|In prehistoric, volcanic activity has occurred, followed by the formation of a 2000 m diameter caldera and form a lake. In 1898 or early 1900 there was a normal eruption and left the sumbatlava which became known as Gunung Colo.
In 1975 a new solfatara / fumarola found on a hill on the northeastern slope, 1,500 m from the peak. The hill is outside the Crater Colo system and is known as Bukit Ambo.
On August 20, 1982 Una Island shook earthquake earthquake, until the end of August as much 41 times the earthquake that the population felt. Meteorology and Geophysics Agency of Winangun, Manado said that the strongest earthquake occurred on August 24, at 00.46.43 WITA, a magnitude of 4.6 SR at a depth of 30 km.
The tectonic earthquake again shook Una-Una Island in early July 1983 which was in power III on MMI slaka. The earthquake is increasingly increasing in number and intensity.
After an 83-year break, on July 23, 1983 there was a terrible eruption that destroyed the lava plug and burned 2/3 of Una-Una Island.
On July 18 the number of earthquakes increased and caused the first phreatic eruption.
Until July 22, 1983 visual observation directly from Una-Una Island, after the eruption of visual observations made from Observation Post (temporary) in Wakai, Togian Island, lk. 40 km from Una-Una Island.
A year later an integrated investigation was conducted, namely seismic investigation and eruption mapping. From seismicity it is known that between February and March 1984 there are still sporadic smoke eruptions from within the crater. In the integrated investigation is also known, that the lava plug was exhausted and left 3 (three) craters with different sizes.
|Una-Una Island is almost rounded with a center line of 20 km. The highest peak is Bukit Sakora (486.9 m) located in the northwest. Preliminary activity in prehistoric times, formed caldera with 2000 m diameter with crater lake. In the activities of 1898 or early 1900 formed a lava plug known as Colo Mountain adjacent to a crater lake known as the Pokai Lake. Before the 1983 Eruption, the existence of Pokai Lake, became a place of freshwater fish maintenance. Pokai Lake measuring 600 m long, 190 m wide 1.5 m deep, is at 250 m above sea level. As long as it is known, the temperature never exceeds the temperature.
From the morphological form, the crater is open to the northeast, located almost in the middle of the island, with the north-south circle along the 1,350 m and the east – west along the 1750 m. The July 1983 eruption changed all that and produced 3 craters, each Crater Main, Crater II, and Crater III.
Crater Main (former sumbatlava) diameter 1100 – 1350 m. In May 1984 flooded water area of 67 ha, but in February 1996 shrank to 30 ha. Crater II is located on the northeast of Bukit Sakora (highest peak of Una-Una Island), rounded in diameter 200 m. Crater III takes place in the southwest of Bukit Sakora. The shape is not perfect because of strong erosion. The lowest wall in the southeast opens towards the Main Crater.