LET Elementary Earth & Space Science — Earth's Structure, Geology, Weather and ClimateDetailed Explanation

Earth's Layers: Structure from Crust to Core

Earth is structured in concentric layers, much like the layers of an onion or the cross-section of a hard-boiled egg. Scientists classify these layers in two ways: by chemical composition and by physical behavior (how the material acts under heat and pressure). Understanding both classifications is important for the LET. CLASSIFICATION BY COMPOSITION: 1. CRUST — the thin, solid, rocky outermost shell of Earth. There are two types: oceanic crust (found under the oceans, thinner at about 5–10 km, denser, made mostly of basalt) and continental crust (the landmasses, thicker at about 30–70 km, less dense, made mostly of granite). The crust is the thinnest layer of Earth. 2. MANTLE — the thick middle layer directly beneath the crust. It is about 2,900 km thick, making it the largest layer by volume. It is composed of hot, semi-solid rock that moves very slowly. The upper mantle is more solid while the lower mantle is more plastic (able to flow). Heat-driven movement in the mantle (called convection currents) is the engine that moves tectonic plates. 3. CORE — the metallic center of Earth, composed mainly of iron and nickel. It has two parts: the OUTER CORE (liquid, about 2,300 km thick) and the INNER CORE (solid, about 1,200 km radius). Even though the inner core is hotter than the outer core, it remains solid because of the enormous pressure squeezing the atoms together. The churning liquid outer core generates Earth's magnetic field, which protects life from harmful solar radiation. CLASSIFICATION BY PHYSICAL BEHAVIOR: 1. LITHOSPHERE — the rigid outer layer made up of the crust plus the very top of the upper mantle. It is broken into tectonic plates. 2. ASTHENOSPHERE — the weak, semi-molten layer just below the lithosphere in the upper mantle. Tectonic plates float and slide on this layer. 3. MESOSPHERE — the strong lower part of the mantle. 4. OUTER CORE — liquid. 5. INNER CORE — solid. A useful analogy for pupils: Think of a hard-boiled egg. The shell is the crust, the egg white is the mantle, and the yolk is the core. For the LET, always remember: CRUST is thinnest, MANTLE is largest, INNER CORE is solid (not liquid) because of pressure.

Plate Tectonics: Moving Plates and Their Boundaries

The theory of plate tectonics is the unifying theory of geology, explaining the origin of mountains, ocean trenches, volcanoes, earthquakes, and the distribution of continents. It builds on Alfred Wegener's earlier hypothesis of CONTINENTAL DRIFT (proposed around 1912), which argued that all continents were once joined as a single supercontinent called PANGAEA (meaning 'all lands'). Wegener's evidence included: (1) matching coastline shapes — South America and Africa fit like puzzle pieces; (2) identical fossils found on continents now separated by oceans; (3) matching rock formations and mountain belts across oceans; and (4) evidence of past climate zones in unexpected places (coal, which forms in tropical swamps, found in Antarctica). Today, plate tectonics adds the mechanism Wegener lacked: CONVECTION CURRENTS in the mantle. Hot rock rises, cools, and sinks in slow circular currents, dragging the lithospheric plates along. Earth's lithosphere is divided into about 15 major tectonic plates (such as the Pacific Plate, Eurasian Plate, Philippine Plate, Indo-Australian Plate) and several minor ones. TYPES OF PLATE BOUNDARIES: 1. CONVERGENT BOUNDARY — plates move TOWARD each other. - Oceanic-continental convergence: The denser oceanic plate sinks beneath the continental plate in a process called SUBDUCTION. The subducted plate melts, creating magma that feeds volcanoes. Example: The Philippine Plate subducting beneath the Eurasian Plate creates the Philippine Trench and the volcanoes of the Philippines. - Oceanic-oceanic convergence: One oceanic plate subducts beneath the other, forming deep ocean trenches and island arc volcanoes. - Continental-continental convergence: Both plates are too light to subduct; they crumple and fold upward, forming mountain ranges. Example: The Himalayas formed when the Indian Plate collided with the Eurasian Plate. 2. DIVERGENT BOUNDARY — plates move AWAY from each other. - New oceanic crust forms as magma rises and solidifies in the gap. This creates mid-ocean ridges (underwater mountain chains). Example: The Mid-Atlantic Ridge. - On continents, divergence creates rift valleys. Example: The East African Rift Valley. 3. TRANSFORM BOUNDARY — plates SLIDE PAST each other horizontally. - No crust is created or destroyed. - Produces frequent, powerful earthquakes. - Example: The San Andreas Fault in California. PHILIPPINE CONTEXT: The Philippines sits at the intersection of the Pacific Plate, the Philippine Plate, the Eurasian Plate, and other microplates, right in the heart of the Pacific Ring of Fire. The convergence of these plates creates the Philippine Trench (one of the deepest ocean trenches in the world), the Marikina Valley Fault System, and a chain of active volcanoes including Mayon, Taal, Bulusan, Kanlaon, and Pinatubo. RING OF FIRE: A horseshoe-shaped zone around the Pacific Ocean where about 75% of the world's volcanoes and 90% of the world's earthquakes occur. It marks the boundaries of several converging plates. The Philippines, Japan, Indonesia, and the western coasts of the Americas all lie within the Ring of Fire.

