Earth Sciences 240A - Lecture 25 - Flood Basalts I

Introduction

Eruption global effects

‘Dust’ to atmosphere (colder, darker)

Aerosols to atmosphere (colder, darker)

CO2 to atmosphere (warmer greenhouse)

Other gases (?) (maybe poisonous?)

Scale of effects

Small: Mt. Pinatubo (150 km3)

1 year cooler climate (no mass extinctions)

Large: Yellowstone (2500 km3)

Vast atmospheric effects (no mass extinctions)

Flood basalts?

Flood Basalts

Fed by ‘superplumes’

Deposits

Flood basalts: Continental/oceanic flood/plateau basalts

Large igneous provinces

‘Coincidental’ deposits?

Deccan Traps (66 mya)

Siberian Traps (245 mya)

Large Igneous Provinces

More primitive composition than MORB

Lower mantle component

Mantle plume only possible source

Continental deposits: common features

Only basalt

Mainly subaerial

Very short eruption time span

Attempted Correlations

Continental Rifting

but not: Columbia River Plateau

Deccan Traps

Siberian Traps (maybe)

Mass Extinction Events

~OK for Deccan Traps, Siberian Traps

Laki, Iceland (1783-84)

The only historical F.B. event

14.7 km3 basalt

Abnormally high (?) content gases

F, SO2 aerosols

Lowered global T by 1oC for 1 year

Was Laki typical of Flood Basalts?

Columbia River Plateau

~175,000 km3 deposit

~ 16 mya

Early deposit from the Yellowstone Plume

Earliest: Imnaha  and Grand Ronde Basalt

Very similar to Hawaiian basalts

Second: Picture Gorge Basalt

Less volume; strong lithosphere component

Third: Saddle Mountain Basalt

Smallest, compositionally very complex

The Roza flow

Traveled 300 km from vent

Flow of 30 m high, 100 km wide, traveling at 5 km/h for 1 week

The Pomona flow

Traveled 600 km from vent

Longest flow on Earth

Lakes of magma created that took years to tens of years to cool

A weird hypothesis?

Large asteroid impacted SE Oregon

Lithosphere fractured

Reduced P produced huge partial melting of asthenosphere

Flood basalts produced

Crater covered

Problem

Impact cannot create a plume

Plume evidence obvious

CRB, SRB, Yellowstone

Comparative eruption volumes

Subduction volcanoes

Mount St. Helens: 1 km3

Mount Pinatubo: 150 km3

Resurgent caldera

Yellowstone: max 2500 km3

Flood Basalt

Laki: 14.7 km3

Columbia River Plateau: 175,000 km3

No mass extinction with any!

 

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Deccan (66 mya)

Siberian (245 mya)