further away from the

crater. E.g.Mt.Vesuvius, Italy

Composite Volcano

Where are Volcanoes Found?

Earthquakes

• A vibration or tremor of the Earth’s surface caused by the sudden release of energy stored in the crust.

1. As tectonic plates move, they exert

friction on one another, causing

pressure to build at their boundaries.

2. Pressure gradually builds up in the plates and

energy is stored up in the Earth’s crust.

3. When the rocks can no longer contain the pressure, energy is released.

This energy radiates out in shock waves through the crust and onto the Earth’s

surface. Large faults will also occur as the rocks break up and move in a series of

sudden jerks.

Tectonic plates exert friction on one another

Pressure builds up as the plates push against one another

Shock waves radiate

Pressure is released in an earthquake

Energy of earthquake dependent on:• Focus – point of release of energy within the

Earth’s crust.• Epicentre – point on the Earth’s surface that lies

directly above the focus.• Energy released as shockwaves.

Earthquakes

Shock waves

Focus (point of origin)

Epicentre

Fault

What determines the strength of an earthquake?

• Shallow focus leads to a stronger earthquake as vibrations are felt more strongly.

• Shock waves of energy are felt more strongly at the epicentre.

Earthquakes

Where are Earthquakes Found?

Just a thought …

Why are shockwaves felt more strongly

near the epicentre and not the focus?