Earthquakes and Volcanoes: Hazards on the Ring of Fire

Because the Philippines is on the Ring of Fire, earthquakes and volcanoes are the most personally relevant Earth Science hazards for Filipino pupils and teachers. The LET tests precise vocabulary, so know these terms thoroughly. EARTHQUAKES: An earthquake is the sudden shaking of the ground caused by the rapid release of energy stored in Earth's crust, usually along a FAULT — a fracture or zone of weakness in rocks where two blocks of rock move against each other. Key vocabulary: - FOCUS (also called HYPOCENTER): the point UNDERGROUND where the earthquake rupture begins and energy is first released. - EPICENTER: the point on the SURFACE directly above the focus. This is where shaking is usually strongest. - SEISMIC WAVES: energy waves that travel outward in all directions from the focus. They are recorded by a SEISMOGRAPH (or seismometer), and the record is called a SEISMOGRAM. - MAGNITUDE: a measure of the total ENERGY released by an earthquake (measured by the Richter scale or moment magnitude scale). It is a single number for the whole event. - INTENSITY: a measure of the STRENGTH OF SHAKING felt at a specific location. Intensity decreases with distance from the epicenter. In the Philippines, the PHIVOLCS Earthquake Intensity Scale (PEIS) is used. Earthquake-associated hazards: - LIQUEFACTION: waterlogged, loosely packed soil behaves like liquid during shaking, causing buildings to sink or tip over. - TSUNAMIS: series of large ocean waves triggered by undersea earthquakes (or underwater landslides and volcanic eruptions) that displace a large volume of water. Tsunamis travel at jet-plane speed across the ocean and slow down but grow in height as they approach shore. - LANDSLIDES: shaking can trigger slope failures, especially on steep or unstable ground. - AFTERSHOCKS: smaller earthquakes that follow the main quake. VOLCANOES: A volcano is an opening (vent) in Earth's crust through which MAGMA (molten rock below the surface), volcanic gases, and ash are expelled. Once magma reaches the surface, it is called LAVA. Types of volcanic activity: - LAVA FLOWS: streams of molten rock moving downhill. - ASHFALL: fine particles of pulverized rock ejected high into the air; heavy ashfall collapses roofs and is a respiratory hazard. - PYROCLASTIC FLOWS: fast-moving (up to 700 km/h), extremely hot mixtures of gas, ash, and rock fragments rushing down volcanic slopes — the most immediately deadly volcanic hazard. - LAHARS: volcanic mudflows formed when volcanic ash and debris mix with water (from rain, melting snow, or crater lakes). In the Philippines, lahars from Pinatubo's 1991 eruption devastated communities in Pampanga and Zambales even years after the eruption. Volcanoes are classified by activity: - ACTIVE: has erupted recently or shows signs of activity. - DORMANT: has not erupted recently but could erupt in the future. - EXTINCT: unlikely to erupt again. PHIVOLCS monitors all active volcanoes in the Philippines and issues Alert Levels (0 to 5) to guide local government evacuations. Teaching note: When discussing these hazards with pupils, disaster preparedness must be integrated. This connects to DepEd's Disaster Risk Reduction and Management (DRRM) program and the school's mandate under RA 10121 (Philippine DRRM Act) to educate pupils about hazard readiness.

