Wednesday 23 December 2020

Four dead and six missing following landslide at copper mine on Cebu Island, the Philippines.

The Mines and Geosciences Bureau of the Philippines is carrying out an investigation at a copper mine on Cebu Island, following a landslide that is known to have killed four miners, as well as injuring another eleven and leaving six more unaccounted for. The incident happened at the Carmen Pit of Carmen Copper Corporation, a subsidiary of Atlas Consolidated Mining Development Corporation, in Toledo City at about 1.00 pm local time on Tuesday 22 December 2020. The incident happened after weeks of heavy rain in the area, which is thought to have led to the limestone and mudstone through which the mine cuts becoming waterlogged, and therefore structurally weakened. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. However, the mine site is known to be close to a fault, the Barot Fault, and the possibility that this played a role in the landslip cannot be ruled out.

The site of a landslide at the Carmen Pit mine on Cebu Island. Alan Tangcawan/Sun Star.

Cebu has a long rainy season, lasting from May to January, driven by the Asian Southwest Monsoon. This has been particularly severe this year, driven by a La Niña weather system and a series of tropical storms, the most recent of which, Tropical Depression Vicky, swept across parts of the Philippines last week.

The spatial relationship between the Barot Fault, the Carmen Pit, and the other pits and faults of the Atlas Copper Complex. Atlas Mining.

Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate. 

Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.

Much of Southeast Asia has two distinct Monsoon Seasons, with a Northeast Monsoon driven by winds from  the South China Sea that lasts from November to February and a Southwest Monsoon driven by winds from the southern Indian Ocean from March to October. Such a double Monsoon Season is common close to the equator, where the Sun is highest overhead around the equinoxes and lowest on the horizons around the solstices, making the solstices the coolest part of the year and the equinoxes the hottest. However Myanmar is largely protected from the Northeast Monsoon by the mountains separating the country from Yunnan Province in China.
The winds that drive the Northeast and Southwest Monsoons in Southeast Asia. Mynewshub.

The La Niña weather system is the opposite of the El Niño weather system, in which unusually cold surface temperatures spread across the equatorial Pacific from the upwelling zone on the South American coast. This traps warm water from the western Pacific, preventing it from spreading east and warming the central Pacific. This leads to lower evaporation over the (cooler) east Pacific, leading to low rainfall on the west coast of South America, and higher evaporation over the (warmer) west Pacific, leading to higher rainfall over East and Southeast Asia and northern Australia.

The effects of a La Niña weather system in December-February. NOAA.

Tropical storms, known as hurricanes in East and Southeast Asia, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

The formation of a tropical cyclone. Natural Disaster Management.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.  

The formation and impact of a storm surge. eSchoolToday.

See also...

Online courses in Palaeontology. 

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