LET Elementary Earth & Space Science — Earth's Structure, Geology, Weather and ClimateRevision Notes

Exam Tips

• LET favorite: 'Which layer is the thinnest?' → CRUST. 'Which is the largest?' → MANTLE. 'Which core layer is solid?' → INNER CORE.
• Remember that Earth's magnetic field is generated by the movement of the LIQUID OUTER CORE.
• The word 'lithosphere' = lithos (Greek for stone/rock) + sphere; think of it as the stony, rigid outer shell broken into plates.
• Oceanic crust is THINNER but DENSER than continental crust — this is why oceanic plates sink under continental plates at convergent boundaries (subduction).

Key Points

• Earth is structured in concentric layers, like the layers of a sibuyas (onion), each with distinct physical and chemical properties.
• By COMPOSITION (chemical makeup), Earth has three main layers: Crust, Mantle, and Core.
• By PHYSICAL STATE (how the rock behaves), scientists further divide Earth into the lithosphere, asthenosphere, mesosphere, outer core, and inner core.
• The CRUST is the thinnest layer — the outermost solid shell of Earth. Oceanic crust is thinner and denser (made of basalt); continental crust is thicker and less dense (made of granite).
• The MANTLE is the thickest and largest layer by volume. It is made of hot, semi-solid silicate rock. The upper mantle, along with the crust, forms the rigid LITHOSPHERE. Below the lithosphere is the softer ASTHENOSPHERE, where rock flows very slowly.
• The CORE is Earth's center, made mainly of iron and nickel. The OUTER CORE is LIQUID — its movement generates Earth's magnetic field. The INNER CORE is SOLID despite being the hottest part, because extreme pressure keeps it solid.
• Memory aid for layers from outside in: Come, Meet, Our Core (Crust, Mantle, Outer core, Core/inner).

Definitions

Section Title

Earth's Layers

Common Mistakes

• Confusing the MANTLE with the CORE. The mantle is the MIDDLE layer; the core is the CENTER.
• Saying the inner core is liquid — it is SOLID. The OUTER core is liquid.
• Thinking the thickest layer is the crust — the MANTLE is the thickest and largest.
• Confusing the LITHOSPHERE (crust + upper mantle) with just the crust alone.
• Saying the inner core is not hot — it IS the hottest part, but pressure keeps it solid.

Exam Tips

• LET loves asking: 'Which agency monitors volcanoes and earthquakes in the Philippines?' → PHIVOLCS. 'Which monitors weather and typhoons?' → PAGASA.
• Convergent = collision = subduction = volcanoes and trenches. Divergent = spreading = new crust = ridges and rift valleys. Transform = sliding = earthquakes only.
• The Philippines is on the Ring of Fire because of CONVERGING TECTONIC PLATES — this causes both frequent earthquakes AND active volcanoes.
• Alfred Wegener's Continental Drift (Pangaea) is the HISTORICAL ROOT of modern plate tectonics theory. Know both terms.
• Convection currents in the MANTLE are the driving force of plate movement — heat rises, cools, sinks, and drags plates along.

Key Points

• PLATE TECTONICS is the scientific theory that Earth's lithosphere is divided into large, moving slabs called TECTONIC PLATES that float on the semi-molten asthenosphere.
• The movement of plates is driven by CONVECTION CURRENTS in the mantle — heat from the core causes hot mantle rock to rise, cool, spread, and sink in a circular pattern, dragging plates along.
• ALFRED WEGENER proposed CONTINENTAL DRIFT — the idea that continents were once joined as one supercontinent called PANGAEA and have since moved apart. Evidence: matching coastlines of South America and Africa, identical fossils on separated continents, and matching rock formations.
• Plates move very slowly — a few centimeters per year (about the rate your fingernails grow).
• Most earthquakes and volcanoes occur at PLATE BOUNDARIES.
• THREE TYPES OF PLATE BOUNDARIES: Convergent (collide), Divergent (separate), Transform (slide past each other).
• CONVERGENT BOUNDARY: plates collide. If one is oceanic and one is continental, the denser oceanic plate sinks (SUBDUCTION). Results: mountains, volcanoes, ocean trenches.
• DIVERGENT BOUNDARY: plates move apart. New crust is formed from magma rising up. Results: mid-ocean ridges (underwater mountain chains), rift valleys on land.
• TRANSFORM BOUNDARY: plates slide horizontally past each other. Results: earthquakes along FAULTS. No new crust formed, none destroyed.
• The PACIFIC RING OF FIRE is the horseshoe-shaped zone around the Pacific Ocean where multiple plates meet, hosting about 75% of the world's active volcanoes and 90% of the world's earthquakes.
• The PHILIPPINES is located on the Ring of Fire, sitting between the Philippine Sea Plate, the Eurasian Plate, and other microplates. This explains why the country has frequent earthquakes and active volcanoes like Mayon, Taal, Pinatubo, and Bulusan.
• PHIVOLCS (Philippine Institute of Volcanology and Seismology) is the government agency under DOST that monitors, studies, and issues warnings about earthquakes and volcanic eruptions in the Philippines.