Rocks and Minerals: The Rock Cycle

A fundamental distinction for the LET: a MINERAL is not the same as a ROCK. MINERALS: A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly internal crystal structure. Key characteristics: (1) naturally occurring — not man-made; (2) inorganic — not from living things; (3) solid — has a definite shape; (4) definite chemical composition — each mineral has a specific formula; (5) crystalline structure — atoms are arranged in a repeating pattern. Examples of common minerals: - QUARTZ (SiO₂): the most common mineral in Earth's crust; clear to white. - FELDSPAR: most abundant mineral group; pink or white; found in granite. - CALCITE (CaCO₃): found in limestone and marble; fizzes with acid. - MICA: shiny, flaky mineral; peels in thin sheets. - IRON PYRITE ('fool's gold'): metallic yellow; often mistaken for gold. Minerals are identified by physical properties: color, streak, luster, hardness (Mohs hardness scale from 1–10), cleavage, fracture, and specific gravity. ROCKS: A rock is a naturally occurring solid made of one or more minerals (or mineral-like materials). Rocks are classified into three types based on how they form, and these three types are connected by the ROCK CYCLE. 1. IGNEOUS ROCKS — form from the cooling and solidification of MAGMA or LAVA. - INTRUSIVE (plutonic) igneous rocks form when magma cools SLOWLY deep underground. Slow cooling = large crystals. Example: GRANITE. - EXTRUSIVE (volcanic) igneous rocks form when lava cools QUICKLY at or near the surface. Fast cooling = small or no crystals. Examples: BASALT (most common oceanic rock), PUMICE (so porous it floats on water), OBSIDIAN (natural glass). 2. SEDIMENTARY ROCKS — form from the compaction and cementation of SEDIMENTS (fragments of rock, minerals, and organic material) that accumulate in layers. Sedimentary rocks are characterized by LAYERS (called strata) and are the only rock type that commonly contains FOSSILS. - Examples: SANDSTONE (sand grains compacted), LIMESTONE (from shells and marine organisms; composed of calcite), SHALE (compacted clay), CONGLOMERATE (rounded pebbles cemented together). - Coal is a sedimentary rock formed from ancient plant material. 3. METAMORPHIC ROCKS — form when existing rocks (igneous, sedimentary, or other metamorphic rocks) are changed by intense HEAT, PRESSURE, or chemically active fluids — without melting completely. Metamorphism changes the texture and sometimes the mineral composition of rocks. - Parent rock → Metamorphic rock: LIMESTONE → MARBLE; SHALE → SLATE; GRANITE → GNEISS; SANDSTONE → QUARTZITE. - Metamorphic rocks often show FOLIATION — parallel alignment of minerals giving a layered or banded appearance. THE ROCK CYCLE: The rock cycle describes the slow, continuous transformation of rocks from one type to another. No rock type is permanent; all can be transformed given the right conditions: - Igneous rock can be weathered and eroded into SEDIMENTS → buried and compacted → SEDIMENTARY rock. - Sedimentary rock can be buried deeper, subjected to heat and pressure → METAMORPHIC rock. - Metamorphic rock (or any rock) can be heated until it melts into MAGMA → cools → IGNEOUS rock. - Any rock can return to magma and restart the cycle. Important exam point: FOSSILS are almost always found in SEDIMENTARY rocks because the quiet, layered depositional environment preserves organic remains. The rapid cooling of igneous rocks and the intense heat/pressure of metamorphism destroy organic material.

The Water Cycle (Hydrologic Cycle)

