Geoscience Review Flashcards

Flashcards based on lecture notes covering observations vs. inferences, Earth's layers, plate tectonics, dating methods, and extinction events.

What is an observation?

Direct measurement (e.g., "CO2 levels rose by 20 ppm")

What is an inference?

Interpretation (e.g., "The rise is due to fossil fuel burning.")

What is an observation?

Direct measurement (e.g., "CO2 levels rose by 20 ppm")

What is an inference?

Interpretation (e.g., "The rise is due to fossil fuel burning.")

How do geoscientists use models to test hypotheses?

Models (e.g., climate simulations) incorporate data to predict outcomes; if predictions match observations, the hypothesis gains support.

Why are models like NASA’s "Earth at Night" useful?

They visualize indirect data (e.g., light pollution = human activity) to test hypotheses about urbanization impacts.

What’s the role of computational thinking in geoscience?

Math/computers quantify relationships (e.g., to simulate natural processes).

How would you interpret a time-series graph of global temperatures?

Rising trend → inference about climate change; variability → natural cycles (e.g., El Niño).

What patterns might you look for in a topographic map?

River valleys (erosion), mountain ranges (tectonic activity), or coastal changes (sea-level rise).

What are Earth’s compositional layers?

Crust (low-density silicates), Mantle (high-density silicates), Core (iron/nickel).

How do mechanical layers differ?

Lithosphere (rigid), Asthenosphere (plastic/flowing), Mesosphere (stiff lower mantle), Outer Core (liquid), Inner Core (solid).

What indirect evidence supports Earth’s layered structure?

Seismic waves (P-waves slow in liquid outer core, S-waves don’t pass through liquids), density calculations, and meteorite composition.

Why do planets have layers?

Differentiation during the molten state—denser materials (iron) sank to form cores, lighter silicates rose to form crust/mantle.

Why is Earth’s crust less dense than the core?

Crust = low-density silicates (e.g., granite, basalt); core = high-density iron/nickel (differentiation during formation).

What hypothesis explains Earth’s iron-rich core?

"Iron sank during the molten state due to high density, supported by seismic data and meteorite comparisons."

How does the nebular model explain planetary differentiation?

Collisions/heat in the early solar system melted materials, allowing density-based layering (supported by asteroid and planet compositions).

How do P-waves and S-waves reveal Earth's internal structure?

P-waves (Primary): Compressional waves that travel through solids and liquids. They slow down in the outer core (liquid) and refract, creating a shadow zone between 105°–140° from the epicenter. S-waves (Secondary): Shear waves that only travel through solids. Their disappearance below 2,900 km depth proves the outer core is liquid.

Why is oceanic crust denser than continental crust?

Oceanic Crust: Made of basalt (mafic: high Fe/Mg, low silica). Forms at mid-ocean ridges via rapid cooling of magma. Density = ~2.9 g/cm³. Continental Crust: Made of granite (felsic: high silica, low Fe/Mg). Forms at subduction zones where melting produces lighter magma. Density = ~2.7 g/cm³.

How does the inner core remain solid despite extreme heat?

Pressure at Earth’s center (~3.6 million atm) raises the melting point of iron-nickel alloy beyond its temperature (~5,700°C).

How does granite form vs. basalt?

Granite: Intrusive igneous rock. Magma cools slowly underground (e.g., subduction zones), allowing large crystals (quartz, feldspar) to form. Basalt: Extrusive igneous rock. Lava cools rapidly at the surface (e.g., mid-ocean ridges), creating a fine-grained texture.

What processes create metamorphic rocks?

Parent Rock Exists (e.g., shale). Heat/Pressure (e.g., at convergent boundaries). Recrystallization (e.g., shale → slate → schist → gneiss).

How does uniformitarianism explain the Grand Canyon?

Slow erosion by the Colorado River over millions of years (supported by layered sedimentary rocks).

When might catastrophism apply?

Sudden events like asteroid impacts (e.g., Chicxulub crater) or megafloods (e.g., Channeled Scablands).

How does the principle of superposition help determine rock layer ages?