Definitions

Section Title

Plate Tectonics and the Philippine Context

Common Mistakes

• Confusing PHIVOLCS (volcanoes and earthquakes) with PAGASA (weather and typhoons) — this is a very common LET trap.
• Saying divergent boundaries produce earthquakes and transform boundaries produce volcanoes — it is the OPPOSITE: transform boundaries cause earthquakes (no magma), and convergent/subduction boundaries create volcanoes.
• Confusing subduction with spreading — subduction is SINKING at convergent boundaries; spreading is at DIVERGENT boundaries.
• Thinking the Ring of Fire is a circle — it is horseshoe-shaped.
• Forgetting that evidence for Continental Drift/Pangaea includes FOSSILS — not just coastline shapes.

Exam Tips

• Focus = UNDERGROUND; Epicenter = SURFACE (directly above focus). Use 'epi' as a hint — epicenter is 'upon' the surface.
• Magnitude = energy (one value per quake, measured by seismograph). Intensity = felt shaking (varies by location, decreases with distance).
• For volcanic hazards, know: lava (molten rock flow), ashfall (ash particles), pyroclastic flow (fast, hot, deadly cloud of gas and debris), lahar (mudflow).
• Remember: the 1991 Mount Pinatubo eruption is the most notable recent example in the Philippines. Years of lahar followed due to typhoon rains washing ash into rivers.
• PHIVOLCS manages the PEIS (Philippine Earthquake Intensity Scale) for measuring intensity in the Philippines.

Key Points

• An EARTHQUAKE is a sudden shaking of the ground caused by the release of stored energy along a FAULT when rocks abruptly slip or break.
• The FOCUS (also called the HYPOCENTER) is the point UNDERGROUND where the earthquake rupture begins and energy is first released.
• The EPICENTER is the point on Earth's SURFACE directly above the focus. This is where shaking is usually strongest.
• Energy from an earthquake travels outward as SEISMIC WAVES, recorded by a SEISMOGRAPH (the instrument) and shown on a SEISMOGRAM (the record/graph).
• MAGNITUDE measures the ENERGY RELEASED by an earthquake. It is a fixed quantity — an earthquake has one magnitude value. Measured on the Richter scale or the moment magnitude scale.
• INTENSITY measures the STRENGTH OF SHAKING felt at a specific location. It varies by distance — closer to the epicenter means higher intensity. Measured using the PHIVOLCS Earthquake Intensity Scale (PEIS) in the Philippines (or the Modified Mercalli Scale internationally).
• Earthquake-related hazards include: ground shaking (primary danger), LIQUEFACTION (saturated soil behaves like liquid, causing buildings to sink or tip), landslides and rockfalls, and TSUNAMIS.
• A TSUNAMI is a series of large ocean waves triggered by an undersea earthquake (or undersea landslide/volcanic eruption). Do NOT call it a 'tidal wave' — it has nothing to do with tides.
• A VOLCANO is an opening or vent in Earth's crust through which molten rock, gases, and ash escape.
• MAGMA is molten rock BELOW the surface; once it erupts and reaches the surface, it is called LAVA.
• Volcanoes are classified by activity: ACTIVE (has erupted recently or shows signs of activity), DORMANT (has not erupted for a long time but could again), EXTINCT (unlikely to erupt again).
• Volcanic hazards include: lava flows, ASHFALL, PYROCLASTIC FLOWS (fast-moving currents of hot gas, ash, and rock — extremely deadly), and LAHARS (volcanic mudflows when ash mixes with water, especially during rainfall or typhoons).
• In the Philippine context, LAHAR is especially dangerous because typhoons often follow volcanic eruptions, washing loose volcanic material into rivers and communities downstream. Pinatubo's 1991 eruption followed by lahar flows for years afterward is the classic example.

