11.4 Earth & Space Science: Earth's Structure, Rocks, Water, Weather & the Solar System

Earth and Space Science

Earth is one of the four Stanford 10 Science content clusters, with 11 of 40 items on a real Intermediate 2 report, the same share as Life and Physical. Pearson's Science subtest moves from general concepts at Primary 3 toward course-specific earth and space content at the TASK levels. Earth and space items often ask students to read a model or diagram, such as a cross-section of Earth's layers, a weather map or a diagram of Sun, Earth and Moon. Reading models like these is the skill behind the Models process cluster.

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Earth Systems, Interior Structure, and Plate Tectonics

Earth is an active, differentiated planet structured into concentric layers based on density and mineral composition:

1. Crust: The cool, brittle outermost layer. Continental crust is thick (30–50 km), buoyant, and predominantly composed of low-density granitic rock. Oceanic crust is thin (5–10 km), dense, and composed of dark basaltic rock.
2. Mantle: Extends to a depth of roughly 2,900 km, composed of dense silicate peridotite. The uppermost rigid mantle combines with the crust to form the lithosphere, which is broken into moving tectonic plates. Directly beneath lies the asthenosphere, a ductile, semi-fluid zone of plastic rock that undergoes slow convective circulation.
3. Outer Core: A liquid layer composed of molten iron and nickel (depth 2,900–5,150 km). Convection currents of molten conductive metal in the outer core generate Earth's geomagnetic field, shielding the biosphere from solar wind radiation.
4. Inner Core: A dense, solid metallic sphere of iron and nickel (depth 5,150–6,370 km) under intense lithostatic pressure that prevents melting despite temperatures exceeding $5{,}400^\circ\text{C}$.

Tectonic Plate Boundaries

Tectonic plates float across the asthenosphere, driven by mantle convection, slab pull, and ridge push. Their interactions at boundaries create major geological features:

Earthquakes originate at the subsurface rupture point called the focus; the point directly above on Earth's surface is the epicenter. Seismic energy travels as primary ($P$) compressional waves (fastest, travel through solids and liquids) and secondary ($S$) shear waves (slower, travel only through solids).

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The Rock Cycle, Weathering, and Geological Time

The rock cycle illustrates the continuous recycling of crustal materials over millions of years through three main rock families:

1. Igneous Rocks: Formed by the cooling and crystallization of molten rock.
   • Intrusive (Plutonic): Magma cools slowly deep underground, permitting large mineral crystals to grow (e.g., granite, gabbro).
   • Extrusive (Volcanic): Lava erupts onto Earth's surface and cools rapidly, producing fine-grained or glassy textures (e.g., basalt, pumice, obsidian).
2. Sedimentary Rocks: Formed through the weathering of pre-existing rock, transport, deposition, compaction, and cementation (lithification) of mineral particles or organic debris.
   • Clastic: Formed from compacted mineral fragments (e.g., sandstone, shale, conglomerate).
   • Chemical: Formed by mineral precipitation from evaporating water bodies (e.g., rock salt, limestone).
   • Organic: Formed from compacted plant or animal remains (e.g., bituminous coal, fossiliferous limestone). Nearly all fossils are found in sedimentary rock, because heat and pressure destroy most remains in igneous and metamorphic rock.
3. Metamorphic Rocks: Pre-existing rocks subjected to intense subsurface heat and tectonic pressure without completely melting, recrystallizing mineral structures.
   • Foliated: Display distinctive parallel bands or mineral alignment (e.g., slate from shale, schist, gneiss from granite).
   • Non-foliated: Uniform crystalline texture lacking bands (e.g., marble from limestone, quartzite from sandstone).

Weathering, Erosion, and Deposition

• Weathering: The in-situ breakdown of rock at Earth's surface. Mechanical weathering physically fractures rock without altering chemistry (frost wedging, root wedging, thermal expansion). Chemical weathering decomposes minerals through chemical reactions (oxidation of iron minerals, carbonic acid dissolution of limestone caves).
• Erosion: The physical detachment and transport of weathered sediments by moving water, wind, glaciers, or gravity.
• Deposition: The laying down or settling of transported sediments as transport fluids slow down, constructing deltas, alluvial fans, sand dunes, and glacial moraines.

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Hydrologic Dynamics and Atmospheric Weather Systems

The water cycle (hydrologic cycle) circulates water through the hydrosphere, atmosphere, and lithosphere powered by solar energy and gravity: evaporation (solar heat transforms liquid water into atmospheric vapor), transpiration (plants release water vapor through leaf stomata), condensation (cooling vapor coalesces into cloud droplets), precipitation (rain, snow, sleet, hail falling to Earth), and runoff / infiltration (water flowing across surfaces into rivers or soaking into groundwater aquifers).

Layers of the Earth's Atmosphere

$\text{Surface} \to \text{Troposphere} \to \text{Stratosphere} \to \text{Mesosphere} \to \text{Thermosphere} \to \text{Exosphere}$

• Troposphere (0–12 km): Contains 75% of atmospheric mass and virtually all water vapor; site of all terrestrial weather phenomena. Temperature decreases with altitude.
• Stratosphere (12–50 km): Contains the protective ozone layer ($O_3$), which absorbs harmful solar ultraviolet (UV) radiation. Temperature increases with altitude due to ozone absorption.
• Mesosphere (50–85 km): The coldest atmospheric layer (temperatures plunge to $-90^\circ\text{C}$); where meteors burn upon atmospheric entry.
• Thermosphere (85–600 km): High temperatures due to X-ray absorption; contains the ionosphere (auroras) and the orbit of the International Space Station.

Air Masses, Fronts, and Weather Maps

Weather conditions depend on the movement and interaction of large bodies of air with uniform temperature and moisture properties (air masses): maritime tropical ($mT$ = warm, humid), continental polar ($cP$ = cold, dry), continental tropical ($cT$ = hot, dry), and maritime polar ($mP$ = cold, humid).

On isobaric weather maps, closely spaced isobars (lines of equal barometric pressure) indicate steep pressure gradients and powerful, gusting winds. High-pressure systems ($H$) bring descending air, divergent winds, and fair, clear skies, while low-pressure systems ($L$) bring converging, ascending air, cloud formation, and stormy weather.

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Celestial Mechanics and Planetary Astronomy

Space science items focus on the geometry of Earth's motion and the Moon's phases:

• Rotation vs. Revolution: Earth rotates on its axis once every 24 hours (causing the day/night cycle, apparent celestial motion, and the Coriolis effect). Earth revolves around the Sun in an elliptical orbit once every 365.25 days.
• Seasons: Caused entirely by Earth's permanent axial tilt of $23.5^\circ$ relative to the ecliptic plane. When the Northern Hemisphere is tilted toward the Sun (summer solstice in June), solar rays strike the surface at high, direct angles, and daylight hours peak. When tilted away (winter solstice in December), solar radiation strikes at low, diffuse angles, producing cold temperatures.
• Lunar Cycle: The Moon revolves around Earth every 27.3 days, displaying eight phases across a 29.5-day synodic cycle as sunlight illuminates varying portions of its Earth-facing hemisphere:

$\text{New Moon} \to \text{Waxing Crescent} \to \text{First Quarter} \to \text{Waxing Gibbous} \to \text{Full Moon} \to \text{Waning Gibbous} \to \text{Third Quarter} \to \text{Waning Crescent}$

• Eclipses:
  • Solar Eclipse: The Moon passes directly between the Sun and Earth (during a New Moon), casting its shadow onto Earth's surface.
  • Lunar Eclipse: Earth passes directly between the Sun and the Moon (during a Full Moon), casting Earth's shadow across the lunar surface.