24.1 Earth's Structure, Plate Tectonics & Changes to Earth's Surface

Overview & Exam Relevance

Competency 015 (Structure and Function of Earth Systems) of the TExES Core Subjects EC-6 Science subject exam (904) covers Earth's internal structure, plate tectonics, constructive and destructive processes such as weathering, erosion, and deposition, and how gradual and catastrophic natural events and human activity change Earth systems. The rock cycle and the formation of rocks, minerals, and soils belong to Competency 016 and follow in the next section. In elementary science, Earth science concepts provide tangible opportunities for students to investigate the world beneath their feet and observe how constructive and destructive geological forces continuously sculpt planetary landforms.

The elementary TEKS progress from observing and sorting rocks, soils, and water in the primary grades, to describing rapid and slow changes to Earth's surface, to modeling how weathering, erosion, and deposition by water, wind, and ice shape landforms (Grade 4) and how sedimentary rocks and fossil fuels form over long periods (Grade 5).

On the TExES 391 exam, you must demonstrate both scientific content knowledge and pedagogical competence. You will be expected to analyze cross-sectional diagrams of Earth's interior, interpret tectonic boundary interactions, identify unknown minerals using standard physical test data, differentiate among rock origins, classify soils using the soil texture triangle, and guide students through hands-on inquiry without reinforcing persistent misconceptions.

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Earth's Layered Interior: Compositional & Mechanical Models

Geoscientists classify Earth's internal layers using two complementary frameworks: compositional layers (defined by chemical and elemental makeup) and mechanical or rheological layers (defined by physical state, rigidity, and behavior under stress).

EARTH'S INTERNAL ARCHITECTURE

COMPOSITIONAL LAYERS                    MECHANICAL (RHEOLOGICAL) LAYERS
┌────────────────────────┐              ┌────────────────────────┐
│ CRUST (0-70 km)        │ ◄──────────► │ LITHOSPHERE (Rigid)    │ (Crust + uppermost solid mantle;
│ Continental: Granitic  │              │ (0-100 km)             │  broken into tectonic plates)
│ Oceanic: Basaltic      │              ├────────────────────────┤
├────────────────────────┤              │ ASTHENOSPHERE (Ductile)│ (Plastic, semi-fluid mantle;
│ MANTLE (70-2,900 km)   │ ◄──────────► │ (100-660 km)           │  thermal convection currents)
│ Silicate rock rich in  │              ├────────────────────────┤
│ Fe and Mg (Peridotite) │              │ MESOSPHERE (Rigid)     │ (Lower mantle under immense
│                        │              │ (660-2,900 km)         │  confining pressure)
├────────────────────────┤              ├────────────────────────┤
│ CORE (2,900-6,371 km)  │ ◄──────────► │ OUTER CORE (Liquid)    │ (Molten Fe-Ni; generates
│ Dense metallic alloy   │              │ (2,900-5,150 km)       │  planetary geomagnetic field)
│ (Iron & Nickel)        │              ├────────────────────────┤
│                        │              │ INNER CORE (Solid)     │ (Solid Fe-Ni sphere under
└────────────────────────┘              │ (5,150-6,371 km)       │  extreme confining pressure)
                                        └────────────────────────┘

