9.1 Earth Systems, Rocks, and Natural Disasters

Why This Matters for the PUPCET

Science makes up about 25% of the PUPCET, and Earth science is a guaranteed slice of it. Because the Philippines sits on the Pacific Ring of Fire and is visited by roughly 20 tropical cyclones a year, exam writers love questions about volcanoes, earthquakes, and typhoons — topics every Filipino student has lived through. Master the vocabulary and the scales (PHIVOLCS intensity, PAGASA signals) and these become some of the easiest points on the test.

Layers of the Earth

The Earth is structured like a giant onion. From the surface inward, the layers are:

Exam tip: A classic trap asks which layer is liquid. Remember: the outer core is liquid, the inner core is solid — opposite of what the names suggest. Another favorite: the crust is the thinnest layer, the mantle the thickest.

The Rock Cycle

Rocks continuously change from one type to another through the rock cycle, driven by melting, cooling, weathering, erosion, compaction, and heat-and-pressure.

Worked logic the exam uses: A rock shows visible layers of compressed sand with a fossil shell embedded in it. What type is it? Layered sediments plus fossils can only mean sedimentary. Fossils almost never survive the heat that creates igneous or metamorphic rock, so the fossil is your giveaway. The chocolate hills and limestone formations of Bohol, and the marble quarries of Romblon, are handy Philippine anchors for sedimentary and metamorphic rock respectively.

Key process terms to memorize: weathering (breaking down rock in place), erosion (transporting the fragments), deposition (dropping sediments), compaction/cementation (turning sediment into rock), and magma vs lava — magma is molten rock below the surface; once it erupts it is called lava.

Plate Tectonics and the Philippine Setting

The theory of plate tectonics says the lithosphere is broken into moving plates that ride on the asthenosphere. Where plates meet, three boundary types occur:

• Convergent boundary — plates collide. Oceanic plates slide beneath continental ones (subduction), creating trenches, volcanoes, and earthquakes.
• Divergent boundary — plates pull apart; magma rises to form new crust (mid-ocean ridges).
• Transform boundary — plates slide sideways past each other, producing fault lines and quakes (like the San Andreas Fault in California).

The Philippines sits on the Philippine Mobile Belt, squeezed between two great plates: the Philippine Sea Plate subducts westward under the archipelago along the Philippine Trench (off the eastern coast), while the Eurasian Plate (Sunda Plate) meets us along the Manila Trench to the west. This double subduction places the country squarely in the Pacific Ring of Fire, the horseshoe-shaped zone around the Pacific Ocean where most of the world's earthquakes and volcanic eruptions happen. The Philippine Fault System — a 1,200 km transform fault running from Luzon to Mindanao — generated the deadly 1990 Luzon earthquake (magnitude 7.7) that devastated Baguio and Cabanatuan. The West Valley Fault cutting through Metro Manila is part of this system.

Earthquakes: Magnitude vs Intensity

Two measurements are constantly confused — and constantly tested:

An earthquake starts at the focus (hypocenter) underground; the point on the surface directly above it is the epicenter. Shaking travels as seismic waves. A magnitude 7 quake is roughly 32 times more energetic than a magnitude 6 quake — each whole number is a big jump. PHIVOLCS (Philippine Institute of Volcanology and Seismology) monitors both quakes and volcanoes — expect its acronym on the exam.

Volcanoes

The three main volcano types:

1. Shield volcano — broad, gently sloping; built by runny lava flows (Mauna Loa, Hawaii).
2. Cinder cone — small, steep-sided; built from ash and cinders.
3. Composite (stratovolcano) — tall, symmetrical cones of alternating lava and ash layers; the most dangerous type.

Mayon Volcano in Albay, Bicol, is a stratovolcano world-famous for its perfect cone. Taal Volcano in Batangas is famous as an island within a lake, on an island (Luzon) — it erupted phreatically in January 2020. Mount Pinatubo's 1991 eruption in Zambales was the second-largest eruption of the 20th century; it cooled global temperatures and created the lahar (volcanic mudflow) hazards that buried towns in Pampanga for years after. PHIVOLCS uses Alert Levels 0 (normal) to 5 (hazardous eruption in progress) for volcanoes.

Typhoons and PAGASA

A tropical cyclone forms over warm ocean water (at least about 26.5 °C) where moist air rises, creating low pressure; the Coriolis effect spins the system (counterclockwise in the Northern Hemisphere). The same storm is called a typhoon in the western Pacific, a hurricane in the Atlantic and eastern Pacific, and a cyclone in the Indian Ocean. Its calm center is the eye, ringed by the violent eyewall. The strongest local examples: Typhoon Yolanda (Haiyan, 2013), one of the strongest landfalling cyclones ever recorded, devastated Tacloban with a deadly storm surge.

PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration) names cyclones inside the Philippine Area of Responsibility (PAR) and raises Tropical Cyclone Wind Signals (TCWS):

Exam tip: Do not confuse PAGASA (weather/astronomy) with PHIVOLCS (volcanoes/earthquakes) — the exam swaps them deliberately.

Disaster Preparedness Facts

Commonly tested readiness items: during an earthquake, duck, cover, and hold; after shaking stops, expect aftershocks and check for gas leaks. Prepare a go bag (water, food for 72 hours, flashlight, whistle, first aid, documents). In floods and typhoons, move to higher ground and monitor PAGASA bulletins. Near volcanoes, heed PHIVOLCS alert levels and avoid the permanent danger zone (4 km around Taal's crater island, 6 km around Mayon). For tsunamis — giant waves triggered by undersea earthquakes — the warning sign is the sea suddenly receding; run to high ground immediately.