The oldest rocks in the Municipality of Guinobatan are the Pre-Tertiary metamorphosed siliceous volcanic rocks mapped by Newhall (1977) in Barangay Bubulusan. These occur as erosional windows avid Quaternary volcanic cones and Quaternary pyroclastics.
An assemblage of late Miocene to Pliocene clastic rocks belonging to the Albay Group Formation (BM, 1965) is found in the Southern part of the municipality. These clastic rocks consist of sandstone and Malama Silt members of Albay Group Formation.
Pliocene to Pleistocene limestone with associated pyroclastic members is found on the hills west of the town proper of Guinobatan. These limestones occur predominantly as massive coralline rock units. The pyroclastic member consists of poorly sorted to well sorted scoriaceous volcanic materials. De Guzman in 1963 has named this rock formation the Ligao Formation.
The most significant volcanic cone in Guinobatan is Mayon Volcano which is audesitic in composition. Six (6) other small basaltic cinder cones have been deposited at the base of these Quaternary volcanic cones.
Volcanic and flurio-volcanic terrains characterize the Northern half of the Municipality of Guinobatan while the Southern half is largely denudational hills and slopes. The land forms of volcanic and flurio-volcanic origin are the cinder cones (VI) , upper volcanic foot slope (FVI) and the lower volcanic foot slope (FV@).
Six (6) small cinder cones are mapped northwest of the proper. Two (2) of these lies on the Southern and Northwestern sides of Barangay Inascan. Another is found west of it. The other cones form cluster of cinder cones in Barangay Tuburan, Ligao City. The average height of this cinder cones is 230 masl. The pyroclasts rest at a moderately steep angle of repose of 34% around an average basal diameter of 710 meters. Ramos, et. al (1988) placed the age of these cinder cones to approximately 5,000 to 10,000 years old based on their erosional and weathering conditions. These cinder cones are surrounded by gently sloping to moderately steep upper volcanic foot slopes. This geomorphologic unit is primarily an accumulational slope is un-dissected to slightly dissect. A significant area is occupied by the lower volcanic foot slope of Mayon Volcano. The lower volcanic foot slope of Mayon end in Guinobatan at the base of the low-lying sedimentary hills. This unit is also un-dissected to slightly dissect by creeks and lahar channels.
The loud forms of denudational origin mostly consist of very low to low hills underlain by varying rock units. Very low hills and slopes underlain by metamorphic rocks (D1) are found North of Guinobatan. The highest elevation is only 235 masl. Newhall (19770 was able to map the underlying rocks as metamorphosed siliceous volcanic rocks probably of trachytic or latite parent material. These hills have short moderately steep slopes. These are differentiated from the gently undulating slopes underlain by sedimentary rocks and pyroclastic (D2) and the low hills and slopes underlain by limestone and associated pyroclastics (D3). These low hills and slopes form apart of an extensive sedimentary basin in western Bicol.
These are six (6) geological hazards affecting the Municipality of Guinobatan. These are earthquake generators, earthquake-related hazards, ground rapture, volcanic eruption-related hazards landslides and flooding. These findings were based on the study on “Geo-Environmental Assessment and Bench marking in Bicol Region” which was prepared in 1998 by save our Future Foundation, Inc. for DENR Regional Office No. 5.
Earthquake Generators. The Legazpi lineament is a left lateral strike slip with a significant normal component. It extends from the Philippine Trench northeast of Samar, passes through Albay Gulf, and cuts obliquely across the Bicol Peninsula and continuous in a WNW direction to join the Philippine Fault in the vicinity of Ragay Gulf. Lake Bato and Mt. Mayon sit on the Legazpi lineament and apparently controlled by it. Geological studies show that the Legazpi Lineament is an active fault.
Earthquake-Related Hazards. Earthquake can be classified as tectonic or volcanic. Tectonic earthquakes are those causes by the sudden displacements along fault fissures in the Lithosphere. Volcanic earthquakes are triggered by the fracturing of rocks and by the rise of molten rock from the earth’s inner core. These usually happen beneath active volcanoes like Mt. Mayon volcano earthquakes are usually less destructive than tectonic ones.
Ground Rapture. Morphological evidences indicated that ground rapture usually occurs at the southwestern margins of the Bicol River Basins from Barangays Guyodan, Bato, Camarines Sur to Barangay Bonbon Libon, Albay. These moderately to steeply structural features are underlain by siltstone-shale.
