Dynamic Earth

Class 13

21 February 2006

Volcanic Imagination(Chapter 4, continued)

Exploring the Earth’s Interior

How do we know about the Earth’s Interior?

• Meteorites

• Direct observation

• High-pressure experimental petrology

• Earthquake waves (seismology)

Layers of the Earth

• Crust

• Rigid upper mantle (+crust = lithosphere)

• Asthenosphere

• Upper mantle

• Lower mantle

• Outer core

• Inner core

Earth’s internal heat•Original heat

•Subsequent radioactive decay

•Conduction

•Convection

Crust as an Elastic Sheet

Continental ice loads the mantle

Ice causes isostatic subsidence

Melting of ice causes isostatic uplift

Return to isostatic equilibrium

The less dense crust “floats” on the less buoyant, denser mantle

MohorovicicDiscontinuity

(Moho)

Convection in Earth’s Mantle

• Convection happens when – Temperature gradient exists

– Heat is directional

– Conduction operates slowly

– Surface area to depth ratio is low

– Viscosity of material not extremely high

Convection in Earth’s Mantle

• Assumptions– Solid mantle behaves fluidly over time

– Mantle and core do not mix

– Heat generated from within the Earth

Temperature vs. Depth

Convection as a Possible Mechanism for Plate Tectonics

Mantle Tomography

• Uses numerous seismic data

• Uses small changes in speed of seismic waves

• Faster wave motion may correspond to denser or colder regions

• Slower wave motion may correspond to buoyant or warmer regions

Basics of Tomography

Tomography of the Mantle

At 2770 km

Tomography at the Base of the Mantle

Tomography Beneath Active Volcanoes

Tomography Beneath Active Volcanoes

Volcanic Activity on Earth

• Spreading centers (ridges)

• Island arc

• Hotspots

Hotspots

• Areas with volcanic activity NOT explained by plate tectonics

• Mantle beneath may be hot, wet, or chemically different

• Commonly active for long time

Global Hotspots

http://www.hvo.usgs.gov

Flood basalts

Hotspot tracks

Oceanic plateaus

Karoo / Etendeka Flood Basalts

Columbia River Flood Basalts

Linear Volcanic Chains

Hawaiian Islands - Emperor Seamounts

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The trail of the Hawaiian Hot Spot goes all the way to the far northwest Pacific

In addition to the Hawaiian Hot Spot track, there are several others in the Pacific

Age Progressions Along Volcano Chains

Hawaiian Islands

Oldest

Youngest

Hotspot Observations

• Volcanic activity, NOT explained by plate tectonics

• Active for long time

• Age varies (youngest in opposite direction of plate motion)

Hot spots are regions in the Asthenosphere that are hotterthan their surroundings

Molten magma rises to the surface to form volcanoes, similar to what happens at

Spreading Centers

Hot spot volcanoes are relatively small, isolated

features.

Hot spots are believed to be fixedrelative to the mantle.

BUT – this is controversial!

When a plate moves over afixed Hot Spot,

a linear chain of volcanoes is formed.

Oceanic crust

Lithosphericmantle

6-9 km

10-60 km

H o t

C o ld

200-400 km

Oceanic flood basalts

Hotspot Origins - Mantle Plumes

Mantle plumes and eruption sizes

Convection in the Mantle

Convection and Mantle Plumes

Why Linear Chains of Volcanoes?

Model of Mantle Plumes

A mantle plume rising beneath a slow-moving plate or continent will “puddle” beneath the lithosphere

Model of Mantle Plumes

When eruptions begin, they are voluminous, causing oceanic plateaus

and flood basalt provinces

Model of Mantle Plumes

Model of Mantle Plume

Mantle Plume Shape Unknown

Model of Mantle Plumes

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Models of Mantle Plumes

Wolfe et al., Nature, [1997]

At 2770 km

Tomography at the Base of the Tomography at the Base of the MantleMantle

Instability Causes Mantle Plumes

How Can Plate Tectonics and Mantle Plumes Work Together?

Why Are Hotspots Important

• Associated with Large Volcanic Eruptions– May inject gas and particles into air– May re-landscape large areas– May decrease habitable areas– May make life difficult for some plants and

animals (and cause mass extinction)

Volcanic Eruptions and the Atmosphere

Life on Earth is Difficult!

• Earthquakes

• Floods

• Climate changes (draught, ice ages)

• Other weather hazards (tornadoes, cyclones)

• Volcanic eruptions

• Meteor / asteroid impacts

Meteor / Asteroid Impacts

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Meteor / Asteroid Impacts

Environmental Catastrophes and Hotspots

End Cretaceous (65 Ma)

Eruption of Deccan Traps (Reunion hotspot)

Extinction Percentages and Hotspots

Extinction is Forever

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Dinosaurs - ~65 Ma

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Giant Ground Sloth ~10 Ka

Trilobites~ 300 Ma

ThursdayVideo:

Death of the Dinosaurs