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Geophysical features of the six
Philippine Turtle Islands

 

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Taganak. Taganak is the largest of the islands in the Turtle Islands Group. It has an estimated land area of 116 hectares, and the highest relief with its maximum elevation of 148 m above sea level (ASL, US Coast and Geodetic Survey, 1940). The substrate of Taganak is basalt, with coralline sand at the southern end. Soil cover of the hilly part is red clay derived from the volcanic bedrock. The coastal plain is covered by coralline sand.

Sandy coastline rims the flatlands at the southern end, while shingles beaches made up of coralline rubbles are common in small coves. At the northern end, the coastline is generally rocky. A short stretch of mangrove coastline is found in Bakkao, the western side of the island. The beach area is generally narrow and in most parts bounded by steep slopes of the volcanic hills. At the southern part of the island, the beach and backshore widen due to the presence of a wide coastal plain.

Erosion scars are evident at the southern end of Limao-limao. Shore erosion at the western side of the southern end of Bakkao, on the other hand, has affected a number of houses along the shore. Bare slopes and landslide scarps indicate high rates of soil erosion. A layer of reddish silt is observed along the coast of Bakkao.

The dominant land cover is grass, which covers about 81 hectares of the island's total land area.

Groundwater is available both in the volcanic substrate as well as in the coralline sand flatlands. Water in the volcanic rocks is available in the weathered portion. Water stored in the fractures of the basalt apparently recharges water along the coastal area.

Boan. With an estimated land area of 76 hectares, Boan is the second largest island in the Turtle Islands Group. It is tadpole-shaped, elongated along the northeast-southwest direction. It has a low relief with topography varying from flat to low hills at the northeastern end. Maximum elevation on the island is about 59 m ASL (US Coast and Geodetic Survey 1940).

The substrate of the island is sandstone. Continued extrusion of mud by diapiric activity built up the hill, though residents report that the outflow of mud has ceased for a number of years now. The flat part of the island is made up of an accumulation of coralline sand, coralline rubbles and some sandstone boulders.

The coastline of Boan is dominated by a rocky and shingles beach made up of coralline rubbles. A small patch of mangrove is located at the northeastern side. The sandy part of the coastline is now occupied by settlements. Shoreline retreat is evident in the toppled coconut trees at the eastern part. Scarp also rims this part of the island. To protect houses from shore erosion, fences of round logs have been set up in some parts of the sandy beach. Several berms made up of corraline rubble have been mapped at the northeastern part.

The land cover is dominantly agriculture in the wooded areas in the northern part of the island. The flat part is made up of an accumulation of coralline sand, coralline rubbles and some sandstone boulders. This alluvial deposit makes up the aquifer of the shallow groundwater, which is essentially freshwater lense floating on seawater. The groundwater is directly recharged by rainfall.

Great Bakkungan. This is the third largest island in the Turtle Islands Group, with a total land area of about 51 hectares. The hilly portion was built up by the extrusion of mud by volcanoes, while the flatland is an accumulation of coralline sand and rubble. The highest elevation is 58 m ASL (US Coast and Geodetic Survey 1940). The vent of the mud volcano is reported to have shifted a number of times in the past. The mud volcano's activity is limited to quiet pulsating extrusion of pure gray mud accompanied by gas bubbling. Dark-colored streaks, which appear to be oil film, sometimes accompany the extruded mud. Gas was sampled in April 1998.

The northern coastline is dominated by a rocky shoreline, with the sandy beach limited to the southern part of the island. Settlements proliferate in the area fronted by the sand beach. Like in the other islands, shore erosion seems to be very active. Beach attrition has already affected a number of houses and the barangay hall.

Agriculture is done in woody areas. There is a sizeable coconut plantation. Fresh water is present in the form of shallow groundwater.

Baguan. Baguan is a 29.1-hectare, bell-shaped island with a coastline of 1.7 km. It is characterized by an expansive beach flat at the southern part and a rolling to gently sloping hill in the north. It is volcanic in origin, later enlarged by the deposition of the coastal plain at the southern part. The volcanic part of the island is elevated, with a maximum elevation of about 40 m ASL. The hill is moderately steep.

The sandy flatland in the southern portion of Baguan makes up the largest part of the island. The flatland is 644 m wide at its broadest part with an average elevation of 3 m ASL. Fringing this flatland is a belt of coralline sand beach.

The coastline at the foot of the hills is rocky, with large volcanic boulders. In the south, the coastal plain is surrounded by wide sandy beaches. Erosion and accretion are active, changing the island's outline over time.

Corals have colonized the substrate in the shallow waters surrounding the island. The reef extends approximately 300 m offshore in the south and is widest in the north where it extends approximately 1.2 kilometers offshore.

The accumulation of non-mineral organic materials that act as ephemeral constituents of the beach may have a marked effect on beach processes and morphology. The incorporation of large tree trunks (transported by ocean currents from nearby Sabah) into the beaches has resulted in some temporary beach stability. These debris serve as traps which inhibit the transport of sand and cause the temporary storage of sand and stability of the beach environment. Beach erosion occur in areas adjacent to the beach.

The composition of beach sediments is derived from the available material in nearby areas and reflects the nature of material brought in from the nearshore or alongshore sources. Transport of sand in the surf and intertidal zone accounts for a significant volume of immediate sand supply. Erosion of the coast itself, reworking and shoreward movement of beach and nearshore material are also important sources of sand.

The source of the coralline beach sand is the surrounding fringing reefs. Areas of reef development generally provide locally derived beach materials which include various biogenic carbonate grains that reflect the composition of living biota adjacent to the beach.

Fresh water is present in the form of shallow groundwater.

Lihiman. Lihiman has a land area of approximately 29 hectares, about the same size as Baguan. It is tadpole-shaped and elongated along the northeast-southwest direction. The circular portion at the northeast end was built up by the extrusion of a mud volcano. Compared to those of Boan and Bakkungan, the mud extrusion is relatively more violent and has created a 20-m crater at the hilly portion. Extrusion activity is said to be accompanied by tremors. Extruded material contains a mixture of mud and angular boulders. Dried mud at the top of the agoho trees rimming the crater shows the high pressure at which mud is vented. Due to the huge volume of material discharged, a drainage way has been carved on the northern hill slope leading to the sea.

The northeastern end (head) of Lihiman is lined by a rocky shoreline, while the southeastern part is rimmed by a shingles beach made up of gravel to boulder size rocks. A sandy beach dominates the southern and western shoreline.

Siltation at the northern shoreline is evident. Gray silt deposit, the same color as the mud coming from the mud volcano, is observed in the foreshore. Some local residents have reported that submarine mud extrusion has been observed offshore. This activity is indicated by the highly turbid water in the area.

Groundwater is present in the sandy flatland. Numerous shallow wells have been dug as source of water for the agricultural plots being cultivated by the residents.

Langaan. Langaan is a sandy islet with a land area of about 7 hectares and a coastline of about 458 m. The island is rimmed with wide sandy beach and backshore. Shoreline retreat has been observed at the western side of the island while accretion is noted at the northeastern side. Dense beach vegetation is observed at the western side.

Shallow groundwater is available.


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