The rock formation along the highway in San Jose, Dulag, Leyte

Have you ever wondered about the rock along the highway in San Jose, Dulag, Leyte? Especially where the road runs close to the sea, travelers see a striking, dark-colored rock formation exposed by the ongoing road widening. 

The andesite rock along the highway in San Jose, Dulag, Leyte

Formed by volcanism between 2.6 and 23 million years ago during the Miocene and Pliocene epochs, this rock in Dulag is known as andesite. Andesite is an intermediate type of volcanic rock, with silica (SiO2) content ranging between 52% and 63%, commonly found in areas with past or present volcanic activity. 

The Dulag andesite belongs to the Pliocene-Miocene intermediate rocks (Jahn & Asio, 2006)

Andesite got its name from the Andes Mountains in South America, where it is abundant. It is also widespread in volcanic regions around the world, especially along the Pacific Ring of Fire, where many volcanoes produce this type of rock. It is an extrusive (volcanic type) and the most widespread igneous rock in the Philippines. 

Andesite typically appears fine-grained, sometimes with small visible crystals embedded in it called phenocrysts. These crystals are often made of minerals like feldspar and dark-colored minerals such as pyroxene or biotite. Because of this, andesite can look gray, pinkish, or slightly dark, depending on its composition. 

Mineral composition of the andesite in Abuyog: Amp- amphibole; Cpx- clinopyroxene;
Opq-opaque minerals; Pl-plagioclase feldspar

Laboratory examination of thin sections using a petrographic microscope (above photo) revealed that the andesite in Dulag, Leyte, consists primarily of plagioclase feldspar (45% of the mineral content), clinopyroxene (20%), and amphibole (5%), along with minor amounts of secondary minerals. It is darker in color compared to the younger andesite rocks (Quaternary volcanics) in the central highlands of Leyte.

The typical andesite rock in the central highlands of Leyte formed by volcanism during the Quaternary period (2 million years ago up to the present). The sample comes
from Cabintan, Ormoc at an elevation of 900 m above sea level.

Acknowledgement

I thank the National Institute of Geological Sciences (NIGS) at UP Diliman for conducting the thin section analysis of my rock samples.

References

Britannica Editors. "andesite". Encyclopedia Britannica, 5 Jul. 2015, https://www.britannica.com/science/andesite. Accessed 3 April 2026.

Jahn, R., & Asio, V. B. (2006). Climate, geology and soils of the tropics with special reference to Southeast Asia and Leyte (Philippines). In Proceedings of the 11th International Seminar-Workshop on Tropical Ecology (pp. 21-25).