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OAE Earth and Space Science study materials
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OAE Earth and Space Science practice questions

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Question 1Nature of Science

A scientist measures the density of five identical rock samples and obtains values of 2.6, 2.7, 2.6, 2.6, and 2.7 g/cm³, while the accepted value is 3.0 g/cm³. Which statement best describes these results?

  1. The measurements are neither accurate nor precise
  2. The measurements are precise but not accurate Correct answer.
  3. The measurements are accurate but not precise
  4. The measurements are both accurate and precise

Explanation

Precision refers to how closely repeated measurements agree with one another, and these values cluster tightly around 2.65 g/cm³. Accuracy refers to closeness to the accepted value of 3.0 g/cm³, which these measurements miss by a consistent margin. Tight clustering with a systematic offset is the classic signature of high precision and low accuracy.

Question 2Nature of Science

In an experiment, a student changes the temperature of water and records how much sugar dissolves. In this investigation, the amount of dissolved sugar is the:

  1. control variable
  2. independent variable
  3. dependent variable Correct answer.
  4. constant

Explanation

The dependent variable is the factor that is measured and that responds to the manipulation. Here the student deliberately changes temperature and measures the resulting amount of dissolved sugar, making sugar solubility the response. The temperature being changed is the independent variable.

Question 3Nature of Science

A hypothesis in a geoscience investigation is best described as:

  1. a testable, falsifiable proposed explanation for an observed phenomenon Correct answer.
  2. a final conclusion reached before data collection
  3. a personal opinion that cannot be tested
  4. a proven scientific law that cannot be revised

Explanation

A scientific hypothesis is a proposed explanation that generates predictions capable of being tested against evidence. The key property is falsifiability: observations must be able to show the hypothesis is wrong. Hypotheses are provisional and can be revised as evidence accumulates.

Question 4Nature of Science

A geologist finds a layer of sedimentary rock containing marine fossils on top of a layer containing land-plant fossils. Applying the principle of superposition, what can be concluded?

  1. The fossils were deposited by tectonic activity rather than sedimentation
  2. The marine environment existed after the land environment at this location Correct answer.
  3. Both environments existed at exactly the same time
  4. The land environment existed after the marine environment at this location

Explanation

The principle of superposition states that in undisturbed sedimentary sequences, younger layers lie above older layers. Since the marine-fossil layer sits above the land-plant layer, the marine conditions came later. This reasoning assumes the sequence has not been overturned by tectonic forces.

Question 5Nature of Science

Which of the following is the best example of a scientific model used in Earth and space science?

  1. A textbook glossary of mineral names
  2. A news report describing a hurricane
  3. A computer simulation of mantle convection used to predict plate motion Correct answer.
  4. A single rock sample displayed in a museum

Explanation

A scientific model is a simplified representation of a system used to explain observations and make predictions. A computer simulation of mantle convection captures key physical processes and generates testable predictions about plate motion. Museum specimens, news reports, and glossaries do not function as predictive representations of a system.

Question 6Nature of Science

A student claims that because a theory is "just a theory," it is less reliable than a law. Which statement best corrects this misconception?

  1. Laws and theories are equally uncertain because both can be disproven
  2. A theory becomes a law once enough data are collected
  3. A scientific theory is a well-tested explanatory framework supported by extensive evidence, not a guess Correct answer.
  4. Theories apply only to biology, whereas laws apply to Earth science

Explanation

In science, a theory is a broad, well-substantiated explanation supported by multiple lines of evidence, such as plate tectonics or evolution. Laws describe consistent relationships, often mathematically, but do not replace theories. Both are subject to revision, yet neither is a mere guess.

Question 7Nature of Science

Which measurement tool would a geologist most appropriately use to determine the mass of a mineral specimen?

