15.1 Plate Tectonics, Rocks, and Natural Disasters

Why This Matters for the USTET

Science is one of four equal-weight USTET subtests (Mental Ability, English, Mathematics, and Science). Within Science, Earth Science draws heavily from the Grade 11 Earth and Life Science strand: Earth's internal structure, the rock cycle, plate tectonics, and geologic hazards. Because the Philippines sits on the Pacific Ring of Fire, exam items often use local volcanoes, trenches, and earthquake vocabulary. Master the boundary types, rock-cycle pathways, and the difference between magnitude and intensity — these are high-yield, concept-based points rather than trivia.

Earth's Internal Structure

Earth is layered by composition and physical behavior. From the surface inward:

Exam tip: The classic trap is asking which core layer is liquid. The outer core is liquid; the inner core is solid. Another frequent item: the crust is thinnest, the mantle is thickest.

Plates are pieces of the lithosphere that ride on the asthenosphere. When you see “plates float on a soft layer,” that soft layer is the asthenosphere — not the outer core.

The Rock Cycle

Rocks are continuously recycled. Three main types and how they form:

Key process vocabulary:

• Weathering — rock breaks down in place (physical or chemical).
• Erosion — fragments are transported by water, wind, ice, or gravity.
• Deposition — sediments settle.
• Compaction and cementation — sediments become sedimentary rock.
• Melting → magma → cooling — pathway into (or back to) igneous rock.
• Magma vs lava — magma is molten rock underground; once it erupts onto the surface it is lava.

Worked logic: A specimen shows horizontal layers and a fossil shell. Fossils and bedding point to sedimentary rock. Fossils almost never survive the temperatures that create igneous or strongly metamorphosed rock. Philippine anchors help memory: limestone landscapes (for example in Bohol) link to sedimentary carbonate rock; Romblon is known for marble, a metamorphic product of limestone.

Any rock type can become any other if the right processes act long enough. That closed loop of melting, cooling, weathering, burial, and metamorphism is the rock cycle — a core Earth and Life Science idea on entrance exams.

Plate Tectonics and Boundary Types

The theory of plate tectonics states that Earth's lithosphere is broken into moving plates. Most earthquakes, volcanoes, and mountain belts concentrate at plate boundaries.

At convergent ocean–continent or ocean–ocean boundaries, the denser oceanic plate sinks in a process called subduction. Subduction creates deep ocean trenches and feeds magma that builds volcanic arcs. At divergent boundaries, seafloor spreading creates new oceanic crust. At transform boundaries, plates grind sideways — energy builds and releases as earthquakes, but large volcanic chains are uncommon.

The Philippine Setting

The archipelago lies within the Philippine Mobile Belt, squeezed between major plates. Broadly:

• The Philippine Sea Plate interacts with the eastern side of the country (including the deep Philippine Trench region).
• To the west, interaction with the Eurasian / Sunda Plate system includes trenches such as the Manila Trench.
• This double-sided tectonic activity places the Philippines on the Pacific Ring of Fire, the nearly continuous belt of volcanoes and earthquakes around the Pacific Ocean.

The Philippine Fault System is a long, largely transform-style fault zone running through much of the archipelago. The West Valley Fault segment near Metro Manila is frequently cited in preparedness materials. Knowing that Philippine seismicity and volcanism are plate-driven — not random — is the conceptual payoff for USTET items.

Earthquakes

An earthquake begins at the underground focus (hypocenter). The point on Earth's surface directly above the focus is the epicenter. Energy travels outward as seismic waves, recorded by a seismograph.

Two measurements are constantly confused — and constantly tested:

A magnitude 7 release is roughly 32 times more energetic than a magnitude 6 release for each whole-number step on the usual logarithmic energy comparison — so “one step higher” is a large jump, not a small one. PHIVOLCS (Philippine Institute of Volcanology and Seismology) monitors earthquakes and volcanoes. Do not mix PHIVOLCS with PAGASA, which handles weather and climate products.

Volcanoes and Related Hazards

Most Philippine volcanoes are tied to subduction. Magma rises above the descending slab, building volcanic arcs. Hot spots (mantle plumes) can also build volcanoes far from plate edges (classic textbook example: Hawaii), but Philippine volcanism is overwhelmingly subduction-related.

Hazard vocabulary worth knowing: lava flows, ash fall, pyroclastic flows (hot gas and rock fragments racing down slopes), lahars (volcanic mudflows, often when heavy rain remobilizes ash), and tsunamis (giant sea waves commonly triggered by undersea earthquakes that displace the seafloor). A sudden, unusual retreat of the sea can be a natural tsunami warning — move immediately to high ground.

Preparedness basics that appear in science and general-awareness style items: during shaking, duck, cover, and hold; expect aftershocks; prepare a go bag (water, food, light, whistle, first aid, documents); heed PHIVOLCS alert levels and permanent danger zones around active volcanoes.

Quick Self-Check Before Moving On

If you can (1) name the three boundary types and what each builds, (2) sort a rock description into igneous / sedimentary / metamorphic, (3) explain why the Philippines has many quakes and volcanoes, and (4) separate magnitude from PHIVOLCS intensity, you are ready for the weather–climate section that follows.