Definitions

Section Title

Earthquakes and Volcanoes

Common Mistakes

• Saying the EPICENTER is underground — it is on the SURFACE. The FOCUS (hypocenter) is underground.
• Confusing MAGNITUDE and INTENSITY — magnitude is constant (energy released), intensity varies by location (felt shaking).
• Calling a tsunami a 'tidal wave' — this term is scientifically incorrect.
• Confusing MAGMA and LAVA — magma is underground, lava is above ground (after erupting).
• Saying LAHARS are caused by lava — lahars are MUDFLOWS of ash/debris mixed with water, NOT lava.

Exam Tips

• Rock type memory trick: I am Ign-ited by fire (Igneous = fire/magma); SEDiments SETTLE in layers (Sedimentary = layers, fossils); META means change (Metamorphic = heat + pressure).
• Parent rock pairs for the LET: Limestone → Marble; Shale → Slate; Sandstone → Quartzite. Memorize all three.
• Fossils = Sedimentary rocks ONLY. This appears on nearly every LET Earth Science section.
• The rock cycle has NO starting or ending point — all rocks can eventually become any other type given enough time and the right conditions.
• Intrusive igneous rocks have LARGE CRYSTALS (cooled slowly underground — slow cooling = more time for crystals to grow). Extrusive igneous rocks have SMALL CRYSTALS or glassy texture (cooled quickly at surface).

Key Points

• A MINERAL is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure. Examples: quartz, feldspar, calcite, mica.
• A ROCK is a solid aggregate (mixture) of one or more minerals. Rocks do NOT have a fixed chemical composition.
• ROCKS ARE CLASSIFIED INTO THREE TYPES based on how they form: IGNEOUS, SEDIMENTARY, and METAMORPHIC.
• IGNEOUS ROCKS form when MAGMA or LAVA cools and solidifies. 'Igneous' comes from Latin for fire (ignis). If magma cools slowly UNDERGROUND, large crystals form (INTRUSIVE/PLUTONIC igneous rock — example: GRANITE). If lava cools quickly at the SURFACE, small crystals or glassy texture forms (EXTRUSIVE/VOLCANIC igneous rock — example: BASALT, PUMICE, OBSIDIAN).
• SEDIMENTARY ROCKS form when sediments (tiny rock fragments, mineral particles, organic material) are compacted and cemented together in layers over time. This process is called LITHIFICATION. Examples: sandstone, limestone, shale, conglomerate. FOSSILS are found mainly in SEDIMENTARY rocks because they form in gentle, layered environments that can preserve organic remains.
• METAMORPHIC ROCKS form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are changed by HIGH HEAT and/or HIGH PRESSURE — without melting completely. Key word: 'metamorphic' = change in form. Examples: marble (from limestone), slate (from shale), quartzite (from sandstone), schist.
• The ROCK CYCLE is the continuous, slow process by which rocks of one type are transformed into rocks of another type through geological processes. It has no beginning or end — it is cyclical.
• KEY ROCK CYCLE TRANSFORMATIONS: Magma cools → Igneous rock. Igneous/metamorphic rock weathers and erodes → Sediments → compacted into Sedimentary rock. Any rock subjected to heat and pressure → Metamorphic rock. Any rock melts → Magma → cycle continues.
• Properties used to IDENTIFY MINERALS: color, luster (shininess), streak (color of powder), hardness (Mohs Hardness Scale — diamond is hardest at 10, talc is softest at 1), cleavage (splitting along flat planes), fracture, specific gravity, and crystal form.