The water cycle, also called the HYDROLOGIC CYCLE, is the continuous movement of water through Earth's systems: the ocean, atmosphere, land surface, and underground. It is powered by two main forces: SOLAR ENERGY (the Sun) drives evaporation and keeps water moving; GRAVITY pulls water downward as precipitation and runoff. Water on Earth is conserved — the total amount stays roughly constant. Water simply changes form (liquid, vapor, ice) and location as it moves through the cycle. This is a key principle. MAIN PROCESSES OF THE WATER CYCLE: 1. EVAPORATION — liquid water (from oceans, lakes, rivers) absorbs heat energy from the Sun and changes into WATER VAPOR (gas) that rises into the atmosphere. The ocean is the largest source of water vapor. Evaporation is the primary way water enters the atmosphere. 2. TRANSPIRATION — plants absorb water through their roots and release water vapor through tiny pores in their leaves called STOMATA. This is essentially evaporation from plants. EVAPOTRANSPIRATION refers to the combined process of evaporation + transpiration. 3. CONDENSATION — as water vapor rises, it cools. Cool air cannot hold as much water vapor as warm air. When vapor cools enough, it condenses into tiny liquid water droplets around tiny particles (dust, pollen) in the air called CONDENSATION NUCLEI. These droplets form CLOUDS and FOG. Condensation is the process that forms clouds. 4. PRECIPITATION — when water droplets in clouds combine and grow heavy enough, they fall back to Earth as RAIN, SNOW, SLEET, HAIL, or FREEZING RAIN, depending on temperature. In the Philippines, precipitation is mostly RAIN. 5. COLLECTION / RUNOFF — precipitation that reaches the surface either: - Flows overland as SURFACE RUNOFF into rivers, streams, and eventually the ocean. - Is absorbed into the ground as INFILTRATION, replenishing GROUNDWATER (water stored in underground aquifers). - Flows into LAKES and RESERVOIRS. - Is absorbed by plant roots (used in transpiration). - Returns to the atmosphere via evaporation. 6. SUBLIMATION — in cold climates, ice and snow can change directly from solid to vapor without melting. Less relevant to tropical Philippines but may appear on exams. IMPORTANT FACTS ABOUT EARTH'S WATER: - About 97% of Earth's water is SALT WATER in the oceans. - Only about 3% is FRESH WATER, and most of that (~70% of fresh water) is locked in glaciers and ice caps. - Less than 1% of Earth's total water is readily available fresh water in rivers, lakes, and groundwater. PHILIPPINE CONTEXT: The Philippines has abundant rainfall due to its tropical location and surrounding oceans. However, the country faces water security issues due to water pollution, deforestation (which reduces infiltration and increases runoff/flooding), and seasonal drought. Typhoons contribute enormous rainfall in short periods, causing flooding rather than replenishing groundwater effectively. Deforestation in the Philippines disrupts the water cycle by reducing transpiration and infiltration, increasing runoff and erosion.

Weathering and Erosion: Breaking Down and Moving Rock

Weathering and erosion are closely related processes that reshape Earth's surface over time, but they are DISTINCT and frequently confused on the LET. The key distinction is whether the rock material is moving or staying in place. WEATHERING is the BREAKING DOWN or DECOMPOSITION of rocks and minerals at or near Earth's surface, IN PLACE — the material does not move to a new location. Weathering is caused by exposure to atmospheric conditions: water, oxygen, carbon dioxide, temperature changes, and biological activity. Two types of weathering: 1. PHYSICAL (MECHANICAL) WEATHERING: Rock is broken into SMALLER PIECES without changing its CHEMICAL COMPOSITION. The rock still has the same minerals; it is just in smaller fragments. Causes include: - FROST ACTION / ICE WEDGING: Water seeps into cracks in rock and freezes, expanding by about 9% and widening the crack. Repeated freezing and thawing eventually splits the rock. Less common in tropical Philippines but relevant to high-altitude areas. - THERMAL EXPANSION AND CONTRACTION: Rocks expand when heated and contract when cooled. Repeated temperature changes can cause surface layers to crack and peel (EXFOLIATION). - PLANT ROOTS: Roots growing into rock cracks exert pressure and widen them. - ABRASION: Rocks and particles carried by water, wind, or ice scrape and scratch other rocks, wearing them down. - PRESSURE RELEASE (UNLOADING): When overlying rock is removed (by erosion), the rock below expands and fractures parallel to the surface. 2. CHEMICAL WEATHERING: The CHEMICAL COMPOSITION of the rock is changed, forming new minerals or dissolving the original ones. Common processes: - OXIDATION (RUSTING): Iron-bearing minerals react with oxygen and water to form iron oxide (rust). Reddish coloring of rocks and soils indicates oxidation. - CARBONATION / ACID RAIN: Carbon dioxide dissolves in rainwater to form weak CARBONIC ACID (H₂CO₃). This acid reacts with carbonate minerals (like calcite in limestone), dissolving them. This creates KARST LANDSCAPES: caves, sinkholes, and stalactites/stalagmites. Puerto Princesa's Underground River in Palawan is a classic Philippine karst feature. - HYDROLYSIS: Water reacts with silicate minerals (like feldspar) to form clay minerals. - BIOLOGICAL WEATHERING: Lichens and mosses release mild acids that dissolve rock; roots of plants also contribute to both physical and chemical weathering. In tropical, humid environments like the Philippines, CHEMICAL WEATHERING dominates because high temperatures, abundant rainfall, and lush vegetation accelerate chemical reactions. EROSION is the PICK-UP and TRANSPORT of weathered material (sediment) from one place to another by AGENTS OF EROSION. Without weathering first loosening material, erosion cannot easily occur — they work as a team. Agents of erosion: - WATER (most important in the Philippines): rivers, rain, waves, currents. - WIND: significant in dry, arid areas; less so in humid Philippines. - ICE (GLACIERS): powerful agents in polar and high-altitude regions; not relevant to the Philippines. - GRAVITY: landslides and mass wasting — very relevant in the mountainous Philippines. DEPOSITION is the DROPPING of eroded material when the transporting agent slows down or loses energy. Deposition creates landforms such as: - DELTAS: fans of sediment at river mouths (e.g., Cagayan River delta). - FLOOD PLAINS: flat areas of deposited sediment along rivers. - SAND BARS and BEACHES: deposited sand along coasts. - ALLUVIAL FANS: fan-shaped deposits at the base of slopes. A MEMORY TOOL: 'Weathering BREAKS IT, Erosion MOVES IT, Deposition DROPS IT.' EROSION VERSUS WEATHERING on the LET: If a question describes rock breaking down or dissolving IN PLACE, it is WEATHERING. If material is being CARRIED AWAY by water, wind, or ice, it is EROSION.