In undisturbed sedimentary sequences, the oldest layers are at the bottom, and the youngest are at the top. This reflects the order of deposition over time.

What do cross-cutting relationships reveal about geologic events?

Features like faults or igneous intrusions must be younger than the rocks they cut through. For example, a dike cutting across sedimentary layers formed after those layers were deposited.

How can you identify an unconformity?

Angular unconformities: Tilted layers overlain by horizontal layers (e.g., Siccar Point). Nonconformities: Sedimentary rocks atop igneous/metamorphic rocks (erosion exposed deep rocks before new sediment accumulated).

Why can’t sedimentary rocks be directly dated with isotopes?

Sedimentary rocks are made of recycled minerals from older rocks. Isotopes date the original mineral formation, not the sedimentary deposition. Use fossils or interbedded volcanic layers for age constraints.

How do igneous rocks help with absolute dating?

Minerals like zircon crystallize from magma and trap radioactive isotopes (e.g., U-Pb). Measuring isotope decay yields the rock’s crystallization age.

What does metamorphism reveal about geologic history?

Metamorphic rocks record tectonic events (e.g., mountain building). Dating them pinpoints when deformation/heat occurred, not the original rock’s age.

What sequence of events created the Grand Canyon’s Great Unconformity?

Deposition of 1.75-billion-year-old metamorphic/igneous rocks. Uplift and erosion (missing ~1.2 billion years of layers). Deposition of horizontal sedimentary rocks ~545 million years ago.

How does isostasy cause uplift?

Removal of weight (e.g., melting glaciers) makes the crust rebound. Example: Scandinavia rising ~1 cm/year after Ice Age glacier loss.

How do the Acasta Gneiss (3.96 Ga) and detrital zircons (4.4 Ga) constrain Earth’s age?

Acasta Gneiss: Oldest intact rock, proving crust existed 3.96 billion years ago. Zircons: Survive erosion and record even older crust (4.4 Ga), suggesting liquid water existed then (cooled surface).

How does the principle of cross-cutting relationships determine the sequence of events in a geologic cross-section?

Any feature that cuts across another (e.g., faults, igneous intrusions) must be younger than the rock it disrupts. For example: A granite dike cutting through sedimentary layers formed after those layers were deposited. A fault displacing rock layers must post-date the layers it offsets.

How can you identify an unconformity in a rock sequence?

Angular unconformity: Tilted layers overlain by horizontal layers (indicating erosion after deformation). Nonconformity: Sedimentary rocks atop igneous/metamorphic rocks (exposed by erosion of overlying material). Disconformity: Missing layers between parallel sedimentary beds (gap in deposition).

How are index fossils used to correlate rock layers across regions?

Species that lived for a short geologic time (e.g., trilobites in the Paleozoic) and were widespread geographically. Their presence dates the layer they’re found in (e.g., a Tyrannosaurus rex fossil indicates Late Cretaceous rocks).

Why can’t fossils alone provide absolute ages?

Fossils give relative ages. Absolute dates come from radiometric dating of volcanic layers above/below fossils (e.g., ash beds).

Why is uranium-lead (U-Pb) dating used for ancient rocks, while carbon-14 is limited to recent fossils?

U-Pb: Half-life = 4.5 billion years (ideal for Earth’s oldest rocks, like zircon grains). C-14: Half-life = 5,730 years (only useful for organic material <60,000 years old).

How do scientists calculate a rock’s age using half-lives?

Measure the ratio of parent (e.g., U-238) to daughter (Pb-206) isotopes. Use the formula: Age = (Half-life × Number of half-lives). Example: If 25% of U-238 remains in a zircon, two half-lives have passed (9 billion years total).

How was the geologic time scale constructed before radiometric dating?

Based on fossil succession and major extinction events (e.g., Cambrian explosion, Permian extinction). Absolute dates were added later via isotopic methods.

Why is the Precambrian less subdivided than the Phanerozoic?

Fewer preserved fossils in Precambrian rocks (complex life arose ~541 Ma).

How do hot spots form, and what geologic features do they create?

