Transcript
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?
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