  1. A digital balance Correct answer.
  2. A thermometer
  3. A hand lens
  4. A graduated cylinder

Explanation

Mass is measured with a balance, which compares the specimen against known masses or uses a load cell. A graduated cylinder measures liquid volume, a thermometer measures temperature, and a hand lens aids visual inspection. Selecting the right instrument is a basic part of experimental design.

Question 8Nature of Science

A scientific claim about the age of a rock formation is considered reliable primarily because it is:

  1. reproducible and supported by independent lines of evidence Correct answer.
  2. stated by a single respected expert
  3. consistent with prior expectations
  4. published in a popular magazine

Explanation

Scientific reliability depends on reproducibility and corroboration by independent evidence, such as radiometric dating combined with stratigraphic relationships. Publication venue, individual authority, and prior expectation are not substitutes for evidence. Claims gain strength when multiple independent methods yield the same result.

Question 9Nature of Science

A geologist collects a sample from a lava flow and uses potassium-argon dating to determine its age. Why is this dating method appropriate for volcanic rock?

  1. Argon accumulates only in sedimentary rocks
  2. Lava flows contain abundant fossils that calibrate the argon ratio
  3. Potassium-argon dating measures the time since the rock was weathered
  4. The eruption resets the argon clock when molten rock cools and crystallizes Correct answer.

Explanation

Potassium-argon dating relies on the accumulation of argon-40 produced by radioactive decay of potassium-40. When lava erupts and solidifies, any pre-existing argon escapes, resetting the clock to zero. The measured argon-to-potassium ratio therefore records the time since crystallization.

Question 10Nature of Science

Two students measure the same fossil's length and obtain 12.4 cm and 12.5 cm. The accepted length is 12.45 cm. Which statement is most defensible?

  1. Their measurements are accurate but imprecise
  2. Their measurements are both accurate and precise Correct answer.
  3. Their measurements are neither accurate nor precise
  4. Their measurements are precise but inaccurate

Explanation

Precision reflects how closely repeated measurements agree, and 12.4 cm and 12.5 cm differ by only 0.1 cm. Accuracy reflects closeness to the accepted value of 12.45 cm, and both measurements are within 0.05 cm of it. When repeated trials agree closely and match the accepted value, the set is both precise and accurate.

Question 11Nature of Science

A researcher wants to test whether a new soil additive increases crop yield. Which experimental design best supports a valid causal inference?

  1. Apply the additive to all plots and compare yields with last year's crop
  2. Apply the additive only to the plots with the healthiest soil
  3. Randomly assign plots to additive or no-additive treatments while controlling light, water, and soil type Correct answer.
  4. Survey farmers who already use the additive and compare their opinions

Explanation

Valid causal inference requires a controlled comparison, so treated and untreated plots must be otherwise equivalent. Random assignment minimizes confounding, and holding light, water, and soil type constant isolates the additive's effect. Without a comparable control group, differences in yield cannot be attributed to the treatment.

Question 12Nature of Science

A scientist observes that a sequence of sedimentary layers is tilted and folded. Which principle must be applied before using superposition to determine relative ages?

  1. The principle of faunal succession, which replaces superposition
  2. The principle of original horizontality, which requires correction for later deformation Correct answer.
  3. The principle of uniformitarianism, which assumes no deformation
  4. The principle of cross-cutting relationships, which always overturns superposition

Explanation

Superposition applies to originally horizontal layers, so folding and tilting must be recognized and accounted for before inferring age order. Original horizontality tells us that deformed layers were once flat, allowing geologists to mentally restore them. Without this correction, younger-over-older logic could be misapplied.

Question 13Nature of Science

A team reports that a meteorite is 4.5 billion years old based on lead-isotope ratios. Which additional step would most strengthen confidence in this conclusion?

  1. Repeating the same lead analysis on the same instrument
  2. Publishing the result without peer review
  3. Obtaining consistent ages from an independent isotope system such as rubidium-strontium Correct answer.
  4. Comparing the result with a single prior estimate

Explanation

Independent corroboration using a different decay system reduces the chance that a systematic error in one method produced the age. Agreement between lead-lead and rubidium-strontium dating, for example, strengthens the conclusion. Repeating the same analysis only tests internal precision, not the accuracy of the method.

