Earth Sciences 240A - Lecture 23 - Continental Hot Spots (Resurgent Calderas)

Introduction

Mantle plumes: ‘burning’ through continental lithosphere

Magma: Commonly all rhyolitic

Craters: Largest of all volcanoes; Resurgent

Yellowstone Resurgent Caldera

Most prominent feature of Wyoming

Stationary Plume/Moving Plate

Track evident as ‘Snake River Basalts’

Current

Largest heat flow anomaly in USA

World’s largest hydrothermal system

Seismically active

Large volume of rhyolite magma at 5-10 km depth

Caldera dome: vertical instability

History

Eruption 2 million years ago: 2,500 km3 volume

Eruption 1.3 million years ago: 280 km3

Eruption 650,000 years ago: 1000 km3

Eruptions over last 150,000 years: Total 1000 km3

Last: 70,000 years ago

Resurgent Caldera Progress

Top of chamber: Highly Si- , volatile-rich

New magma inserted at base: Usually basaltic

Failure of ‘cap’

Plinian eruption

Emptying of chamber (blows, flows)

Collapse of dome walls; 2nd eruptive stage

Rebuilding of dome (resurgent stage)

Current Magma Chamber

End of last stage? No!

Beginning of new stage? Yes!

Plume and hot spot still active

Seismic data: new magma moving in

Next large eruption

As catastrophic as a major asteroid impact to North American civilization

Long Valley Caldera

Mantle plume/hot spot location

Biggest eruption 760,000 years ago: 1500 km3; Ash over 9 states

Caldera area dropped 2 km vertically

Magma chamber then: 19 km diameter; 5 km below surface

Current

Magma chamber now: 10 km diameter; 8 km below surface

Mammoth Mountain, Mono-Inyo craters

Formed in last 400,000 yrs; still growing

1980: 6 large (M6) earthquakes (3 in 1 day); Dome lifted 60 cm

1997: 8000 earthquakes; >1000 in 1 day

Hazard

Most drinking water for LA from Mammoth Lakes; aqueduct transfer

Escape route: new highway built

“Next to Mount St. Helens, the Long Valley area is the location in the US picked by geologists as most likely to have an eruption soon”

Continuous GPS measurement

Vert: ±8 mm, Horiz: ±3 mm

Tiltmeters, seismographs, magnetometers, strain gauges, etc.

Mono-Inyo Craters

Most recent eruption: 250 years ago

CO2 hazard

Eruption

Highly likely

Probably minor volume

Valles Caldera

Control

NOT plume

Basement structure (inter-plate boundary)

Eruption:

1.4 mya; 1 mya

Current

Magma chamber 1: 13 km diameter; 6.5 km depth

Magma chamber 2: ? Diameter; 37 km depth

No surface activity

Next

Eruption Prediction