Definitions

Section Title

Rocks and Minerals and the Rock Cycle

Common Mistakes

• Confusing MAGMA (underground) with LAVA (at or above the surface) when classifying igneous rocks.
• Saying fossils are found in IGNEOUS or METAMORPHIC rocks — FOSSILS are in SEDIMENTARY rocks.
• Thinking metamorphic rocks form by MELTING — they do NOT melt; if they melted, they would become MAGMA and form igneous rocks.
• Confusing GRANITE (intrusive igneous — large crystals, formed slowly underground) with BASALT (extrusive igneous — fine-grained, formed quickly at the surface).
• Forgetting the parent rock pairs: shale → SLATE (not marble); limestone → MARBLE (not slate).

Exam Tips

• Quick sequence of the water cycle: EVAPORATION (Sun heats water → vapor) → CONDENSATION (vapor cools → clouds) → PRECIPITATION (water falls) → RUNOFF/INFILTRATION (water moves on or into land) → back to the ocean.
• Transpiration is the water cycle's link to BIOLOGY — plants release water. LET may connect this to photosynthesis lessons.
• The SUN is the ENERGY SOURCE for the water cycle (drives evaporation). GRAVITY brings precipitation down and drives runoff.
• In the K-12 curriculum, Grades 3–4 cover the water cycle. As a future teacher, connecting this to local watersheds and deforestation issues in the Philippines adds real-world relevance.
• Philippine relevance: Deforestation reduces transpiration, increases runoff, causes floods and droughts — a common LET integrative question linking water cycle to environmental issues.

Key Points

• The WATER CYCLE (HYDROLOGIC CYCLE) is the continuous movement of water on, above, and below Earth's surface. It is driven by SOLAR ENERGY (the Sun) and GRAVITY.
• The total amount of water on Earth remains roughly CONSTANT — it only changes form (liquid, gas, solid) and location.
• About 97% of Earth's water is SALT WATER in the oceans. Only about 3% is FRESH WATER, and most of that is locked in ice caps and glaciers. Less than 1% is readily available liquid fresh water.
• MAIN PROCESSES OF THE WATER CYCLE:
• 1. EVAPORATION — The Sun heats liquid water (oceans, lakes, rivers), causing it to change into WATER VAPOR (gas) that rises into the atmosphere. This is the MAIN source of atmospheric water.
• 2. TRANSPIRATION — Plants absorb water through roots and release it as WATER VAPOR through tiny pores in their leaves called STOMATA. The combined process is sometimes called EVAPOTRANSPIRATION.
• 3. CONDENSATION — As water vapor rises and cools in the upper atmosphere, it changes back into tiny liquid water DROPLETS that form CLOUDS and FOG. This is the reverse of evaporation.
• 4. PRECIPITATION — Water falls from clouds to Earth's surface as RAIN, SNOW, SLEET, or HAIL, depending on atmospheric temperature. In the Philippines, precipitation is almost always RAIN.
• 5. RUNOFF — Precipitation that flows over land surface into streams, rivers, and eventually back to the ocean. The rate of runoff is increased by deforestation (fewer plants to slow water) and steep slopes.
• 6. INFILTRATION — Water that seeps into the soil and becomes GROUNDWATER, stored in underground reservoirs called AQUIFERS.
• 7. COLLECTION — Water gathers in oceans, lakes, rivers, and underground as groundwater.
• In the Philippines, forests play a critical role in the water cycle — they increase infiltration, reduce runoff, and contribute to transpiration, maintaining water supply in watersheds.

Definitions

Section Title

The Water Cycle (Hydrologic Cycle)

Common Mistakes

• Confusing EVAPORATION (water bodies → vapor) with CONDENSATION (vapor → liquid droplets/clouds).
• Forgetting TRANSPIRATION — plants are part of the water cycle. Water released by plants is also an important source of atmospheric vapor.
• Thinking precipitation only means RAIN — it also includes snow, hail, and sleet (though rare in the tropics).
• Forgetting that the Sun drives evaporation and gravity drives precipitation and runoff.
• Mixing up INFILTRATION (water seeping into soil) with RUNOFF (water flowing over land surface).

