2. The Sleeping Giant

 Where the cataclysmic events of 73,000 years ago took place, there is now an idyllic landscape with a picturesque lake at its centre. The area has great beauty, friendly locals and many potential tourist attractions. Only the occasional slight tremor, or puff of smoke and a few hot springs give a tiny hint of the violence that is lurking below. In fact,

Toba is likely to have been the place where modern Homo sapiens most dangerous and fiercest struggle for survival began. Countless other species faced the same trial - and many did not make it. Our ancestors did.

 

 Fig. 2-1. Today's topography around Lake Toba, the site of the last major Toba eruption 73,000 years ago.

 

Vocanic features in and around Lake Toba today:

grey area Present-day topographic depression
green area Updomed areas 

1 Sibandung caldera: made 73,000 years ago by the Toba YTT event (Young Toba Ash)
2 Haranggaol caldera: made 500,000 years ago by the Toba MTT event (Middle Toba Ash)
3 Sibandung caldera: made 800,000 years ago by the Toba OTT event (Old Toba Ash)

The MTT and OTT events were not as large as the YTT event of 73,000 years ago
but were still major eruptions of at least VEI 7. 

V1 Tandukbenua (Sipisopiso) - young dacit-andesite volcano
V2 Pusubukit volcano - young dacit-andesite volcano
D1 Pardepur dacite domes
D2 Tuk-tuk rhyolite dome
HS Hot springs 

Fig. 2-2.  The eruption of 73,000 years ago left the Sibandung caldera.  Lake Toba is surrounded by two small, active volcanos as well as several updomed areas and hot springs. These features indicate that there is activity below the surface today and that pressure is rising. Samosir island, too, is evidence for upthrust from below. From the record it seems that Toba produces major eruptions every 300-400,000 years and that it will erupt again - but not any time soon.

 

 

 Fig. 2-3. Lake Toba today:, an idyllic scene.

 

 Fig. 2-4. A satellite picture of Lake Toba. 

 

1.
Over millions of years, magma rises from below; a volcanic cone develops on the surface.

2.
Magma pressure rises until the volcano explodes in a major eruption.
73,000 +/- 4,000 years ago.

3.
The eruption exhausts itself when pressure from below is relieved. A depression forms which is gradually filled with rainwater and forms a lake.
Some thousands of years after the eruption has died down.

4.
Magma pressure begins to rise again from below and an island is pushed up in the lake.
Around 10,000 years ago.

Fig. 2-5. Lake Toba developed its characteristic geography during the last 73,000 years as shown (highly simplified) here.