Introducing Volcano Mount Natib

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Volcano Mount Natib

Reason for Interest
There are not many active volcanoes with mothballed nuclear power plants on them that some are proposing to reopen. While the debate about that goes on other risks might be slightly ignored. Whilst I doubt Mount Natib will produce a really big eruption it can potentially disrupt the lives of a very large population.

The Setting
Luzon is the largest and most populous island in the Philippines. It is the economic and political center of the nation, being home to the country's capital city, Manila, as well as Quezon City, the country's most populous city. 

The central section of Luzon is characterized by a flat terrain, known as the Central Luzon plain, and is the country's largest producer of rice. The western coasts of Central Luzon are typically flat extending east from the coastline to the Zambales Mountains, the site of Mount Pinatubo. These mountains extend to the sea in the north, forming the Lingayen Gulf, and to the south, forming the Bataan Peninsula. The peninsula encloses the Manila Bay, a natural harbor considered to be one of the best natural ports in East Asia, due to its size and strategic geographical location.

Bataan is a province situated in the Central Luzon region of the Philippines and occupies the entire Bataan Peninsula. The City of Balanga is the capital of the province of Bataan with a population of approximately 100,000.  Bataan is bordered by the provinces of Zambales and Pampanga to the north. The peninsula faces the South China Sea to the west, Subic Bay to the north-west and encloses Manila Bay to the east. 

 

The US Naval base at Subic bay base covers 262 square miles and was the largest overseas military installation of the United States Armed Forces. Following its closure in 1992, it was transformed into the Subic Bay Freeport Zone by the Philippine government. It is now one of the busiest ports in the Philippines and host the Subic bay international airport.

 

Located in Orani on the slopes of Mount Natib is Bataan National Park which is the last remaining closed canopy dipterocarp forest in the whole Central Luzon. It is home to giant fruit bats, wild monkeys, wild boars, civet cats, eagles, and pygmy woodpeckers along with fruit-bearing trees, including coffee, jackfruit, guyabano, and lanzones. Covering about 23,688 hectares, the Bataan National Park is also the home to the Pasukulan falls and the Dunsulan falls.

 
Forests extend from Subic Bay National Park up the north-western slope of Mt Natib in Bataan National Park. These are one of the few remaining undisturbed forests that face the South China Sea. The lowlands around Subic Bay National Park are now predominantly agricultural land and human settlements. The lower slopes of the mountains are covered by grasslands and croplands while old growth forest is mainly confined to the steep slopes and gullies at higher altitudes. Much of the forest was damaged by ash from the Mt Pinatubo eruption in 1992. 

The Tectonic environment
The Philippine Sea plate is bordered by the larger Pacific and Eurasia plates and the smaller Sunda plate. The Pacific plate is subducted into the mantle from south of Japan extending down the eastern margin of the Philippine Sea plate. This subduction zone is characterized by rapid plate convergence and high-level seismicity extending to depths of over 600 km.

Along its western margin, the Philippine Sea plate is associated with a zone of oblique convergence with the Sunda Plate. This highly active convergent plate boundary extends along both sides the Philippine Islands, from Luzon in the north to the Celebes Islands in the south. On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north.

 

Studies have identified the Lubao Fault in Pampanga from a prominent surface lineament trending southwestward across deltaic plains north of Manila Bay. Field mapping and remote sensing has shown that the fault extends southwestward through Mount Natib Volcano into the South China Sea, and presently may be undergoing 3.2 cm/y of right-lateral motion. The active Lubao Fault extends southwestward from the plains north of Manila Bay to cut through Natib and emerge at the BNPP inactive nuclear power plant.

The Volcano
Mount Natib is a massive stratovolcano located at the northern end of the Bataan Peninsula, Luzon Island, Philippines. Mount Natib lines up with Mount Mariveles to the South and Mount Pinatubo to the North .  It is topped by a 7 kilometer acorn shaped Caldera that is open to the northwest with the smaller younger 2 kilometer wide Pasukulan Caldera to the east of the main caldera. The highest peak of Mount Natib has an elevation of 1,253 metres and is at the point of convergence of the two calderas. The breach in the caldera is the most likely source of a pyroclastic deposit in the eastern which indicates  a Natib eruption occurred much more recently than previously thought. Current activity on Natib is through five thermal areas with hot springs have temperatures ranging from 30-56 °C.
 

Mount Pinatubo
Mount Pinatubo is located north east of Mount Natib which on 15th June 1991 had one of the largest volcanic eruptions of the 20th century. This is thought to have been triggered by a large earthquake in July 1990 60 miles north on a fault line that passes through Pinatubo. It is estimated that ash was propelled more than 30 km into the air and pyroclastic flows extended more than 10 km from the summit of Mount Pinatubo. Pyroclastic flows traveled as much as 12 to 16 kilometers from the vent in all sectors. Pyroclastic flows crossed a 400m high ridge 3km away and a further 100m ridge further away. Before the eruption Pinatubo was considered dormant much like Mount Natib.

Krakatoa
 The tsunamis which accompanied the eruption of Krakatoa were believed to have been caused by gigantic pyroclastic flows entering the sea. This caused several cubic kilometers of material to enter the sea, displacing an equal volume of seawater. The town of Merak was destroyed by a tsunami which was 46 meters high. Some of the pyroclastic flows reached the Sumatran coast as much as 40 km away having apparently moved across the water on a cushion of superheated steam.

Nuclear Power Station
On the South West part  of the volcano is the site of a mothballed nuclear power plant that has caused arguments about proposals to activate. The site is 5km from the volcano caldera and is underlain by lava flows, lahar deposits and at least six pyroclastic deposits. It is argued that because the area is now on a ridge compared to the surrounding land that the lahar risks are low and that pyroclastic flows are more likely to flow down the breach in the caldera to the north west. Global standards do recommend that nuclear facilities are not within 1km of an active fault so it remains mothballed for now until further investigations can be done.

 

Manila
Metropolitan Manila lies in a coastal region along the Manila Bay about 80km across the bay from Mount Natib. It is flat and low-lying terrain with elevations ranging from 0 to 1 meter above mean sea level. Metropolitan Manila faces the South China Sea and is thus exposed to the south-west monsoon. Metropolitan Manila has suffered serious flood damage mainly caused by overtopping of the main rivers and inland water bodies. In September 2011, Typhoon Nesat passed about 200 km north of Manila Bay and  was one of the most destructive storms to affect the coastal areas of Metro Manila. The storm surge began early in the morning, with seawater spilling on streets by 8am  then huge waves caused  portions of the sea wall to collapse. It has been reported that more than 6,000 flood control projects have been completed to protect flood-prone areas across the country yet still parts of Manila flood regularly.

 

Discussion
Not being an expert I prefer not to comment on the nuclear power plant, but I think you could argue that any activation of Mount Natib whether through earthquakes or other means could pose a flooding threat to the Manila Bay area. The omission in the discussions for me is the threats to the Subic Bay area , to the City of Balanga and the towns around the volcano. I Guess it interests me that there is a distraction from the obvious hazards of this volcano. It is not likely to erupt on the same scale as Krakatoa or Pinatubo but seems to me a sleeping danger to a large population.

 

Edited April 30, 2021 by BrickFielder