Exam Tips

• LET KEY DISTINCTION: Weathering = IN PLACE (no movement). Erosion = MOVEMENT (by water, wind, ice, gravity). Deposition = DROPPING in new location.
• For physical vs. chemical weathering: Physical = same material, smaller size. Chemical = NEW material/composition.
• Classic LET question: 'Ice cracking a rock apart is an example of _____ weathering.' Answer: PHYSICAL (freeze-thaw).
• Another classic: 'Acid rain dissolving limestone is an example of _____ weathering.' Answer: CHEMICAL (carbonation).
• In the Philippine context, WATER is the most important agent of erosion. Heavy rains + deforested slopes = dangerous landslides — connect to disaster preparedness themes.

Key Points

• WEATHERING, EROSION, and DEPOSITION are three sequential processes that break down rocks, move the material, and drop it in a new location. They work together to reshape Earth's surface.
• WEATHERING is the BREAKING DOWN of rocks IN PLACE. Rocks are broken into smaller pieces or chemically changed, but the material does NOT move during weathering itself.
• TWO TYPES OF WEATHERING: (1) PHYSICAL (MECHANICAL) WEATHERING — rock is broken into SMALLER PIECES without changing its chemical composition. The pieces are the same material as the original rock, just smaller. (2) CHEMICAL WEATHERING — the rock's chemical makeup is CHANGED, forming new substances. Often involves water, oxygen, and acids.
• EXAMPLES OF PHYSICAL WEATHERING: Freeze-thaw action (water enters cracks, freezes, expands, widens the crack — common in cold climates); ABRASION (rocks grind against each other, e.g., in riverbeds or by wind-blown sand); thermal expansion and contraction (heating and cooling cycles crack rocks); BIOLOGICAL weathering — plant roots growing into cracks, widening them over time; burrowing animals.
• EXAMPLES OF CHEMICAL WEATHERING: Oxidation (rusting — iron in rocks reacts with oxygen and water, forming reddish iron oxide); CARBONATION — carbon dioxide dissolves in rainwater to form weak carbonic acid that dissolves limestone (forming caves and sinkholes — KARST topography); HYDRATION — water chemically combines with minerals.
• EROSION is the TRANSPORTATION (movement) of weathered rock material (sediment) from one place to another by AGENTS: Water (rivers, rain, waves — the most important agent in the Philippines), Wind (important in deserts and coasts), Ice/GLACIERS, Gravity (landslides).
• DEPOSITION is the DROPPING or settling of eroded material in a new location when the agent of erosion loses energy. Deposited material forms features such as DELTAS (at river mouths), BEACHES, SANDBARS, ALLUVIAL FANS, and FLOODPLAINS.
• SIMPLE MEMORY AID: WEATHERING breaks it → EROSION moves it → DEPOSITION drops it.
• In the PHILIPPINES, WATER is the dominant agent of chemical weathering and erosion due to the tropical climate, heavy rainfall (especially typhoons), and steep terrain. Deforestation makes slopes even more vulnerable to LANDSLIDES.
• SOIL FORMATION (pedogenesis) is closely linked to weathering — physical and chemical breakdown of rock, combined with organic material from decomposing plants and animals, produces SOIL over very long periods.

Definitions

Section Title

Weathering, Erosion, and Deposition

Common Mistakes

• Confusing WEATHERING and EROSION — weathering is breaking DOWN (in place), erosion is MOVING the broken material.
• Saying acid rain causes physical weathering — acid rain (carbonation/oxidation) is CHEMICAL weathering, not physical.
• Thinking erosion can happen without prior weathering — in reality, weathering must first break down the rock before erosion can transport it.
• Confusing deposition with decomposition — deposition is DROPPING sediment; decomposition is biological breakdown of organic material.
• Saying erosion only involves water — it also involves WIND, ICE (glaciers), and GRAVITY (mass movement/landslides).

Exam Tips

• AGENCY DISTINCTION — one of the most tested items: PAGASA = weather, typhoons, climate. PHIVOLCS = earthquakes, volcanoes. Both are under DOST.
• Weather = short term and local. Climate = long term and regional. 'What should I wear today?' = weather. 'What wardrobe do I need for this region year-round?' = climate.
• Typhoon = same as hurricane (Atlantic) and cyclone (Indian Ocean). Regional name only differs.
• El Niño → DROUGHT in Philippines. La Niña → FLOODING in Philippines. Both are caused by changes in PACIFIC OCEAN temperatures.
• STORM SURGE killed more people during Typhoon Yolanda than winds did — teach pupils about storm surge as a distinct hazard from typhoon winds.
• As a future teacher, reinforce that disaster preparedness (RA 10121) is a school responsibility. Teachers must model and teach safe practices related to Philippine natural hazards.