1. Compositional Layers

• Crust: The outermost, thinnest chemical layer, accounting for less than 1% of Earth's total volume. The crust is split into two distinct varieties:
  • Continental Crust: Relatively thick ($30\text{ to }70\text{ km}$ beneath mountain ranges), low average density ($\approx 2.7\text{ g/cm}^3$), and composed predominantly of light-colored, felsic igneous rocks such as granite, rich in silica and aluminum (often termed sial).
  • Oceanic Crust: Relatively thin ($5\text{ to }10\text{ km}$ beneath ocean basins), high average density ($\approx 3.0\text{ g/cm}^3$), and composed predominantly of dark, mafic igneous rocks such as basalt and gabbro, rich in silica and magnesium (often termed sima). Because oceanic crust is denser than continental crust, it subducts beneath continental margins during tectonic collisions.
• Mantle: Extending from the base of the crust to a depth of approximately $2,900\text{ km}$, the mantle comprises roughly 82% of Earth's volume and 68% of its mass. It is composed of dense, ultramafic silicate rocks rich in iron and magnesium, primarily peridotite.
• Core: Earth's central sphere, extending from $2,900\text{ km}$ to the center at $6,371\text{ km}$. It is a dense metallic alloy that is mostly iron ($\text{Fe}$), with a smaller share of nickel ($\text{Ni}$, roughly 5%) and lighter elements such as sulfur, oxygen, or silicon.

2. Mechanical (Physical) Layers

• Lithosphere: The strong, rigid, brittle outermost mechanical layer comprising the entire crust and the very top portion of the upper mantle. Ranging from $10\text{ to }100\text{ km}$ thick under oceans and up to $250\text{ km}$ thick under older continental cratons, the lithosphere is broken into roughly a dozen major and minor tectonic plates that move relative to one another.
• Asthenosphere: The semi-fluid, ductile, mechanically weak layer of the upper mantle lying directly beneath the lithosphere ($100\text{ to }660\text{ km}$ deep). Elevated ambient temperatures and confining pressures cause the solid peridotite rock to behave plastically, flowing slowly over geological timescales. Thermal convection currents circulating within the asthenosphere act as a primary engine driving the motion of overlying lithospheric plates.
• Mesosphere (Lower Mantle): Extending from $660\text{ to }2,900\text{ km}$, this deep mantle zone experiences extreme confining pressure that counteracts high temperatures, forcing the rock into a more rigid, solid mechanical state than the ductile asthenosphere.
• Outer Core: A liquid metallic layer spanning from $2,900\text{ to }5,150\text{ km}$. S-waves (shear seismic waves) cannot travel through the outer core, confirming its liquid state. Convective circulation of molten conductive iron coupled with Earth's planetary rotation produces electrical currents that generate Earth's geomagnetic field via the geodynamo mechanism, deflecting harmful solar wind and cosmic radiation.
• Inner Core: A solid metallic sphere at Earth's center, extending from $5,150\text{ to }6,371\text{ km}$. Although temperatures reach $5,000^\circ\text{C}$ to $6,000^\circ\text{C}$ (comparable to the surface of the Sun), immense confining pressures (over $3.3\text{ million atmospheres}$) prevent the iron-nickel atoms from melting, forcing them into a solid crystalline metallic lattice.

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Plate Tectonic Theory & Dynamic Crustal Processes

1. From Continental Drift to Plate Tectonics

In 1912, German meteorologist Alfred Wegener proposed the hypothesis of continental drift, asserting that all present-day continents were once united in a supercontinent named Pangaea that fragmented approximately 200 million years ago. Wegener supported his claim with four lines of empirical evidence:

1. Continental Jigsaw Fit: The complementary matching shorelines of South America and Africa.
2. Fossil Correlations: Identical terrestrial fossil organisms found across separated ocean basins, such as Mesosaurus (a small freshwater aquatic reptile) and Glossopteris (a subpolar seed fern).
3. Stratigraphic and Mountain Belts: Continuous geological rock formations and mountain chains abruptly terminating at one continental coast and resuming across the ocean (e.g., the Appalachian Mountains matching the Caledonian Mountains in Scotland).
4. Paleoclimatic Evidence: Glacial striations and tillite deposits dating to the late Paleozoic found in tropical regions of southern India, Australia, South America, and southern Africa.

