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 There is a huge debate right now about hotspots, about how they form and what they are. The word hotspot is a kind of generic term for a region of volcanic activity of unknown cause, meaning volcanic activity not typical of mid-ocean ridge type rifting, nor of islands arcs or of volcanism associated with tectonic boundaries. This map shows the global distribution of hotspots. Many hotspots are located near mid-ocean ridges, but some are isolated and in the middle of tectonic plates. Hawaii is one of these hotspots unassociated with a plate tectonic boundary. I won’t go into the debate much in this talk but rather will talk about the most common explanation for 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) This is a diagram from Courtillot and Renne showing the excellent fit between the timing of environmental crises, recognized as period changes in the geological time scale and the timing of LIP emplacement. The correlation is very good and it’s difficult to ignore that there might be some relationship; however, we don’t know if the relationship is causal or coincidental - whether the LIP emplacement is the actual reason for the environmental crises or whether they both occur as a result of some larger force. End Cretaceous (65 Ma)

Extinction Percentages and Hotspots This diagram shows the correlation between extinction percentages at the genus level, in the filled gray pattern, with eruption of continental flood basalts superimposed in the dark gray bars. The timing of oceanic plateau are also noted in text only. There does seem to be some correlation between extinction events and flood basalt eruptions, at least in the case of the largest extinction events at the end of the Permian and the Siberian Traps, at the end of the Triassic, and the central Atlantic Magmatic province, and at the Cretaceous tertiary boundary, and the Deccan Traps.

Extinction is Forever Dinosaurs - ~65 Ma Trilobites ~ 300 Ma Giant Ground Sloth ~10 Ka Trilobites ~ 300 Ma

Thursday Video: Death of the Dinosaurs