Formation: Mantle plumes (stationary upwellings of hot rock) melt crust, forming volcanoes. Features: Chains of volcanoes (e.g., Hawaiian Islands) as plates move over the plume and flood basalts (massive eruptions, e.g., Deccan Traps) from prolonged hot spot activity.

Why are flood basalts linked to mass extinctions?

They release CO₂ (causes rapid global warming/ocean acidification) and SO₂ (forms acid rain and blocks sunlight → cooling).

What evidence supports asteroid impacts as extinction triggers?

Chicxulub crater (66 Ma): Iridium layer (rare in Earth’s crust but common in asteroids) and shocked quartz and tektites (melted rock droplets) are found globally. Pattern: Sudden fossil disappearance (e.g., non-avian dinosaurs).

How do volcanic eruptions differ from impacts in extinction patterns?

Volcanoes: Gradual decline (e.g., foram fossils weaken before extinction). Impacts: Immediate catastrophe (e.g., "nuclear winter" from debris).

How did the Deccan Traps contribute to the K-Pg extinction?

Erupted ~66 Ma (same time as Chicxulub). Released CO₂ (warming) and SO₂ (cooling), destabilizing ecosystems. Debate: Did volcanism weaken life before the asteroid finished it off?

What made the Permian extinction unique?

Cause: Siberian Traps eruptions + ozone destruction (malformed plant spores). Impact: 96% of marine species lost; it took 10M years to recover.

How does modern CO₂ release compare to flood basalts?

Deccan Traps: ~300–900 million tons CO₂/year (over 1M years). Humans: 32 billion tons CO₂/year (2023) → 100x faster than historic volcanism.

How do hot spots differ from divergent boundaries in magma generation and geologic features?

Hot Spots: Cause: Mantle plumes (stationary heat anomalies) melt crust. Features: Volcano chains (e.g., Hawaii), flood basalts (e.g., Deccan Traps). Magma: Mafic (basalt) from decompression melting. Divergent Boundaries: Cause: Plate separation allows mantle upwelling. Features: Mid-ocean ridges (e.g., Mid-Atlantic Ridge), rift valleys. Magma: Mafic (basalt) from partial melting of rising asthenosphere.

How does paleomagnetism provide evidence for seafloor spreading?

Igneous rocks record Earth’s magnetic field orientation when they cool. Evidence: Symmetrical magnetic stripes (normal/reversed polarity) parallel to mid-ocean ridges. Example: The crust is younger near ridges and older near subduction zones (the oldest oceanic crust is 280 Ma).

Why is no oceanic crust older than 300 million years preserved?

Oceanic crust subducts due to high density (mafic composition), while buoyant continental crust resists subduction.

How does mantle convection drive plate tectonics?

Upwelling: Hot, less dense mantle rises at divergent boundaries, creating new crust. Lateral Flow: Mantle drags plates laterally. Downwelling: Cold, dense slabs sink at subduction zones (e.g., Pacific Plate).

Why do some plates move faster than others?

Speed depends on slab pull (subduction force) and ridge push (gravitational sliding from elevated ridges).

What evidence supports Wegener’s continental drift hypothesis?

Fossils: Identical species (e.g., Mesosaurus) on separate continents. Rock Types: Matching mountain belts (e.g., Appalachians-Caledonides). Paleoclimate: Glacial striations in tropical regions (e.g., India).

How does the Hawaiian chain demonstrate plate motion?

Progression: Older volcanoes (e.g., Kauai, 5.6–4.9 Ma) to younger (e.g., Kilauea, active). Direction: NW-SE alignment matches Pacific Plate movement.

How does seafloor spreading create new oceanic crust, and what features are formed?

Mantle upwelling at mid-ocean ridges (e.g., Mid-Atlantic Ridge) causes decompression melting → mafic (basaltic) magma erupts, forming new crust. Features: Symmetrical paleomagnetic stripes (youngest crust at the ridge). Rift valleys (continental divergence, e.g., East African Rift). Passive margins (transition from continental to oceanic crust; no tectonic activity).