Weather versus Climate: Elements, Patterns, and the Philippine Setting

WEATHER and CLIMATE are related but fundamentally different concepts, and the LET consistently tests the distinction between them. WEATHER is the current or short-term state of the ATMOSPHERE at a specific place. Weather changes from hour to hour and day to day. 'It is raining heavily in Cebu City today' describes weather. CLIMATE is the long-term AVERAGE PATTERN of weather conditions in a region over many years (typically 30 years or more). Climate describes what is NORMAL for a place across seasons. 'The Philippines has a tropical climate with a wet and dry season' describes climate. A useful memory aid: 'Climate is what you EXPECT; weather is what you GET.' ELEMENTS OF WEATHER AND CLIMATE (same elements, different time scale): 1. TEMPERATURE: degree of hotness or coldness of the air. 2. HUMIDITY: amount of water vapor in the air. HIGH humidity = more moisture in the air; the Philippines has high humidity due to its warm ocean surroundings. 3. AIR PRESSURE (ATMOSPHERIC PRESSURE): the weight of air pushing down on a surface. Measured by a BAROMETER. Low pressure often brings cloudy, rainy weather; high pressure brings clear skies. 4. WIND: horizontal movement of air from areas of HIGH pressure to areas of LOW pressure. Measured by an ANEMOMETER (speed) and WIND VANE (direction). 5. CLOUD COVER: amount of clouds in the sky (affects temperature and precipitation). 6. PRECIPITATION: water falling from the atmosphere (rain, snow, hail). PHILIPPINE CLIMATE: The Philippines has a TROPICAL MARITIME CLIMATE — warm, humid, and rainy due to its location near the equator and its surrounding warm seas. The country experiences two main seasons: 1. WET SEASON (RAINY SEASON): roughly June to November, characterized by heavy rainfall, often brought by the HABAGAT (southwest monsoon) — warm, moisture-laden winds blowing from the southwest during these months. Typhoons are most frequent during this period. 2. DRY SEASON: roughly December to May. The AMIHAN (northeast monsoon) blows cool, dry winds from the northeast from November to February or March, bringing cool and relatively dry conditions, especially to the northern and eastern Philippines. PHILIPPINE CLIMATE TYPES (PAGASA Classification): - TYPE I: Two pronounced seasons — dry from November to April, wet the rest of the year (western coast). - TYPE II: No dry season; very pronounced wet season from December to February (eastern coast, e.g., Eastern Samar). - TYPE III: No pronounced maximum rainfall period; short dry season (3 months or less). - TYPE IV: Rainfall more or less evenly distributed throughout the year. CLIMATE CHANGE: Climate change refers to long-term shifts in global climate patterns, primarily caused by increasing concentrations of GREENHOUSE GASES (especially CO₂ from burning fossil fuels, methane, and nitrous oxide) in the atmosphere. These gases trap heat radiated from Earth's surface, warming the atmosphere — the enhanced GREENHOUSE EFFECT. Consequences for the Philippines: - More intense and frequent TYPHOONS (stronger storms due to warmer ocean temperatures). - Rising SEA LEVELS (thermal expansion of oceans + melting of glaciers) threatening low-lying islands like those in the Visayas and Malabon, NCR. - More severe DROUGHTS linked to EL NIÑO (a periodic warming of the central and eastern Pacific Ocean that reduces rainfall in the Philippines and Southeast Asia). - Disrupted agricultural seasons, affecting rice and corn production. - Coral reef bleaching (Philippine seas are biodiversity hotspots). EL NIÑO and LA NIÑA: - EL NIÑO: Periodic warming of the Pacific Ocean's surface. Effects in the Philippines: DROUGHT, reduced rainfall, dry spells that harm agriculture. - LA NIÑA: Periodic cooling of the Pacific surface. Effects in the Philippines: MORE RAINFALL, stronger typhoons, flooding. MEASURING WEATHER — Weather Instruments: - THERMOMETER: measures temperature. - BAROMETER: measures air pressure. - HYGROMETER: measures humidity. - ANEMOMETER: measures wind speed. - WIND VANE / WEATHER VANE: measures wind direction. - RAIN GAUGE: measures amount of rainfall. PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration) is the national agency responsible for weather forecasting and typhoon warnings in the Philippines. It issues the Public Storm Warning Signal (PSWS) system (Signals 1–5 now used). PAGASA also maintains climate records and monitors seasonal changes.