Volcanic Eruption-Related Hazards. Lava flows are relatively large, coherent and elongated streams of incandescent molten volcanic rock materials. The red hot molten materials usually coze non-explosive from the volcano’s summit crater or from a point near the summit area and then cascaded along the ravines and gullies. The Barangays in Guinobatan that were affected by the 1993 Mt. Mayon eruptions are as follows, Doña Tomasa, Maninila, Masarawag, Muladbucad Grande and Pequeño, Sitio Mabalod and Barangay Tandarora.
Pyroclastics lflows sometimes called ash flows are super hot, often incandescent and turbulent blasts of volcanic fragments (boulders, pebbles, sand and dust) and hot gases that sweep along close to the ground at hurricane speed, sometimes as great as 500 kph. Being ground huggers, they can devastate large areas along the topographic depressions and gullies which they encroach.
Lahar, an Indonesian term sometimes calles mudflows or volcanic debris flows are flowing mixtures of volcanic debris and water. Lahar are classified by geologist into two types: primary or hot lahar which are usually associated with volcanic eruptions and the secondary or cold lahar which are caused by heavy rainfall.
Volcanic ash is a kind of tiny or powdery tephra. It is produced when rising magma in the upper conduit if fragmented or pulverized due to expansion of compressed gases. These pulverized materials are then blasted from the crater during explosive eruptions.
The Hazard Zone Map of Mt. Mayon is as follows:
6 km. radius from the summit as Permanent Danger Zone, where the danger is present at all times and the hazard involved is high during eruption;
8 km. radius from the summit as moderate Hazard Zone where people within the zone are alerted at the onset of eruption; and
10 km. radius from the summit where people are alerted for possible evacuations once signs of violent eruptions are noticed.
Landslide. The term landslide includes a wide variety of processes that result in the downward and outward movement of slope-forming materials composed of natural rocks, soil, artificial fill or a combination of these materials. Landslides in the region are triggered by either a strong typhoon (rain-induced landslides) or by a strong earthquake. In the province of Albay, major landslides occurred only at Manito and Rapu-Rapu. Ten (11) barangays in the Municipality of Guinobatan that are vulnerable to landslides, includes Balite, Batbat, Malabnig, Cabaloaon, Lomacao, Minto, Morera, Ongo, Palanas, Quitago and Sinungtan.
Flooding. Tropical cyclones are the fountain heads of floods. Aside from these cyclones, heavy rains due to seasonal monsoons also cause severe flooding. Simultaneous occurrences of tropical cyclones and heavy rains have devastating effects. Based on 1998 data provided by the Provincial Disaster Operation Center of the Provincial Disaster Management Office of the Province of Albay, 32 out of 44 barangays of Guinobatan are vulnerable to floods. These barangays are Agpay, Inamnan Pequeño, Inamnan Grande, Inascan, Iraya, Ilawod, Lomacao, Lower Binogsacan, Maipon, Maninila, Masarawag, Minto, Morera, Muladbucad Pequeño, Muladbucad Grande, Ongo, Poblacion, Pood, Quibongbongan, Quitago, San Francisco, San Jose, Sinungtan, Tandarora and Travesia.
The most frequently identified tree/woody species in the project area is alim (Mallotus multiglandulusos). This was followed closely anonang (Condia dichotama), nangka (Artocarpus heterophullus) and coconut (cocos Nocefera). Avocado (Persia Americana) and cacao (Theoloroma cacao) are two other frequently encountered fruit tree crops, Hagcysit and havili are two most prevalent woody shrubs in the area (Table 2.4).
In terms of density, coconut planted at 10×10 meters distance is the densed with a relative density (RD) value 22.54. Gemelina with RD value of 22.42, kakawate (Gliriciclea septem) at RD value of 2.38, tanag (Kleinhovia hospital) at RD value of 2.15 are the next densest vegetation in the area, but at a much lower density compared to coconut. Of the shrubs, hagupit and havili are most densely distributed with RD values of 1/37 and 1.29 respectively.
Among the grasses, herbs and non-woody shrub in the area, the most common are makahiya (Mimosa pudica), kudzu (Mucumoides), centrosema (Centrosema pubescens), lapis-lapis (Stachytorheta jamaicensis) and carabao grass (Axonopus compressus)
Severe species of birds were observe during the conduct of transect surveys (Table 5). The most conspicuous of them are the Bulbus, sunbirds, shrikes and swiftlets. Dogus large-billed crows were observed, but several other species are reported to frequent the area. Several Volant mammals are reported but the Kalabidong or common short-nosed bat can be seen regularly in the area and was in fact observed during late afternoon. Paniki or the Geoffroy’s common Rousette is reported to be something fading fruiting period of trees.
