The Eruption of the Toba

Supervolcano ca. 72,000 B.C.

and Its Effects on the Early Human Population

72,000 B.C.Humankind may have been nearly wiped out forever by the volcano under what would become Lake Toba;a massive flooded volcanic crater (caldera) today.

It was by far the largest volcanic eruption ever witnessedin the history of man. (woman man)

Discovery of Volcano1929: Ignimbrite (rock deposited by pyroclastic flows) discovered around sides of lake by Dutch geologist Rein van Bemmelen.

Evidence for massive Plinian, rhyolitic (explosive) volcanic activity around/involving Lake

Toba sometime in the distant past. van Bemmelen reasoned that Lake Toba was a gigantic

caldera (mountaintop collapsed by volcanism).

A more familiar example of a volcanic caldera: Crater Lake, Oregon

Evidence Elsewhere -Layer of ash identified with Toba eruption found all over Asia and worldwide-Called “Young Toba Tuff”

-Up to 20 feet deep in certain locations on Indian subcontinent

How big was it? A modern comparison

Mt. St. Helens (WA), 1980:most well-known example of Plinian-style eruption. -Highly explosive -Large ash cloud -Pyroclastic flows

Eruption deposits/pyroclastic flows devastated230 square miles of forest

Mount St. Helens:A fairly substantial eruption.

Ash fell over 12 U.S. states;carried halfway across the continent

And that’s basically what happened.

Mount St. Helens: 0.26cubic miles ejected.

Toba Supervolcano: 670cubic miles ejected(estimate).

The Toba eruption was thus(670)/(0.26) =

2,577 times as big as Mount St. Helens.

So basically, picture this going on

for nine hours, which was the Mount St. Helens

eruption…

and then picture 2,576 other volcanoes just like iterupting alongside for that duration.

That’s how much ash Mount Toba produced.

Crater of Mount St. Helens

We’ve seen what kind of eruption could produce a crater like this.

1.2 miles wide1.8 miles long

Lake Toba

60 miles long18 miles wide

So, huge eruption. So what?

-Keep in mind: Eruption of Mount Tambora in 1815 blocked out enough sunlight for a year to cause

worldwide “Year Without a Summer.”

-Crops failedworldwide

-70-90,000 perishedfrom starvation

in 1815-16

Yet Tambora was only a VEI 7 and only erupted~38.6 cubic miles of tephra.

Toba is estimated to have erupted 670 cubic miles.

(Probable) Climatic Effects of Toba

Ice core samples from Antarctica and Greenland:-6-year period of sulphur deposition far above normal levels – sulphur and volcanic ash remained in atmosphere, blocking out substantial amounts of sunlight. – Nuclear Winter for6—10 years.

Global circulation of ash from

Mount Pinatubo, 1991 –atmospheric presence of Toba

Tuffwas undoubtedly far more

extensive.

Toba was also situated at the worst location possiblefor a supervolcano.

Proximity to equator ~ ability to affect all

latitudes of globe, and for tephra circulation to be affected by trade winds.

Here is Toba,at 2 degrees northof the equator.

Resulting Scenario

-Past temperatures on Earth can be determined by measuring the ratio of Oxygen-16 atoms to Oxygen-18 within ice cores.

-Conducting this process with ice cores taken from Greenland and Antarctica: Toba event coldest 1,000 years in last glacial period (last ~110,000 years), possibly with exception of last ice age.

Greenland: Temperature dropped16* C in 160 years.

Effect on Humankind(controversial theory)

-Homo sapiens is remarkable for lack of genetic diversity in comparison to other primates.

-Two possible explanations:1)Evolution slowed by ability

to artificially protect oneself –but too recent?

2) Normal evolution; resultingdiversity cut off by recent genetic “bottleneck.”

“Bottleneck” scenario:Colossal near-species-extinction level event leaves small # of individualsremaining, who become the genetic root of the species.

Mount Tobaeruption:

-Recent (only 74,000 years)-Global temperature drop of 5-15* C-Up to 20 feet of ash deposited in places

How it might have happened

No more convincing suspect for “bottleneck” scenario.

Most well-known proponentof this theory: Stanley H. Ambrose,University of Illinois at Urbana.

If this theory is correct:

-Eruption of Mount Toba 74,000 years ago may have nearly killed off humankind.

-Volcanic winter lasted six years, according to sulfur deposits; at the end of those six years

only a few thousand or ten thousand humans might have remained.

Stone found in Blombos Cave, South Africa,dating from 77,000 years ago (3,000 years before Toba).

Did any of the carver’s descendants survive?

-Will a geological catastrophe ever push humankind to the brink again?