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An underwater volcano off the coast of Tonga is spewing ash high into the air - causing air travel chaos and turning the surrounding ocean the colour of blood. 

The Hunga Tonga-Hunga Ha'apai underwater volcano is around 40 miles (65km) north of Tonga's capital Nuku'alofa.

Its ash plume has reached heights of more than 14,765ft (4,500 metres) and the eruption has disturbed algae in the region, causing what's known as a red tide.

The Hunga Tonga-Hunga Ha'apai underwater volcano is located about 40 miles (65km) north of the capital Nuku'alofa. Its ash has reached heights of more than 14,765ft (4,500 metres) and the eruption has disturbed algae in the region, causing what's known as a red tide

All international flights around the island have been cancelled, but domestic flights were continuing as scheduled, according to Tonga Airports chief executive Viliami Ma'ake. 

The volcano, which first erupted in 2009, had been rumbling in recent weeks before exploding violently in the past few days. 

Brad Scott, a volcanologist at New Zealand agency GNS Science, said volcanic activity had been recorded for several weeks in a stretch of ocean and small islands around 35 miles (60km) north of Nuku'alofa.

He believes the volcano may have started beneath the ocean and grown until it reached the surface, essentially creating a new island.

But he said details of the eruption still remain unclear. 

The Wellington Volcanic Ash Advisory Centre (VAAC) confirmed the height of the volcanic ash plume earlier today. 

A short-lived but powerful eruption of the volcano, in March of 2009, lasted for almost a week.

At the time, geologists said the eruption originated from two volcanic vents.

One was located on Hunga Ha'apai, while the other was around 330ft (100m) offshore.

The two vents spewed so much debris, however, they created a new land mass.

And now scientists want to establish whether the latest eruption is related to this. 

Mr Scott continued that Tonga has asked New Zealand for technical assistance, and that GNS plans to send two volcanologists to assess what's happening as soon as flights resume.

A short-lived eruption in March 2009, (pictured) lasted for almost a week. At the time, geologists said the eruption originated from two volcanic vents - one was located on Hunga Ha'apai, while the other was around 330ft (100m) offshore.The two vents spewed so much debris, however, they created a new land mass

Hunga Tonga-Hunga Ha'apai (marked) sits beneath the ocean 39 miles (62km) northwest of Tonga's capital Nuku'alofa. The volcano gets its name from the two islands it is situated between - Hunga Tonga and Hunga Ha'apai. It is said to be part of the volcanic Pacific Ring of Fire and is one of a 36 undersea volcanoes locally

The so-called red tide is also known as an algal bloom. 

An algal bloom occurs when there is a sudden increase in the amount of algae in water. 

When levels of nitrogen and phosphorus rise, which can be caused by underwater eruptions such as the one off the coast of Tonga, algae can multiply. 

Typically, this causes water to turn green, but the effect also causes reds and yellows to be released. 

In particular, a bloom of dinoflagellate algae causes water to turn blood red in colour. 

These natural-colour images from 29 December and 31 December show the waters around Tonga, Hunga Tonga, and Hunga Ha'apai. They were taken by the Moderate Resolution Imaging Spectroradiometer (Modis) on Nasa's Terra satellite. Dense cloud cover prevented other remote observations for several days

The so-called red tide (pictured) is also known as an algal bloom, which occurs when there is a sudden increase in the amount of algae in water. In particular, a bloom of dinoflagellate algae causes water to turn blood red in colour