Wegener's hypothesis was largely dismissed by contemporary geologists because he could not propose a physically viable mechanism capable of plowing rigid continental masses through solid oceanic seafloor. The hypothesis was revived and transformed into modern Plate Tectonic Theory during the 1960s through discoveries of seafloor spreading (proposed by Harry Hess), paleomagnetic symmetrical zebra-stripe reversals recorded in basaltic crust along the Mid-Atlantic Ridge, and precise mapping of global earthquake epicenters along plate margins.

FORCES DRIVING TECTONIC PLATE MOTION
│
├── Slab Pull ─────────► Cold, dense subducting oceanic lithosphere sinks into the
│                       asthenosphere under gravity, pulling the trailing plate (dominant force)
├── Ridge Push ────────► Elevated topography of mid-ocean ridges creates gravitational sliding
│                       downward and outward away from the spreading ridge axis
└── Mantle Convection ─► Thermal plumes and convective cells in the asthenosphere transfer heat
                        from the core-mantle boundary toward the surface

2. Types of Plate Boundaries

Tectonic plates interact along three primary boundary geometries, each producing diagnostic landforms and seismic profiles:

3. Earthquakes & Seismic Waves

An earthquake occurs when accumulated elastic strain energy within deformed crustal rocks suddenly overcomes static friction along a fault plane, releasing mechanical energy that propagates outward as seismic waves:

• Focus (Hypocenter): The subterranean point of initial rupture along the fault where stored elastic strain is liberated.
• Epicenter: The point on Earth's geographic surface located vertically directly above the focus.
• Seismic Body Waves:
  • P-Waves (Primary / Compressional): Longitudinal waves that compress and expand rock parallel to the direction of wave propagation. They are the fastest seismic waves, arrive first at seismograph stations, and can travel through both solids and liquids.
  • S-Waves (Secondary / Shear): Transverse waves that displace rock particles perpendicular to the direction of propagation. They are slower than P-waves, arrive second, and can only travel through solids, proving that Earth's outer core is molten liquid.
• Seismic Surface Waves: Rayleigh and Love waves that travel along Earth's exterior crust. Slower than body waves, but their large mechanical amplitudes and complex ground motions produce the vast majority of surface destruction.
• Measurement Scales:
  • Moment Magnitude Scale ($M_w$): Quantifies total physical energy released based on fault surface area, displacement distance, and rock rigidity. It is logarithmic: each whole-number increase represents a $\approx 32$-fold increase in released energy.
  • Modified Mercalli Intensity Scale: Evaluates observable structural destruction, ground shaking, and human subjective impact at specific locations using Roman numerals (I to XII).

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Weathering, Erosion, and Deposition (WED)

Earth's surface landforms are continuously reshaped by the constructive and destructive triad of weathering, erosion, and deposition:

THE WED CONTINUUM
┌────────────────────┐         ┌────────────────────┐         ┌────────────────────┐
│    WEATHERING      │ ──────► │      EROSION       │ ──────► │     DEPOSITION     │
│ In-situ breakdown  │         │ Transport/removal  │         │ Settling/dropping  │
│ (Breaks it)        │         │ (Moves it)         │         │ (Drops it)         │
└────────────────────┘         └────────────────────┘         └────────────────────┘

1. Weathering (In-situ Breakdown)

• Mechanical (Physical) Weathering: Breaks rocks into progressively smaller fragments without altering elemental or mineral chemistry:
  • Frost Wedging: Water penetrates rock fractures, freezes, and expands by approximately 9% in volume, exerting outward mechanical pressure that levers fractures open.
  • Root Wedging: Plant roots infiltrate bedrock joints, expanding as they grow.
  • Thermal Exfoliation / Unloading: Overlying rock is stripped away by erosion, releasing confining pressure and causing deep plutons to expand outward and shed curved rock sheets (e.g., Enchanted Rock in Texas).
  • Abrasion: Friction caused by wind-borne sand grains, river gravels, or glacial ice scouring exposed rock faces.
• Chemical Weathering: Decomposes or transforms rock minerals into new chemical compounds through atmospheric and aqueous reactions:
  • Oxidation: Oxygen dissolved in water reacts with iron-bearing minerals, generating reddish iron oxide (rust) that weakens crystalline integrity.
  • Hydrolysis: Hydrogen and hydroxide ions in water react chemically with silicate minerals; for example, orthoclase feldspar decomposes into clay minerals and soluble silica.
  • Carbonation / Dissolution: Atmospheric carbon dioxide dissolves in rainwater to yield weak carbonic acid ($\text{H}_2\text{CO}_3$). This acidic rainwater dissolves limestone bedrock (calcite), carving subterranean caverns, sinkholes, and producing distinctive karst topography across the Texas Hill Country.

