Tectonic Hazards Revision

Layers of the Earth

Inner core, outer core, mantle, crust, lithosphere

Lithosphere

The rigid crust and upper mantle, broken into tectonic plates

Asthenosphere

The semi-molten mantle below the lithosphere where convection occurs

Density of oceanic crust

~3.0 g/cm³

Density of continental crust

~2.7 g/cm³

Main tectonic plates

Pacific, North American, South American, African, Eurasian, Antarctic, Indo-Australian

Plate boundary

The edge of tectonic plates where most earthquakes and volcanoes occur

Movement of tectonic plates

Via convection currents in the mantle, causing divergence, convergence, or lateral movement

Types of plate boundary

Divergent/constructive, convergent/destructive/collision, conservative/transform

Constructive/divergent boundary example

Eurasian Plate and North American Plate at the Mid-Atlantic Ridge

Destructive/convergent boundary example

Nazca Plate and South American Plate at the Andes subduction zone

Collision boundary example

Indian Plate and Eurasian Plate at the Himalayas

Conservative/transform boundary example

North American Plate and Pacific Plate at the San Andreas Fault

Location of most earthquakes

At plate boundaries, along fault lines

Earthquake

A sudden shaking of the Earth caused by stress release at a fault

Focus

The point underground where the earthquake starts

Epicentre

The point on the surface above the focus

Seismic waves

Energy waves that travel through the Earth after an earthquake

Types of seismic waves

P-waves (fast, compressional), S-waves (slower, shear), Love-Waves, Rayleigh-Waves

Love waves

Seismic surface waves causing horizontal ground movement; very destructive

Rayleigh waves

Seismic surface waves causing rolling, elliptical motion, damaging to buildings

How do earthquakes happen?

Plates stick, pressure builds, sudden slip, seismic waves radiate

Scale measuring earthquake magnitude

Moment Magnitude Scale (or Richter Scale)

Scale measuring earthquake intensity

Mercalli Scale

Secondary effect of an earthquake

Tsunamis, landslides, building collapse

Tsunami

A giant sea wave triggered by underwater earthquakes

Magma

Molten rock beneath the Earth's surface

Lava

Molten rock on the surface

Main features of a volcano

Crater, vent, magma chamber, secondary cone (parasitic cone)

Shield volcano

A wide, gently sloping volcano with basaltic lava flows

Example of a shield volcano

Mauna Loa, Hawaii

Composite/strato volcano

Tall, steep-sided volcano made of layers of ash and lava

Example of a composite volcano

Mount Fuji, Japan

Cinder cone volcano

Small, steep volcano built from cinders and tephra

Example of a cinder cone

Paricutin, Mexico

Classification of volcanoes by activity

Active, dormant, extinct

Volcanic hazards

Lava flows, pyroclastic flows, ash falls

Lahar

A mudflow of volcanic ash and water

Tephra

Material ejected from a volcano during an eruption

Volcanic hazard speed impact

Faster hazards cause more destruction and less time to evacuate

Importance of eruption size/frequency

Larger/frequent eruptions increase risk and damage

Example of toxic gas from volcanoes

Sulphur dioxide (SO₂)

Pyroclastic flow

A fast-moving, hot gas and ash cloud

Difference between lahars and pyroclastic flows

Lahars are mud/ash, slower; pyroclastic flows are hot, fast, gas

Positive effect of volcanic eruptions

Fertile soils or tourism/research opportunities

Secondary cone

A smaller vent/cone forming on the side of the main volcano

Tohoku earthquake date

11th March 2011

Tohoku earthquake magnitude

9.0 (Moment Magnitude Scale)

Plates causing Tohoku earthquake

Pacific Plate subducting under North American Plate

Depth of Tohoku earthquake focus

30 km

Main primary hazard of Tohoku earthquake

Tsunami

Height of tsunami waves

Up to 40 metres

Secondary effect of Tohoku earthquake

Fukushima nuclear meltdown

Number of deaths in Tohoku earthquake

Around 20,000

Economic cost of Tohoku earthquake

$235 billion

Long-term response to Tohoku earthquake

Higher sea walls, tsunami warning systems

Location of Montserrat

Caribbean island

Start date of Montserrat eruption

18th July 1995

Type of Montserrat volcano

Composite/Stratovolcano

Plates causing Montserrat eruption

Atlantic Plate subducting beneath Caribbean Plate

Main hazard of Montserrat eruption

Pyroclastic flows

Number of deaths in Montserrat eruption

19 people

Capital affected by Montserrat eruption

Plymouth buried

Percentage of Montserrat uninhabitable

Two-thirds

Population migration from Montserrat

Many to the UK

Monitoring installed for Montserrat

Montserrat Volcano Observatory (MVO)

Rift valley

Lowland formed at divergent boundaries

Mid-ocean ridge

Underwater mountain range at constructive margins

Ocean trench

Deep depression where a plate is subducted

Fold mountains

Mountains formed at continental collision zones

Accretionary wedge

Sediments scraped off subducting plate at destructive boundary

Benioff zone

Zone of earthquake focus down a subducting plate

Difference between continental & oceanic crust

Oceanic: dense, basalt; Continental: less dense, granite

Orogeny

Mountain-building process

Slab pull

Force from dense sinking plate pulling rest of plate

Ridge push

Force from elevated mid-ocean ridge pushing plates apart

Conservative boundary

Plates slide past each other (e.g., San Andreas)

Collision boundary

Plates crash, forming mountains (e.g., Himalayas)

Destructive boundary

Oceanic subducts under continental (e.g., Andes)

Constructive boundary

Plates move apart, magma rises (e.g., Mid-Atlantic Ridge)

Primary response

Immediate actions: rescue, medical aid, food, shelter

Secondary response

Longer-term: rebuilding, planning, infrastructure repair

Technology for earthquakes

Seismometers, GPS

Technology for volcanoes

Gas sensors, satellite imagery

Why do volcanoes erupt at destructive boundaries?

Subduction melts mantle, magma rises

Why do earthquakes occur at conservative boundaries?

Plates stuck, pressure builds, sudden slip

Pyroclastic flow speed

Up to 600 km/h

Positive human reason for living near a volcano

Fertile soil, geothermal energy, tourism

Importance of magnitude in disaster planning

Determines scale of damage, response required