Dynamic Earth Class 13 21 February 2006 Volcanic
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 Mohorovicic Discontinuity (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
Tomography at the Base of the Mantle At 2770 km
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
Hotspot tracks Flood basalts Oceanic plateaus
Karoo / Etendeka Flood Basalts
Columbia River Flood Basalts
Linear Volcanic Chains
Hawaiian Islands - Emperor Seamounts
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 hotter than 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 fixed relative to the mantle. BUT – this is controversial!
When a plate moves over a fixed Hot Spot, a linear chain of volcanoes is formed.
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
Models of Mantle Plumes Wolfe et al. , Nature, [1997]
Tomography at the Base of the Mantle At 2770 km
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
Meteor / Asteroid Impacts
Environmental Catastrophes and Hotspots Eruption of Deccan Traps (Reunion hotspot) End Cretaceous (65 Ma)
Extinction Percentages and Hotspots
Extinction is Forever Dinosaurs - ~65 Ma Trilobites ~ 300 Ma Giant Ground Sloth ~10 Ka
Thursday Video: Death of the Dinosaurs
- Slides: 61