2. Erosion & Deposition Dynamics

• Erosion: The physical detachment and mobilization of weathered sediments by mobile geological agents:
  • Running Water: The single most powerful erosional agent on Earth. Swiftly moving streams dislodge and carry sediments, carving steep V-shaped river valleys and deep canyons.
  • Wind: Deflates fine silt and sand across arid plains, sculpting desert pavements and ventifacts.
  • Glacial Ice: Continental and alpine ice sheets pluck rock fragments from valley walls and scour underlying bedrock, transforming narrow river valleys into broad, steep-sided U-shaped glacial valleys.
  • Gravity: Direct driver of mass wasting events including landslides, mudflows, rockfalls, and slow subterranean soil creep.
• Deposition: Occurs when transporting agents lose kinetic energy, causing carried sediments to settle onto landforms:
  • Deltas: Fan-shaped accumulations of sediment formed at river mouths where flowing water enters standing bodies of water (lakes or oceans) and abruptly decelerates.
  • Alluvial Fans: Cone-shaped sediment deposits formed where high-gradient mountain streams emerge onto broad, flat valley floors.
  • Moraines: Unsorted ridges of glacial till dumped along the margins and termini of retreating glaciers.
  • Sand Dunes: Asymmetrical sediment ridges deposited by decelerating wind currents.

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Gradual and Catastrophic Changes to Earth Systems

The framework asks how human activity and natural processes, both gradual and catastrophic, can alter Earth systems.

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Reading Earth's History in Rock Layers

• Law of superposition: In undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest are at the top.
• Fossils reveal past life and environments. Marine fossils in the rocks of the Texas Hill Country show that a shallow sea once covered the area. Dinosaur tracks preserved in the riverbed at Dinosaur Valley State Park near Glen Rose show where dinosaurs walked along an ancient shoreline.
• Geologic time is so long that scientists divide it into eras and periods. Elementary students can model it with a long strip of paper on which human history occupies only a tiny piece at the end.

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Classroom Instructional Strategies & Scenario Application

Overcoming Common Student Misconceptions

Exemplary Classroom Inquiry Scenario

Classroom Context: Ms. Garcia's 4th-grade class is modeling slow changes to Earth's surface caused by weathering, erosion, and deposition (TEKS 4.10(B)). Students are investigating how flowing water changes land features over time.

Investigation Setup: Each cooperative team sets up a plastic stream table filled with a blend of sand, silt, and pea gravel. The table is elevated at one end to model slope. Students run a controlled flow of water from a siphon reservoir onto the upper end for 10 minutes.

Observations & Deductions:

1. Erosion Phase: Fast-moving water near the source dislodges fine silt and sand, carving a miniature river channel and small canyons with steep walls.
2. Transport Phase: Students observe that large pea gravel remains near the source or rolls slowly, while lighter sand and silt are carried down-gradient in suspension.
3. Deposition Phase: When the stream reaches the flat basin at the bottom of the tray, the water slows down immediately, depositing the sediment in a fan-shaped delta.
4. Pedagogical Synthesis: Ms. Garcia facilitates a reflective discussion connecting their stream table models to the formation of the Mississippi River Delta and the Brazos River Basin in Texas, establishing a direct connection between laboratory models and real-world planetary landforms.