This week's topic is plate tectonics. The discussion will cover how the theory developed, starting with continental drift, then seafloor spreading, and finally the unified theory of plate tectonics. It will also explore plate boundaries, the mechanism behind plate tectonics, and the specific tectonics of the Philippines.

Alfred Wegener, a meteorologist, proposed the continental drift theory in 1915, suggesting a supercontinent called Pangaea existed 200 million years ago and drifted apart. Evidence included the jigsaw-like fit of continents (especially South America and Africa), similar rock ages and mountain belts across continents, distribution of land reptile and plant fossils (Glossopteris), and paleoclimatic indicators like glacial deposits in currently warm regions and coal deposits in Antarctica.

Despite compelling evidence, Wegener couldn't explain the mechanism of continental movement, leading to initial rejection of his theory. With advances in technology after World War II, mapping of the seafloor in the 1950s and 1960s led Harry Hess to propose the seafloor spreading hypothesis. This theory explained that new oceanic crust forms at mid-oceanic ridges and spreads outwards, with the youngest rocks found at the ridges and older ones near trenches. This process doesn't expand the Earth because old crust is recycled at subduction zones in trenches.

During this period, the concept of paleomagnetism emerged. Magnetic minerals in rocks align with the Earth's magnetic field during formation, recording its direction. Studies showed that rocks formed at the same time across different continents had consistent magnetic alignments, supporting that continents move. Furthermore, geomagnetic reversals, where the Earth's magnetic poles flip, leave characteristic 'magnetic stripes' on the seafloor, further confirming seafloor spreading.

All these ideas culminated in the plate tectonics theory, often called the unifying theory of geology. In 1968, Isaacs, Oliver, and Sykes' paper linked global seismicity (earthquake locations) to plate boundaries. The theory posits that the Earth's lithosphere is divided into rigid plates that move and interact, causing volcanism, earthquakes, and mountain building. There are seven major plates and numerous smaller ones, with the Philippines situated on a complex plate boundary.

Divergent boundaries are where plates pull apart, leading to decompression melting and the formation of new crust. Examples include mid-oceanic ridges, like the Mid-Atlantic Ridge which surfaces in Iceland. Continental rifting, such as the East African Rift Valley and the Red Sea, also exemplifies divergent boundaries, where continental landmasses stretch and eventually become new ocean basins.

Convergent boundaries involve plates crashing together. There are three types: oceanic-continental, where the denser oceanic plate subducts beneath the continental plate (e.g., the Andes Mountains); oceanic-oceanic, where the older, denser oceanic plate subducts beneath the younger one, forming island arcs (e.g., the Philippines and Japan) and causing significant volcanism (like Mount Pinatubo); and continental-continental, where both low-density continental plates collide, causing massive deformation and mountain building (e.g., the Himalayas).

Transform boundaries are areas where plates slide horizontally past each other, creating and destroying no crust. The San Andreas Fault in California, where the Pacific Plate slides past the North American Plate, is a prime example. These boundaries can also connect segments of divergent ridges, accommodating differential spreading rates.

The primary mechanism driving plate tectonics is mantle convection. Hot, less dense material from the deep mantle rises (upwelling), while cooler, denser material sinks (downwelling). This convection helps facilitate plate movement. While the exact pattern of convection (whole mantle vs. layered) is still debated, the rising hot material correlates with magmatism and divergent boundaries, and sinking cool material with subduction zones.

Modern technology like GPS provides direct evidence of plate movement. GPS stations worldwide measure the precise movement and velocity of plates, confirming Wegenar's theory with quantitative data. For instance, the Philippines plate moves 7-9 cm per year. Hotspots, which are fixed plumes of abnormally hot mantle material, also serve as evidence. As plates move over these fixed hotspots, they leave a trail of extinct volcanoes, like the Hawaiian island chain, with the active volcano remaining over the hotspot.

The Philippines is a complex island arc system situated on a plate boundary, primarily between the Sunda Plate (west) and the Philippine Sea Plate (east), forming the Philippine Mobile Belt. This region experiences multiple subduction zones, causing significant faulting, volcanism, and earthquake activity. Features include the Manila Trench, Negros Trench, Sulu Trench, and the Philippine Trench. The presence of microcontinental blocks like Palawan (believed to have originated from South China) further adds to its geological complexity. The Philippines is characterized by numerous active and inactive volcanoes and frequent earthquakes.