Question 14Nature of Science

Which statement best describes the role of peer review in Earth and space science?

  1. It replaces the need for replication
  2. It is conducted only by government agencies
  3. Independent experts evaluate the methods, evidence, and reasoning before publication Correct answer.
  4. It guarantees that published results are correct

Explanation

Peer review subjects a manuscript to scrutiny by independent specialists who assess whether the methods and evidence support the conclusions. It improves quality but does not guarantee correctness, and replication remains essential. Peer review is conducted across academia, industry, and government.

Question 15Nature of Science

A student predicts that a hotter magma will flow farther than a cooler magma of the same composition. Which observation would best test this hypothesis?

  1. Recording the color of cooled lava samples
  2. Measuring the distance traveled by magma samples of the same composition at different temperatures Correct answer.
  3. Measuring the temperature of a single lava flow at one location
  4. Comparing two lava flows of different compositions at the same temperature

Explanation

A valid test must vary the proposed cause (temperature) while holding other factors constant, such as composition and volume. Measuring flow distance across a temperature range directly tests the prediction. Varying composition or measuring only one flow introduces confounds or provides no comparison.

Question 16Nature of Science

A scientist uses a global climate model to project sea-level rise. Which statement best describes a key limitation of this model?

  1. Its projections depend on assumptions and simplifications that introduce uncertainty Correct answer.
  2. It is valid only for a single year
  3. It eliminates the need for empirical measurements
  4. It cannot be tested against any observational data

Explanation

All models simplify reality and rely on assumptions, such as future emission scenarios and ice-sheet behavior, that introduce uncertainty. Model projections are still tested against historical observations to build confidence. Recognizing uncertainty is essential for interpreting model output responsibly.

Question 17Nature of Science

A geologist maps a fault that cuts across several sedimentary layers. Using the principle of cross-cutting relationships, what can be concluded?

  1. The fault is older than the layers it cuts
  2. The fault is younger than the layers it cuts Correct answer.
  3. The fault formed at the same time as the layers
  4. The fault's age cannot be determined relative to the layers

Explanation

The principle of cross-cutting relationships holds that a geologic feature cutting another must be younger than the feature it cuts. A fault displacing sedimentary layers therefore postdates those layers. This principle allows relative dating of faults, intrusions, and veins.

Question 18Nature of Science

Which of the following best represents a quantitative observation in an Earth science investigation?

  1. The mineral sample looks like mica
  2. The mineral sample is shiny and dark
  3. The mineral sample feels smooth
  4. The mineral sample has a mass of 24.6 grams Correct answer.

Explanation

Quantitative observations involve numerical measurements with units, such as mass in grams. Qualitative observations describe properties like luster, texture, or appearance without numbers. Recording mass numerically allows statistical analysis and comparison.

Question 19Nature of Science

A researcher studying earthquake recurrence wants to estimate the probability of a major quake in the next 30 years. Which approach best reflects sound scientific practice?

  1. Rely on a single historical earthquake record
  2. Combine paleoseismic records, instrumental data, and fault-mechanics models while stating uncertainty Correct answer.
  3. Predict an exact date for the next earthquake
  4. Assume earthquake probability is constant and independent of fault history

Explanation

Probabilistic forecasting is strongest when multiple independent data sources are integrated, including prehistoric rupture records, modern seismicity, and physical models. Explicitly stating uncertainty allows decision-makers to interpret the forecast appropriately. Single records or exact-date predictions overstate what science can provide.

Question 20Nature of Science

A scientist wants to determine whether a newly discovered rock layer is older or younger than a nearby intrusion. Which principle provides the most direct answer?