Natural Hazards in the Philippines: Typhoons, Floods, and Disaster Preparedness

The Philippines is one of the world's most disaster-prone countries. Its location on the Ring of Fire and in the western Pacific typhoon belt exposes it to a wide range of natural hazards. Future teachers must understand these hazards both scientifically and in terms of their responsibilities to keep pupils safe, consistent with RA 7836 (Philippine Teachers Professionalization Act) and DepEd's DRRM mandate. TYPHOONS (TROPICAL CYCLONES): A TROPICAL CYCLONE is a large, rotating storm system that forms over warm tropical oceans when sea surface temperatures reach 26–27°C or higher. It draws energy from the warm water, creating a system of spiral rain bands, strong winds, and a calm EYE at the center. In different ocean regions, the same type of storm is called: - TYPHOON: in the western Pacific (Philippines, Japan, China) - HURRICANE: in the Atlantic and eastern Pacific (Americas) - CYCLONE: in the Indian Ocean and South Pacific They are classified by maximum sustained wind speed. In the Philippines, PAGASA uses the following categories: - Tropical Depression (TD): winds below 63 kph - Tropical Storm (TS): 63–88 kph - Severe Tropical Storm (STS): 89–117 kph - Typhoon (TY): 118–184 kph - Super Typhoon (STY): above 185 kph Hazards associated with typhoons: 1. DESTRUCTIVE WINDS: damage to structures, trees, and infrastructure. 2. HEAVY RAINFALL: causing floods and landslides. 3. STORM SURGE: an abnormal rise in sea level along the coast caused by the typhoon's low pressure and strong onshore winds pushing water inland. Storm surges were responsible for the greatest loss of life during Super Typhoon Yolanda (Haiyan, 2013), which struck the Eastern Visayas. Storm surges are DIFFERENT from tsunamis — surges are wind-driven, tsunamis are seismically driven. FLOODS AND LANDSLIDES: Floods occur when waterways overflow their banks, often due to heavy monsoon rains, typhoons, or upstream deforestation. FLASH FLOODS are rapid flooding in low-lying areas, often with little warning. LANDSLIDES (mass movements) are triggered by heavy rains saturating steep slopes, by earthquakes, or by deforestation that removes stabilizing root systems. The Philippines has several landslide-prone regions including the Cordillera, Leyte, and many parts of Mindanao. EARTHQUAKES, VOLCANIC ERUPTIONS, AND TSUNAMIS: (covered in detail in previous concepts — see above) DROUGHT: Prolonged below-normal rainfall, often linked to EL NIÑO events. Drought reduces water supply for drinking and irrigation, causes crop failures, and increases wildfire risk. TEACHER'S RESPONSIBILITY — DRRM IN SCHOOLS: Under RA 10121 (Philippine Disaster Risk Reduction and Management Act) and DepEd Order No. 23, s. 2015 (Comprehensive School Safety Framework), schools are required to: 1. Conduct regular EARTHQUAKE DRILLS (duck, cover, hold) and FIRE DRILLS. 2. Maintain updated SCHOOL DRRM PLANS and evacuation maps. 3. Teach DRRM concepts integrated across subjects. 4. Suspend classes based on PSWS issued by PAGASA. As a professional teacher under RA 7836 and the Code of Ethics for Professional Teachers, protecting pupils' safety during disasters is not optional — it is a professional and ethical obligation. Article IX of the Code of Ethics states that teachers must be vigilant in looking after the health and welfare of their pupils.