Rats are said to be coomon near households and farm lots in the area. With this abundance of rats, snakes such as the reticulated python, the Philippine cobra and the common wolf snake thrives in the area. The common house lizard is very common inside households, while tokway geckos can be heard frequently. According to personal interviews with rsidents, monitor lizards are also seen frequently.
Several types of insects were also netted in the area. Various colors and sizes of moth and butterflies, dragonflies/damselflies were captured within fine insect nets. Bugs belonging to class coleopteran and homoptera were also found. Obviously, there are honeybees and wasps in the area as well as houseflies and mosquitoes. Several types of spiders are present as evidenced by webs cell over the place.
A total of 340 individual birds were observed during the transect survey, belonging to 5 species. The survey showed that the Big Species Richness (BSR) is very low since only 5 species were observed, although the results could have been different if more days were allotted for more transect walks.
TOPOGRAPHY AND SLOPE
Topographical conditions in the Municipality of Guinobatan are broadly classified in these categories: (1) the alluvial flood plains of San Francisco and Cabagsay Rivers; (2) flat to slightly rolling hills; and (3) the rugged and steep terrain of Mt. Mayon.
The flood plains of San Francisco River are located at the mid eastern area of the Municipality while flood plains of Cabagsay River are located at the Southern most tip of the Municipality. These areas have a level to nearly slope ranging from 0 to 30% (Table 2.1). The elevation of this flood plain ranges from 0 to 100 meters above mean sea level. The area covered by the flood plains is estimated at 14,231.40 hectares, which comprises 70.11% of the total land area of the municipality. These area are generally irrigated and/or rainfed field of about 358 hectares.
The rolling hills located at the western to southern section of the municipality have elevation ranging from 100 to 500 meters above sea level. Slope description of these hills ranges from greatly sloping (4%) to undulating (8%). These areas covering 2,119.98 hectares are generally planted with coconut, corn and small [patches of rain fed and upland rice. Mt. Mayon is located at the Northern flank of the Municipality. This area is characterized by a slope gradient of 30% and above with an elevation greater than 1,000 meters above mean sea level. The total area is approximately 185 hectares located within the 6 kilometers danger zone in cases of volcanic eruption.
PHYSIOGRAPHIC AND SOIL
Based on the Comprehensive Land Use Plan of the Municipality of Guinobatan, the soil series in the municipality consist of four (4) types. These are (1) Mayon gravely sandy loam found in the Northeastern portion of the municipality; (2) Leagzpi sandy loam found in barangays along slope of Mt. Myon; Guinobatan sandly loam, and (4) Mauraro gravely sandy loam. The two latter soil types are the predominant soil types in the rolling hills located at the southern flanked of the municipality.
The predominant soil type of the alluvial plains is also the Legazpi sandy loam. The internal drainage of this soil type varies from very poor to moderately well drain. The pH varies from 5.5 to 7.6. The content or organic matter varies from low to medium (1.7% to 2.9%). The base saturation percentage is medium (range from 42% to 52%) and action exchange capacity is from low to medium (8 to 19 meg/100 gm. of soil).
The organic matter content of Guinobatan sandy loam is between low to medium (1.6% to 2.6%). The base saturation varies from medium to high (42% to 69%), and action exchange capacity from medium to high (22 to 60 meg/100 gm. of sol).
More than one third of the land area of Guinobatan is susceptible to severe soil erosion. The geographical feature, specifically its slope and soil type, coupled with the existing vegetation cover of the project area makes it vulnerable to soil erosion hazards.
The soil in the municipality consists of four (4) types. They are: (1) Mayon gravelly sandy loam, (2) Legazpi sandy loam, covering the barangays in the Mayon Sector, (3) Guinobatan sandy loam (4) Mauraro gravelly sandy lam, covering the areas toward the highland Sector. The proximity of Mayon Volcano influences greatly the soil typology of Guinobatan. In the northeastern portion of the municipality, the prevalent soils found are the Mayon gravelly sandy loam and legazpi sandy loam while on the southern hilly sides, the Guinobatan sandy loam and Mauraro gravelly sandy loam cover the area.
Past volcano eruptions of Mayon Volcano have a boon and bone for Guinobatan soils, while each eruption had caused damages to particular agricultural areas due to mudflows, siltation of sand and stones, the accumulation of the more beneficial volcanic deposits over the centuries have generally enriched the land for multi-crop uses.