Living with Tectonic HazardsPart 1: Introduction to plate tectonics

Natural Hazard vs. Natural

Disaster

What are hazards?• Hazard: A threat (whether natural or human) that has the potential to cause loss of life, injury, property damage, socio-economic disruption or environmental degradation.

Disaster

Disasters occur when hazard events (i.e. the catalyst) meet human vulnerabilities

Overview of topic:• Examples of tectonic hazards:

• Volcanoes, Earthquakes (EQs)• Tsunamis, Landslides

• Why do some places experience these hazards more so than others?

• Why do some places suffer more than others?

• Are disasters inevitable?

Part One:• Understanding Plate Tectonics Theory• How it shapes our world• Resulting landforms and features?

Plate Tectonics 1:Theory & MovementRefer to Handout

What did the Earth look like in the past? What will it look like in the future?

Why is the Earth ‘broken’?

Mirovia

Rodina

Gondwana

Laurasia

What is causing the continents to

move?

CRUSTUPPERMOST MANTLE

ASTHENOSPHERE

LOWERMANTLE

OUTERCORE

INNERCORE

Core Hottest layer: over 5000°C Divided into two layers:

Solid inner core Molten outer core

Made of iron and nickel

Mantle Temperatures btw 500°C nearer

to the crust and 3000°C near the core

3 diff layers: Made up of mostly malleable

rocks of different viscosities. Mainly in a semi-molten state

Crust Thinnest layer: 1 to 70km thick Two types :Continental and

Oceanic

Inside the earth:

Heat

Q: why is the Earth’s crust

broken?

Convection Currents

How long does one cycle of convection currents take?How fast do the plates move every year?

The Earth’s crust is made up of around thirty plates that floats on the mantle.

Tremendous heat found in the Earth’s interior causes rocks in the mantle to melt and become magma.

Heated magma expands and rises and spreads out beneath the earth’s surface, generating convection currents that push plates away from each other.

Magma nearer to the Earth’s surface cools, contracts and sinks, bringing plates towards each other.

This constant rising and sinking of magma thereby result in plate movements over the Earth’s surface!

See: http://education.sdsc.edu/optiputer/flash/convection.htm

At the same time…• Most scientists working today do not believe that

the convection currents in the mantle alone drives plate movements.

• RATHER, they propose that it is the weight of cold, dense plates sinking into the mantle at some plate boundaries that drives plate movements:

Q: why do plates move?Through both convection currents and slab-pull effect

How do the properties and phenomena that occur in the interior

layers of the earth affect us, surface-dwellers?

What happens at the boundary of a plate?

Types of plates• Seven major plates:

• African Plate, Antarctic Plate, Indo-Australian Plate, Eurasian Plate, North American Plate, South American Plate, Pacific Plate.

• Plates converge, diverge or grind past each other.• The landforms that result depend on the TYPE of plate

invlolved:• Oceanic plates vs. Continental plates

Continental vs. Oceanic

Continental vs. Oceanic

Continental vs. Oceanic

(basalt)

3 types of boundariesDivergent Convergent TransformOceanic-oceanic

Oceanic-oceanic

All types of plates

Continental-continental

Continental-continental

Oceanic-continental

Divergent plate movement (p.10) • Constructive boundary is formed when two

plates move away from each other.• New land is create here.

DivergenceOccurs at• Oceanic Oceanic• Continental Continental

Sea Floor Spreading(see video: http://www.youtube.com/watch?v=GyMLlLxbfa4 )

1: Cracks develop along the boundary where two oceanic plates move apart from one another.

2: Magma escapes from the mantle and rises through the cracks to cool and solidify on the surface, forming new lands.

3. Overtime, the new sea floor piles up to form oceanic ridges near the cracks. Older floors are pushed further away. Hence the process sea-floor spreading

Landform: Ocean ridges• A an underwater mountain system that consists of various mountain chains.

• The mid-ocean ridges of the world are all connected and form a single chain that is part of every ocean, making it the longest mountain range in the world!

• The continuous mountain range is over 65,000 km

Can you name some examples?

See: https://www.youtube.com/watch?v=fFVCZkWhPE4

Iceland

Question 1Name an example of an oceanic-oceanic divergence (landforms, plates etc).Explain what occurs at such a boundary using a well-labelled diagram.

Draw out and annotate• Oceanic Oceanic

Convection Currents

Convection Currents

North American Plate

Eurasian Plate

South American Plate

African Plate

What happens at a

continental continental

(bear in mind that these plates are thicker)

Divergent Plate Boundary

+ 50 million years

Divergent Plate Boundary

+ 100 million years

+ 100 million years

The East AfricanRift Valley

• Called the cradle of human life

• Home to the oldest known human fossils

Newly forming divergent plate boundary!

Rift Valley &Block MountainsThe East African Rift Valley was formed via the divergence of the Nubian Plate from the Somalian Plate. As two plates diverged, tensional forces pulled the rock layers apart in opposite directions.

