The story is almost always told through its size. It is worth more telling it through its weakest point.
01 The largest explosion measured
The eruption began on 14 January and peaked the following day. The eruptive column rose 58 kilometres and entered the mesosphere: the highest recorded since Krakatoa in 1883. It was also the largest eruption since Pinatubo in 1991.
Energy estimates vary widely between studies, between 61 and 200 megatons of TNT equivalent, and the explosivity index is put at between 5 and 6 depending on who calculates it. That very wide range is itself a fact: this was a phenomenon outside the usual measuring range.
One number illustrates the violence better than the megatons. Between 05:00 and 06:00 UTC on 15 January, 200,000 lightning discharges were recorded in the ash cloud. Two hundred thousand strikes in an hour, over one point in the ocean.
02 The wave that went round the world
The atmospheric pressure wave spread across the whole planet and barographs recorded it circling as many as four times, according to observatories in Japan and Utah; in Massachusetts at least two passes were counted. In New Zealand the peak amplitude reached about 7 hectopascals.
The sound travelled the same way. It was heard 840 kilometres away in Samoa; in New Zealand, more than 2,000 kilometres away, it arrived two hours later. And in Anchorage, some 9,300 kilometres off, people heard a series of bangs lasting around half an hour. It was audible in the Yukon too, about 9,700 kilometres away.
The tsunami arrived before the news did. In Nuku'alofa the first wave measured 1.2 metres and tide gauges recorded 1.5 to 2; elsewhere in the archipelago waves of up to 20 metres were measured, and on the island of Tofua the water ran up 45 metres above sea level. Four people died in Tonga and seven in all, counting Peru and Fiji.
03 A country with a single cable
All of Tonga's international data traffic ran through a single submarine fibre-optic cable. The eruption broke it at two points, 37 and 47 kilometres from Nuku'alofa, and it was not a clean break: entire sections were found displaced five kilometres from their position and buried under thirty centimetres of sediment.
The repair required replacing 87 kilometres of cable, with 55 written off. The cable ship had to locate the ends, lift them and splice them at sea. International service did not return until 22 February: five weeks of a country effectively disconnected.
In the meantime, contact with the outside world went wherever it could: satellite links of very limited capacity, radio, and messages that travelled physically on the aircraft. For a diaspora used to speaking to family daily, five weeks of silence is a long time.
04 Five to ten centimetres on the runway
A layer of about two centimetres of ash fell on the capital and the sun was blotted out. On the runway of the international airport, in the south of the island, the layer was thicker: between five and ten centimetres, enough to stop anything landing.
With machinery destroyed or unreachable, the runway was cleared by hand. Hundreds of volunteers, workers and Tongan defence personnel advanced at a rate of one or two hundred metres a day. The first aid flight landed five days after the eruption.
That detail sums up the real scale of the episode: the largest bang modern science has measured was dealt with, on the ground, using shovels and people in a line.
05 What it reveals
An island country can be joined to the world by a thread the thickness of a hose, laid along hundreds of kilometres of seabed. When that thread is cut there is no alternative route; and when a volcano cuts it, hurrying does not help either: someone has to go down and find it.
For a traveller the reading is concrete, and it applies to the luggage rather than the conversation: download the maps before you arrive, carry cash, keep your tickets on paper and do not assume you will be able to sort anything out online as you go. Here, coverage is a convenience, not a guaranteed utility.