  1. The principle of lateral continuity
  2. The principle of cross-cutting relationships Correct answer.
  3. The principle of superposition alone
  4. The principle of uniformitarianism

Explanation

Cross-cutting relationships establish that an intrusion is younger than the rocks it cuts. If the intrusion cuts the layer, the intrusion is younger; if the layer rests on eroded intrusion rock, the layer is younger. Superposition alone addresses layered sequences, not intrusive contacts.

Question 21Nature of Science

A geologist is analyzing a dataset of earthquake magnitudes recorded by three different networks. Which action is most important before comparing the datasets?

  1. Assume all networks measure identically
  2. Discard the dataset with the fewest records
  3. Verify that all networks used the same magnitude scale and calibration Correct answer.
  4. Average all values without checking units

Explanation

Comparing measurements across sources requires consistent scales, units, and calibration; otherwise differences may reflect methodology rather than the phenomenon. Verifying scale and calibration prevents systematic error from contaminating the analysis. Assumptions of identical measurement are unjustified without verification.

Question 22Nature of Science

A study reports a correlation between solar activity and global temperature over the past century. Which additional analysis is most critical before inferring causation?

  1. Increasing the number of data points by extending the record backward
  2. Confirming that both variables were measured in the same units
  3. Reporting the correlation coefficient to more decimal places
  4. Testing whether the correlation persists when other known forcings, such as greenhouse gases, are accounted for Correct answer.

Explanation

Correlation alone cannot establish causation because a third variable may drive both trends. Multiple regression or similar analysis that controls for other known forcings tests whether solar activity adds explanatory power. Greater precision or more data points do not resolve confounding.

Question 23Nature of Science

A geochronologist obtains an isochron with a very low slope from a rubidium-strontium analysis. What does the slope of the isochron primarily indicate?

  1. The mass of the sample
  2. The age of the sample, with shallower slopes indicating younger ages Correct answer.
  3. The temperature of the sample at formation
  4. The mineral composition of the sample

Explanation

On a rubidium-strontium isochron, the slope is proportional to the elapsed time since isotopic homogenization, so a shallow slope corresponds to a young age. The isochron also yields an initial isotope ratio from its intercept. Slope is therefore an age indicator, not a measure of temperature or mass.

Question 24Nature of Science

Two research groups study the same volcanic province and reach different conclusions about eruption timing. Which response best reflects the nature of scientific inquiry?

  1. Compare their sampling methods, analytical uncertainties, and assumptions to identify the source of disagreement Correct answer.
  2. Accept the conclusion of the more senior group without further review
  3. Average the two conclusions to produce a compromise age
  4. Discard both conclusions as unreliable

Explanation

Scientific disagreement is resolved by examining methodology, uncertainty, and assumptions rather than by authority or compromise. Differences may stem from sampling bias, analytical error, or differing interpretive models. This scrutiny often drives new measurements that resolve the discrepancy.

Question 25Nature of Science

A researcher proposes that a mass extinction was caused by a meteorite impact. Which combination of evidence would most strongly support this hypothesis?

  1. A single local fossil gap and one unverified crater
  2. A change in ocean temperature with no extraterrestrial signature
  3. A gradual decline in biodiversity with no impact markers
  4. A worldwide iridium anomaly, shocked quartz, and a crater of matching age Correct answer.

Explanation

Strong support requires multiple independent impact markers that converge on the same time: an iridium anomaly, shocked quartz, and a crater dated to the extinction horizon. Converging evidence from different sources is far more persuasive than any single observation. Gradual-change evidence would instead support a different causal hypothesis.

Question 26Nature of Science

A geologist finds that a granite intrusion contains abundant large crystals, while a nearby basalt flow contains only microscopic crystals. Which conclusion best follows from these observations about the cooling histories of the two rocks?

