Geological history
The islands of the Philippines were developed through volcanic activity in the sea floor millions of years ago. The above figure shows how the islands have formed over the last 30 million years. The Philippines first started to form in the mid-Oligocene, around 30 million years ago, when the land of mass that is now Australia started to drift northward, causing an uplift in the Pacific sea floor. At the same time, a part of the Asian continental shelf drifted southward, creating the landform which now consists of modern day Palawan and Mindoro islands. The years of the Miocene epoch saw a rise in a series of volcanic islands in shallow waters, and further subduction of converging plates in the Pacific formed other larger islands such as Visayas and Mindanao. It was only after the Pilocene that the smaller islands emerged above sea level, forming the chain of 7,000 islands that make up the country today. The tectonic movements caused complex deformation in the crust and developed geologic structures such as thrust faults and strike-slip faults. Compression due to central Luzon's westward movement along with the rest of the Philippine Sea Plate caused the formation of large thrust components in the norther Philippines (Rimando et al, 2006).
Geology of Manila today
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The city of Manila rests upon alluvial deposits from Pasig River, which is a tidal estuary that connects Laguna de Bay to Manila Bay. The direction of flow of water depends on the relative water levels of Laguna de Bay and Manila Bay.
Metro Manila is built on an extensive flood plain, and has little variation in elevation. According to the Metropolitan Manila Development Authority (MMDA), Manila is "relatively flat of about 0.81% to very gently sloping at 3.17%". The flatness of the region makes Metro Manila prone to flood. The city of Manila is especially vulnerable to flood because of its location on the opening of Pasig River, and the fact that it is built on a riparian system of streams, tributaries, rivulets, creeks and canals (Ketsana Ondoy Watch).
Metro Manila is built on an extensive flood plain, and has little variation in elevation. According to the Metropolitan Manila Development Authority (MMDA), Manila is "relatively flat of about 0.81% to very gently sloping at 3.17%". The flatness of the region makes Metro Manila prone to flood. The city of Manila is especially vulnerable to flood because of its location on the opening of Pasig River, and the fact that it is built on a riparian system of streams, tributaries, rivulets, creeks and canals (Ketsana Ondoy Watch).
Stratigraphy
The Philippine Islands are located on the outer edge of a continental shelf, and therefore have relatively modern sediments. Older formations are buried deep under the surface, and not enough time has passed for erosion to expose the rocks. Throughout Luzon Island, igneous rock consisting of diorites, andesites, diorite intrusions and other granites are buried by tertiary sediments. Although these rocks are more visible in northern Luzon where more vigorous erosion has taken place, it is less exposed in Manila where it is seen only in isolated locations such as Ambos Camarines, the Loboo Mountains of Batangas, etc (Warran, 1915).
![Picture](/uploads/2/7/2/4/27249943/8760287.jpg?487)
The map on the left shows the geomorphological classification of Metro Manila. It also shows the locations of active faults in the region. Manila is situated on the Coastal Lowland region, which consists of soft sand and clay deposits that can reach 40m in depth (Miura et al., 2008).