Toba Volcano 

by George Weber


Eruption of Pinatubo in the Philippines, 12th June 1991, the largest volcanic eruption since 1912. The Toba eruption 73,000 years ago would have looked rather similar, but much larger. While Pinatubo erupted 4 km3 of ash, Toba produced at least 800 km3.

Photo curtesy United States Geological Survey (USGS), photographer D. Harlow

1. Introduction

When Toba volcano in western Sumatra erupted 73,000 +/- 4000 years ago it was (and still is) the largest vocanic cataclysm to have taken place on planet earth for the last 28 million years.

The eruption happened at a crucial time in the development of anatomically modern humans. We will explore how, whether and to what extent the Toba eruption and its climatic aftermath has influenced the development and spread of modern Homo sapiens.

Therefore, the last Toba eruption (also called the YTT event , see chapter 3 below) is of enormous scientific and general interest. Ths article asttempts to give an overview and introduction to the subject, based on the ongoing work of many scientists in many fields. It is also hoped that the article will raise interest among both lay people and scientists in the subject.

Fig. 1-1. Toba eruption compared to some earlier and some more recent eruptions.

The comparison is based on estimated DRE (Dense Rock Equivalent) in cubic kilometers, i.e. on the amount of material expelled during the eruption.

The Volcanic Explosivity Index (VEI) is used to classify eruptions:

VEI

Plume height

DRE estimated
volume expelled

Class name

Example

Remarks

0

<100 m

>1000 cu m

Hawaiian

Kilauea, repeated

1

100 - 1000 m

>10,000 cu m

Hawaiian / Stombolian

Stromboli, repeated

2

1 - 5 km

>1 million cu m

Strombolian /Vulcanian

Galeras 1992

3

3 - 15 km

>10 million cu m

Vulcanian

Ruiz 1985

4

10 - 25 km

>100 million cu m

Vulcanian / Plinian

Galunggung 1822

Killed 4,000 local people directly

5

>25 km

>1 cu km

Plinian

Mt. St. Helens 1980

Killed 57 local people directly

6

>25 km

>10 cu km

Plinian / Ultra-Plinian

Krakatoa 1883

Earthquakes and streams drying up observed and distant rumblings heard in the Andamans;killed ca. 36,000 people, mostly through tidal wave along the Java coast

7

>25 km

>100 cu km

Ultra-Plinian

Tambora 1815

Killed 12,000 local people directly and more than 80,000 worldwide through starvation (crop failures in the "Year without Summer")

8

>25 km

>1000 cu km

Ultra-Plinian

Toba 73,000 years ago

 

 

Fig. 1-2. Many of the most active and dangerous volcanoes on earth are in Indonesia. Only those active since 1900 are shown in the map below - plus currently dormant Toba. The long line of volcanoes along the Indonesian island arc (which also includes Narcondam and Barren island volcanoes east of the Andaman islands not shown on the map) is the result of plate tectonics: to the west of Sumatra, the Indo-Australian below the Indian Ocean is pushing towards the Sumatra. 

 

Fig. 1-3. The deep Java trench marks the line where the Indo-Astralian plate subducts, i.e. slips under, the section of the Eurasian plate on which Indonesia sits. While sinking, the Indo-Australian plate heats up and its water content turns to superheated steam under enormous pressure.. Prodigious energies are generated and a part of these energies are released by the volcanoes on the fault line. The speed of that push is 70 mm (2.75 in.) per year, adding up to more than 5 km (3.1 miles) in the 73,000 years since the last major Toba eruption.