Earth's layers from outside to inside: (1) Crust — the thin, solid outermost shell; (2) Mantle — the thick middle layer of hot, semi-solid rock; (3) Outer Core — liquid iron-nickel that generates Earth's magnetic field; (4) Inner Core — solid iron-nickel kept solid by extreme pressure. The inner core is the hottest but the pressure keeps it solid.

Identical fossils of land or freshwater organisms on widely separated continents is key evidence for continental drift — these organisms could not have crossed a wide ocean, so the continents must have once been connected. Wegener used this type of evidence (along with matching coastlines and rock formations) to propose that all continents were once joined as Pangaea. The modern theory of plate tectonics provides the mechanism for how they drifted apart.

Pyroclastic flows are the most immediately deadly volcanic hazard — superheated (up to 700°C), fast-moving, and leaving no time to escape. Lahars form when volcanic ash and debris are saturated with water from rain or melting snow/ice. Mt. Pinatubo's 1991 eruption created lahars that devastated Pampanga and Zambales communities for years after the eruption ended.

The two defining characteristics of sedimentary rocks are: (1) formation in distinct LAYERS (strata) and (2) preservation of FOSSILS. Fossils are almost always found in sedimentary rocks because the slow, gentle depositional environment preserves organic material, unlike the rapid cooling of igneous rocks or the intense heat/pressure of metamorphic formation which would destroy fossils.

Step 1: EVAPORATION — the Sun heats ocean water, converting liquid water to water vapor that rises into the atmosphere. Step 2: CONDENSATION — as water vapor rises and cools, it condenses into tiny water droplets around dust particles, forming clouds. Step 3: PRECIPITATION — water droplets in clouds combine and grow heavy enough to fall as rain (or snow at higher altitudes). Step 4: RUNOFF — the precipitation that falls on land flows overland into rivers, streams, and eventually back to the ocean; some infiltrates the ground as groundwater.

Key distinction: the marble is not just being broken into smaller pieces — its CHEMICAL COMPOSITION is being changed. This is chemical weathering, specifically carbonation. This same process forms limestone caves (karst landscapes) like Puerto Princesa's Underground River in Palawan. Acid rain from industrial pollution accelerates this process in urban areas.

WEATHER refers to the current, short-term atmospheric conditions at a specific place and time — 'today's temperature' is a snapshot of the present. CLIMATE refers to the long-term average pattern of atmospheric conditions for a region over many years — 'known for cool temperatures throughout the year' is based on decades of accumulated data and represents what is TYPICAL for Baguio. Memory device: Climate is what you EXPECT; weather is what you GET.

This is a classic LET question type that tests agency function: PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration) = everything atmospheric: typhoons, weather forecasts, seasonal outlooks, rainfall advisories. PHIVOLCS (Philippine Institute of Volcanology and Seismology) = everything geological: volcanic eruptions, earthquakes, tsunamis. The boundary is clear: atmospheric = PAGASA; geological/seismic/volcanic = PHIVOLCS.

El Niño (Spanish for 'The Child') is a climate pattern that recurs every 2–7 years and significantly affects Philippine weather. Its opposite, La Niña (cooling of the Pacific), brings above-normal rainfall and stronger typhoons to the Philippines. LET questions about El Niño in the Philippines always focus on DROUGHT as its main local effect. The contrast with La Niña (more rain/stronger typhoons) is also frequently tested.

The rock cycle is not a simple one-way process — any rock type can potentially become any other type given the right conditions. Limestone, a sedimentary rock, under increasing heat and pressure becomes marble (metamorphic transformation). With even greater heat, rock melts into magma. When that magma cools slowly deep underground over thousands of years, large crystals form, producing granite (an intrusive igneous rock). This demonstrates that the rock cycle can go in any direction and that rocks are in constant, albeit very slow, transformation.