Key Points

• WEATHER is the current, SHORT-TERM state of the atmosphere at a specific PLACE at a specific TIME. It changes rapidly — from hour to hour or day to day.
• CLIMATE is the LONG-TERM AVERAGE of weather patterns for a particular REGION over many decades (typically 30+ years). Climate changes very slowly.
• MEMORY AID: 'WEATHER is what you WEAR today; CLIMATE is what CLOTHES you keep in your CLOSET year-round.'
• BOTH weather and climate describe the ATMOSPHERE using the same elements: TEMPERATURE, HUMIDITY (moisture in the air), AIR PRESSURE (measured by a barometer), WIND SPEED AND DIRECTION, CLOUD COVER, and PRECIPITATION.
• The PHILIPPINES has a TROPICAL MARITIME CLIMATE — generally hot and humid throughout the year, with a WET (rainy) SEASON and a DRY SEASON.
• MONSOON WINDS shape Philippine seasons: HABAGAT (Southwest Monsoon) blows from the southwest, bringing heavy rains from approximately May to October — this is the WET season. AMIHAN (Northeast Monsoon) blows from the northeast, bringing cooler, drier weather from approximately November to February — this is the COOL DRY season.
• CLIMATE CHANGE refers to long-term shifts in global climate patterns, largely caused by the buildup of GREENHOUSE GASES (carbon dioxide, methane, water vapor) in the atmosphere that trap heat — the GREENHOUSE EFFECT.
• Consequences of climate change relevant to the Philippines: RISING SEA LEVELS (threatens coastal communities and low-lying islands), more INTENSE TYPHOONS, disrupted agricultural seasons, and MORE EXTREME DROUGHT (El Niño) and FLOOD (La Niña) events.
• EL NIÑO is a periodic warming of Pacific Ocean surface temperatures that typically causes REDUCED RAINFALL and DROUGHT in the Philippines. LA NIÑA is the opposite — cooler Pacific temperatures cause ABOVE-NORMAL RAINFALL and flooding.
• TYPHOONS: The Philippines is struck by an average of 20 typhoons per year, more than any other country. A TROPICAL CYCLONE is a rotating storm system with strong winds and heavy rain that forms over warm tropical oceans. It is called a TYPHOON in the western Pacific/Philippines region, a HURRICANE in the Atlantic/eastern Pacific, and a CYCLONE in the Indian Ocean — same phenomenon, different regional name.
• STORM SURGE is a sudden, dangerous rise in sea level along the coast caused by strong typhoon winds pushing water onshore. It is often MORE DEADLY than the wind itself (e.g., Typhoon Yolanda/Hainan in 2013).
• PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration) is the government agency under DOST that monitors and forecasts weather, issues typhoon warnings, and provides rainfall advisories.
• PAGASA uses a PUBLIC STORM WARNING SIGNAL (PSWS) system with four levels (Signal No. 1 to 4) to communicate typhoon strength and expected impact to the public.
• DISASTER PREPAREDNESS: Teachers have a responsibility to teach pupils about hazard awareness, evacuation routes, and heeding official warnings. This aligns with RA 10121 (Philippine Disaster Risk Reduction and Management Act).

Definitions

Section Title

Weather versus Climate and Natural Hazards

Common Mistakes

• Confusing PAGASA (weather/typhoons) with PHIVOLCS (volcanoes/earthquakes) — this is one of the most common LET mistakes.
• Saying 'typhoon' and 'hurricane' are different phenomena — they are the SAME type of storm, just called different names in different regions.
• Confusing HABAGAT and AMIHAN — Habagat is the SW monsoon (WET season); Amihan is the NE monsoon (COOL DRY season).
• Thinking STORM SURGE is just strong waves — it is a sustained rise in SEA LEVEL along the coast, not just wave action.
• Confusing EL NIÑO (drought/less rain in Philippines) with LA NIÑA (flooding/more rain in Philippines).