New Estimation of the Height of Mount Tambora before Great Eruption 1815

The great eruption of Mount Tambora in 1815 was the largest volcanic eruption in history recorded by humans. It was a super colossal eruption, having a VEI scale (volcanic explosion index) of 7. Around 100 km3 of materials were released during the eruption. After the eruption, most of the mountain part at the top has disappeared.

The current height of the Mount Tambora is 2,850 m [1]. Before the eruption, the height of the Mount Tambora was certainly greater than this value. But how much? Some scientists claimed that the height of the Mount Tambora before the eruption was around 4,300 m, and it was the second-highest mountain in Dutch colonial after Puncak Jaya [2]. Was this value correct?

Now I propose a simple method for estimating the height of the Mount Tambora before the eruption. This estimation is based on the old Balinese record that before the eruption, the Mount Tambora appeared close to the Mount Rinjani [3]. This note clearly informs that the Mount Tambora could be seen from the Bali island. What is the distance of the Bali island from the Mount Tambora? Figure 1, that has been obtained from Google Map, shows that the distance of the easternmost location in the Bali island to the Mount Tambora is around 251 km. it is also clear that the line connecting the Bali with the Mount Tambora passes near the Mount Rinjani. This proves that the old Balinese record that the Mount Tambora looks close to the Mount Rinjani was correct.

Let us use this data to estimate the height of the Mount Tambora before the eruption. I used a simple geometrical procedure for the estimation. Figure 2 illustrates the position of Mount Tambora on the Earth globe. O is the Earth center, R is the Earth’s radius, P indicates the Tambora peak, B is the easternmost location in the Bali island, h is the height of the Mount Tambora peak from the sea level, and s is the distance of the Mount Tambora from the easternmost location in the Bali island. Notice that the OBP triangle is a right triangle so that so obtain the following equation

Since h << R then cos A -> 1. This means that A -> 0 so that we can approximate

with A is in radian and satisfying

Then we apply the binomial approximation for the right side of Eq. (1) to have

Substituting Eqs. (2), (3), and (4) into (1) one obtains

We used the available data: s = 251 km and R = 6,400 km to produce

h = 4.9 km.

The above calculation result has been obtained based on the assumption that the Mount Tambora was seen from the sea level at the easternmost part of the Bali island. However, if the Mount Tambora view were found in the records of old society, those view must not been made in a special place. People in daily life must be able to see the mountain.

In the past, the most developed region in the eastern part of the Bali island was the Karangasem Kingdom. As shown in Figure. 3, the distance of this kingdom from the Mount Tambora is 261 km. By using Eq. (5), the height of the Mount Tambora so that it can be seen from this distance (if it is assumed that the Karangasem Kingdom was located at 0 meter above sea level) is 5.3 km.

However, the parts of the Karangasem that have most population are located between 0 m to 500 m above the sea level [4]. Therefore, in order to see the Mount Tambora from such locations (with altitude of up to 500 meters), the Mount Tambora height does not have to be 5.3 km. It is only need to have an altitude of

h = 5.3 km – 0.5 km = 4.8  km.

Thus, our estimation is larger than the previous report of 4.3 km [2].

References:

[1] https://en.wikipedia.org/wiki/Mount_Tambora

[2] R.B. Stothers, The great Tambora eruption in 1815 and its aftermath, Science, 224, 1191-1198 (1984).

[3] M. Abdullah, Letusan Tambora 1815: Letusan Terbesar dalam Catatan Sejarah Manusia, Bandung: ITB Press (2018).

[4] RPI2-JM Bidang Cipta Karya Kabupaten Karangasem 2015 – 2019

Image source: https://scitechdaily.com/

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