Tonga - Geography

Polynesia's oldest and last surviving kingdom consists of some 170 islands, of which 45 are inhabited. Tonga is divided into three main regions: Tongatapu ("sacred Tonga") and its nearby islands in the south; the Ha'apai Group, some 100 kilometers to the north; and the Vava'u Group, yet another 100 kilometers farther northward. Niuatoputapu, Niuafo'ou, and their surrounding islands (called the Niuas for short) lie some 600 kilometers to the northeast of Tongatapu, which alone accounts for 35 percent of the land area and is the major population center. The islands to the east are of coral formation, the Lifuka and Nomuka groups with many small coral islands and reefs. The islands to the west are volcanic. Except for a chain of smaller islands along the western edge of the country, which are of recent volcanic formation, the islands are raised limestone or coral limestone structures that have few, if any, hills or valleys. They spread over an expanse of ocean that gives the country a potential EEZ of about 700,000 square kilometers.

There are active volcanoes on four of the islands, including Tofua Island whose crater is filled with hot water. Falcon, an active volcano under the sea, sends up lava and ash from time to time.

The Tongan Islands are located on the crest of the Tonga Ridge, an active fore-arc bordering the Tonga Trench at the Pacific Plate boundary, to the west lies the volcanically active Tofua arc. The ridge is divided into separate fault blocks. The surfaces of individual blocks are less than 200 m deep and have an irregular limestone cap, the emergent portions of which form islands. The southern (Tongatapu) block has the islands of Tongatapu and 'Eua at its southern end and a lagoon to the north.

The islands of Tongatapu and Vava'u are composed of emerged and tilted limestones of Pliocene and Quaternary age with a volcanic soil mantle. Their morphologies and surface geology are mainly the result of subaerial and marine erosion. A marine dissolution process, termed solution cliffing, is thought to be responsible for excavating depressions and channel-ways below present sea level in the interiors of the islands. Factors that promote solution cliffing include (1) tilting of the atoll surface which provided connections between lagoon and open sea at virtually all eustatic sea levels; (2) tidal dispersal of the dissolved limestone products from the interior of the atoll; (3) a rate of biogenic sedimentation in the interior waterways that is slower than the rate of erosion.

Erosion, and especially solution cliffing, has reached a more advanced stage of development in Vava'u than Tongatapu, due probably to Vava'u's greater rate (or duration) of uplift. In Tongatapu, many primary depositional features - reef rim, patch reefs and lagoon bed - are still evident and some may be associated with relict deposits of construction material. This is not the case in Vava'u where primary depositional features are absent. Here new sources of sand and gravel are likely to be found in large overwash lobes on the modern reef flat.

On March 17, 2009, a surtseyan eruption occurred around Hunga Ha'apai Island, Tonga. The island was formed by a surtseyan eruption, which is a relatively modest explosive eruption (compared to say, Mt St Helens or Mt Pinatubo) occurring in shallow waters. They are relatively common along the active Tonga trench. But it is less common for such eruptions to construct stable landmasses that survive for more than a few months. Over the past century, only two other surtseyan events have resulted in lasting edifices: Surtsey island in Iceland (erupted in the late 1960s; the type event), and Capelinhos on Faial in the Azores (mid/late 1950s).

The eruption start time was estimated to be between 01:50 and 11:10 local time, on March 17, 2009 (i.e., between 12:50 and 22:10 UTC, March 16). The initial explosive phase lasted 3–5 days and consisted of multiple steam and tephra explosions from two distinct vent sources, one on the northwest side, and another about 100 m off the south shore of the pre-existing island. The eruption plume reached 4.0 to 7.6 km altitude above sea level, and tephra added new land around each of the vents, initially tripling the area of the pre-existing island. The next phase of steaming from newly formed crater lakes around the vents lasted a few days. Three warm crater lakes formed initially, but disappeared with time as the shoreline eroded. After ~ 2 months, vegetation that was initially buried by tephra was recovering; after ~ 10 months, the size of the island had eroded down to ~ twice that of the pre-existing island.

The Hunga Tonga-Hunga Ha'apai underwater volcano created new land and eruptions in 2014-2015. It created enough new land to connect between the two nearby islands of Hunga Tonga and Hunga Ha'apai. The island first formed between Dec. 2014 and Jan. 2015, when an underwater volcano explosively erupted. When all the dust, rock, and ash settled, a newly-formed island remained between two older islands, with a summit reaching 400 feet high. It was the first of its kind to form in 53 years—as well as the first to form during the modern satellite era. Thus, scientists have been able to study its birth and evolution in vivid detail from space. Since its formation, the island has erupted intermittently.

In Tonga, there are several examples of such eruptions forming short-lived islands over the past century, with the most recent erupting from the same submarine caldera as HTHH in 2009, only 1-2 km from the current cone; it washed away within half a year or so. The current cone may be persisting perhaps due to a larger volume of material ejected (giving it more time to stabilize before the oceanic wave action and pluvial (rain-caused) erosion erode it away), or perhaps its position between the pre-existing islands has provided a level of protection against oceanic wave erosion.

An undersea volcanic eruption was observed off the South Pacific island of Tonga on 16 January 2022. Tsunami waves were recorded soon after in areas of the Pacific. The Japan Meteorological Agency said the eruption occurred at the Hunga Tonga-Hunga Ha'apai underwater volcano, shortly after 04:00 UTC. The eruption led to heavy rainfall, thunder and lightning in the island. Meanwhile fumes of smoke blackened the skies sending powerful waves through the region. The eruption was so intense that Fiji which is more than 800 kms from Tonga also heard loud thunder sounds. The eruption had a radius of 260 km (161.5 miles), and sent ash, steam, and gas 20 kilometers (12.4 miles) into the air. It was also about seven times more powerful than the previous eruption on Dec. 20, 2021. Additionally, a tidal gauge in Nuku'alofa, the capital of Tonga, measured a 30 cm (one foot) tsunami wave that resulted from the blast.

Images from the Himawari weather observation satellite show smoke from the volcano spread in a concentric manner and had covered an area of about 200 kilometers in diameter about one hour later. Officials at the Wellington Volcanic Ash Advisory Center in New Zealand say the cloud of ash reached a height of about 15,000 meters.

The Pacific Tsunami Warning Center says tsunami waves measuring about 80 centimeters were observed in the capital of Tonga... and about 60 centimeters in the capital of American Samoa. Tsunami warnings were issued across wide areas of the South Pacific. Australia has also urged local residents in the south eastern coastal area to be cautious. The US Tsunami Warning System initially said a magnitude 7.6 earthquake had been recorded in Tonga at 06:40 UTC.