Lecture11_PlateTectonics.pdf

Plate Tectonics

Lecture 11

What Are the Earth’s Major Geological Hazards?

´ Dynamic processes move matter within the earth and on its surface´ Cause volcanic eruptions,

earthquakes, tsunamis, erosion, and landslides

Plate Tectonics

´ Unified theory: Study the dynamic creation, movement, and destruction processes of plates

´ Plates: lithosphere fragments

´ Plates move in relation to each other at varied rates

´ No major tectonic movements within plates

´ Dynamic actions concentrated along plate boundaries

3 major types of plate boundaries

 Divergent: plates moving apart and new lithosphere produced in mid-oceanic ridge

 Convergent: plates collide, subduction and mountain building

 Transform: two plates slide past one another

The Earth Beneath Your Feet Is Moving

Divergent Margins and the Red Sea

Figure 2.21Seafloor Spreading

Convergent Margins When plates collide

´Continent-Continent

´Ocean-Ocean

´Ocean-Continent

Transform Faults

Transform Margins along the Seafloor

San Andreas fault

Driving Mechanism´ What drives plate motion?

´ Old idea: plates are dragged atop a convectingmantle.

´Convection does occur; not the prime driving mechanism.

´ Modern thinking: two other forces drive plate motions. ´ Ridge-push—elevated MOR pushes lithosphere away.

´ Slab-pull—gravity pulls a subducting plate downward.

Volcanoes Release Molten Rock from the Earth’s Interior

´ Volcano´Magma rising through the

lithosphere reaches the earth’s surface through a crack (fissure)

´Eruption–release of lava, hot ash, and gases into the environment

• ~1500 active volcanoes on Earth

• 400 erupted in the last century

• ~50 eruptions per year

• Most activity concentrated along major plate boundaries

• Impact risks depend on the type of volcano

Introduction

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Viscosity of Magma and Explosiveness

Silicon-oxygen tetrahedron

Hot Spots

´ Volcanic centers with magma source from deep mantle, perhaps near the core-mantle boundary

´ A chain of volcanoes over a stationary hot spot

´ The bend of a seamount chain over a hot spot representing the change of plate motion

What are Earthquakes?

Faulting

´Occurs when rock breaks accompanied by displacement

´Occurs along zones of weakness in the crust, fault zones

´Fault lines´Begin as sudden ruptures, but

can result in large (hundreds of km) faults over millions of year

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Faulting

´Earthquakes´Vibration in Earth resulting from

sudden displacement along a fault

´Earthquake waves´Energy released by earthquakes

moves in several types of seismic waves that originate at the center of fault motion, the origin

´Ground above origin experiences strongest jolt, the epicenter

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Body Waves: P and S waves

´ Body waves´ P or primary waves

´fastest waves´travel through solids, liquids, or gases´compressional wave, material

movement is in the same direction as wave movement

´ S or secondary waves´slower than P waves´travel through solids only´shear waves – move material

perpendicular to wave movement

Surface Waves: R and L waves

´ Surface Waves´ Travel just below or along the ground’s surface´ Slower than body waves; rolling and side-to-side

movement´ Especially damaging to buildings

Surface Waves: R and L waves

´ L-waves (Love waves)´Waves that intersect the land surface´Move the ground back and forth like a

writhing snake

´ R-waves (Rayleigh waves)´ Waves that intersect the land surface´ Particles underground follow a circular

path as the wave passes´ Cause the ground to ripple up and down

Where Do Earthquakes Occur and How Often?~80% of all earthquakes occur along the “Ring of Fire”

Earthquakes Are Geological Rock-and-Roll Events

´ Richter scale ´Insignificant: <4.0´Minor: 4.0–4.9´Damaging: 5.0–5.9´Destructive: 6.0–6.9´Major: 7.0–7.9´Great: >8.0

´ Largest recorded: 9.5 in Chile, 1960

Tsunamis

Earthquakes on the Ocean Floor Can Cause Tsunamis

´ Tsunami´Series of huge waves generated when ocean floor

suddenly rises or drops´Travels several hundred miles per hour

´Slows down as it approaches coastline´ December 2004–Indian Ocean tsunami

´Magnitude 9.15 earthquake´Over 230,000 people lost their lives

´No warning system in place

Earthquakes on the Ocean Floor Can Cause Tsunamis

Where they Occur

(Setake and Atwater, 2007)