Somalian Plate

Nubian Plate

Divergent Plate Boundary

+ 50 million years

Rift ValleyThe East African Rift Valley was formed via the divergence of the Nubian Plate from the Somalian Plate. As two plates diverged, tensional forces pulled the rock layers apart in opposite directions.

Somalian Plate

Nubian Plate

Rift ValleyOvertime, the stresses deepened to form fault lines that grew an deep into the surface of the Earth. In between lay a central mass of rock bounded by the two faults. This was called a fault block.

Somalian Plate

Nubian Plate

Tensional forcesTensional forces

Rift ValleyOvertime, as the two plates continued to diverge, the fault block in the center gradually slid downwards and sunk, forming the East African Rift Valley.

Somalian Plate

Nubian Plate

Fault BlockSinks

Tensional forcesTensional forces

Block MountainDue to successive faulting, this gave rise to numerous fault blocks along the plate.The blocks that were left standing become known as block mountains (or horsts).

BlockMountain

BlockMountain

BlockMountain

Tensional forces

Tensional forces

Grabens: On a large scale these features are known as Rift Valleys

Future…?

This whole area will split apart and form a separate island!

Future (10 million years)

The Horn of Africa (sitting on the Somali plate) would become a continental island, like Madagascar or New Zealand.

Sample Question:• Using a series of diagrams, illustrate the formation of the East African Rift Valley in Ethopia.

Draw out and annotate• Continental Continental

Lines of weakness

East African Rift Valley

Nubian Plate Somalian Plate

MantleConvectionCurrents

Volcano

ConvectionCurrents

What happens when two plates

collide?

Can you name some examples?

Convection Currents

Where new land is formed, old land is destroyed.

Convergent plate movement• Destructive plate boundary is formed when two

plates move towards each other.• Folding and/or subduction occurs.• Where there is subduction, land is destroyed.

Subduction (write this down)

• Refers to the downward movement of one edge of a plate beneath another.

• Happens due to differing plate densities• The denser plate will always sink• This motion is partly driven by the weight

of cold, dense plates (i.e. gravity)• Giving rise to the slab-pull effect

3 types of boundariesDivergent Convergent TransformOceanic-oceanic

Oceanic-oceanic

All types of plates

Continental-continental

Continental-continental

Oceanic-continental

SubductionOccurs mainly at• Oceanic Oceanic• Oceanic Continental

Subduction• Oceanic Oceanic

The Mariana islands were formed this way, through the convergence of the Pacific Plate into the Philippine Plate.

Draw out and annotate• Oceanic Oceanic

Marina Islands

Some faulting occurs due to compressional

forces.Magma rises and builds

up to form volcanic islands

Marina Trench

Subduction• Oceanic Continental

SubductionOceanic Continental

The Andes from space:

Pacific Plate

South AmericanPlate

Andes Mountains

Questiona) With the aid of a well-labelled

diagram, explain how the Barisan Mountain Range was formed.

Draw out and annotate• Oceanic Continental

As the edge of the continental plate bends and folds, cracks

occur along the crust.Magma rises and builds up to

form volcanic mountain ranges.

What happens when two continental plates meet?

Folding

Folding• Folding refers to the geologic process

where layers of rock bend or fold due to compressional forces along convergent plate boundaries.

Himalayas

Fold Mountains

Fold MountainsAnticline

Syncline

Folding + Faulting• COMPLEX

folding is typical of the Alps and Himalayan Mountains. 

• Here you can see a number of asymetrical folds as well as faulting. 

Question 3:a) With the aid of a well-labelled

diagram, explain how the formation of the Himalayas.

b) Why are there no volcanoes found in the Himalayan region?

Draw out and annotate• Continental Continental

As the plates bend, fold and crack, they

push each other upwards, creating fold

mountains.

Indian PlateEurasian Plate

Himalayan

MantleLittle to no subduction occurs as the plates

are of similar densities.

San Andreas Fault, North America

Where are most of them found? Land or ocean?

Transform Plate Movement (p.16)• Plate boundary is formed when two plates slide

laterally past each other.• No land is destroyed or formed but sudden

release of build-up pressure gives rise to earthquakes.

Landform: Transform Fault• Transform (lateral) plate movements creates fault lines known as tear faults or strike-slip faults.

Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

KeyWords:In summary• Slab-pull

effect• Convection

currents• Folding• Subduction• Divergent• Convergent• Transform• Faulting• Rift valley• Fold

Mountains• Block

mountains• Ridge• Trench

• Plates interact with one another at boundaries in one of three ways: they diverge, converge, or slide past one another.

• Plates are made up of two types of crust – oceanic and continental:• oceanic crust is thinner and denser than continental

crust. • A single plate can have both continental and oceanic

crust.• Gravity (slab-pull effect) and mantle convection

are two driving forces for the movement of plates.• Earthquakes and volcanoes occur primarily along

plate boundaries; the frequency and type of events vary with the type of boundary.

Recap Questions:• How do the properties and phenomena that occur in the interior layers of the earth affect us, surface-dwellers?

• Divergent boundaries are most common… where?

• Mountains are all folded pieces of the earth’s crust. True/false?

• Subduction occurs at all convergent plate boundaries. True/false?