Indonesia yesterday raised the danger alert level for the Anak Krakatau volcano amid signs of increased activity, after its eruption last weekend triggered a deadly tsunami.
Here is a look at Indonesia's volcanic hot spots.
Q Why is Indonesia prone to quakes and eruptions?
A Indonesia is a vast archipe-lago that sits on the Pacific Ring of Fire. The ring is a 40,000km arc that stretches from New Zealand to Papua New Guinea, Indonesia, the Philippines and Japan, across to Alaska, the United States and Mexico, to the southern tip of South America.
It is where tectonic plates collide and a large portion of the world's volcanic eruptions and earthquakes occur.
The United States Geological Survey says more than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the Ring of Fire, while about 90 per cent of the world's earthquakes occur along the ring.
Q How many volcanoes does Indonesia have, and how many of them are active?
A Indonesia has 127 active volcanoes, with seven of them currently in the "eruption phase", the Sydney Morning Herald reported, citing volcanologist Janine Krippner of West Virginia's Concord University.
A volcano is deemed as no longer in the eruption phase when it remains inactive for a period of three months.
Anak Krakatau, one of the world's most active volcanoes, is in the eruption phase, and has been rumbling on and off since July.
Formed from the eruptions of the legendary Krakatau volcano, it was among 20 volcanoes in In-donesia listed by the govern-ment as alert zones for travellers last Wednesday, the Jakarta Post reported.
It is currently at level 3, the second highest danger alert level on the four-level scale, which goes from normal (level 1) to caution (level 2), alert (level 3) and danger (level 4).
Other volcanoes on the list include Mount Soputan in North Sulawesi, also at level 3, and Mount Sinabung in North Sumatra, which is currently at the highest level.
Mount Sinabung, which has been rumbling since 2010, saw deadly eruptions in 2014 and 2016. An eruption in February this year saw it spewing massive clouds of smoke and ash.
Another recently active volcano is Mount Merapi, currently at alert level 2. Located north of Yogyakarta, its series of small eruptions in May prompted the evacuation of residents and the closure of the city's airport.
Meanwhile, Mount Agung, on the island of Bali, has been erupting periodically since it rumbled back to life last November.
It is currently at alert level 3. An eruption in June temporarily shuttered the holiday island's airport and grounded hundreds of flights. Mount Agung's last major eruption in 1963 killed around 1,600 people.
Q How do volcanoes and earthquakes occur?
A Along the Ring of Fire, a number of major tectonic plate boundaries collide. The interconnected series of fault lines are constantly under pressure from super-hot molten rock beneath. Occasionally, the fissures give in and explode, creating volcanic eruptions and causing the land on either side of the fault line to shift and buckle violently, triggering earthquakes.
Q How can a volcanic eruption trigger a tsunami?
A Tsunamis are often triggered when large amounts of water are displaced quickly by earthquakes. But Indonesian geologists say that last Saturday's tsunami likely occurred when an eruption caused a section of the volcano to collapse and slide into the ocean, triggering high waves.