  1. The granite cooled slowly at depth, while the basalt cooled rapidly at the surface. Correct answer.
  2. Both rocks cooled at the same rate but from magmas of different composition.
  3. The granite cooled rapidly at the surface, while the basalt cooled slowly at depth.
  4. The basalt cooled slowly because it was buried beneath the granite.

Explanation

Crystal size in igneous rock reflects cooling rate: slow cooling allows ions time to organize into large crystals, whereas rapid cooling produces fine-grained or glassy textures. A coarse granite therefore indicates slow cooling, typically within a pluton at depth, and a fine-grained basalt indicates rapid cooling at or near the surface.

Question 27Nature of Science

A seismology team reports that a fault segment has a recurrence interval of about 200 years and that the last major rupture occurred 150 years ago. Which statement best describes what this information supports?

  1. A major earthquake on that segment is guaranteed within the next 50 years.
  2. The recurrence interval proves that no earthquake will occur for another 200 years.
  3. The segment has a higher conditional probability of rupture soon, but timing remains uncertain. Correct answer.
  4. The segment can no longer produce a major earthquake because stress has been released.

Explanation

A recurrence interval is a long-term average derived from the paleoseismic and historical record, not a schedule. When elapsed time approaches or exceeds that average, the conditional probability of rupture rises, yet earthquakes remain inherently stochastic, so no specific date can be guaranteed.

Question 28Nature of Science

A student reads that a mineral's hardness is reported as 6.5 on the Mohs scale and concludes that this value means the mineral is 6.5 times harder than talc. Which statement best evaluates this reasoning?

  1. It is correct, because hardness values are derived from absolute indentation measurements.
  2. It is correct, because Mohs values are ratio-scale measurements of resistance to scratching.
  3. It is incorrect, because the Mohs scale is ordinal and its intervals are not equal or proportional. Correct answer.
  4. It is incorrect, because hardness cannot be quantified and must be described qualitatively.

Explanation

The Mohs scale ranks minerals by relative scratching resistance, making it an ordinal scale in which the numerical gaps between successive minerals are unequal. Comparing Mohs numbers by ratio, such as claiming one mineral is 6.5 times harder than talc, is therefore invalid; quantitative hardness requires tests such as Vickers or Knoop indentation.

Question 29Earth's Structure and Internal Processes

Which layer of Earth's interior is composed primarily of solid iron and nickel and lies below the outer core?

  1. The lithosphere
  2. The asthenosphere
  3. The mantle
  4. The inner core Correct answer.

Explanation

Earth's core is divided into a liquid outer core and a solid inner core. The inner core is solid iron-nickel alloy despite extreme temperature because pressure is so great. The mantle and lithosphere lie above the core, not within it.

Question 30Earth's Structure and Internal Processes

The Mohorovičić discontinuity marks the boundary between which two regions?

  1. The crust and the mantle Correct answer.
  2. The lithosphere and asthenosphere
  3. The mantle and outer core
  4. The outer core and inner core

Explanation

The Moho is a seismic velocity boundary marking the base of the crust and top of the mantle. Seismic waves speed up abruptly there because mantle rock is denser and more rigid than crustal rock.

Question 31Earth's Structure and Internal Processes

Which type of seismic wave travels fastest and arrives first at a distant seismograph station?

  1. Love waves
  2. Surface waves
  3. S-waves
  4. P-waves Correct answer.

Explanation

P-waves are compressional body waves and travel fastest through rock, so they arrive first. S-waves are slower shear waves, and surface waves are slowest. Seismologists use the arrival-time difference to locate an earthquake.

Question 32Earth's Structure and Internal Processes

The S-wave shadow zone exists because S-waves cannot pass through the outer core. What does this observation indicate about the outer core?

  1. It is composed of solid rock
  2. It is cooler than the mantle
  3. It behaves as a liquid Correct answer.
  4. It has no density contrast with the mantle

Explanation

S-waves are shear waves and cannot propagate through fluids. Their absence in the shadow zone beyond about 103 degrees demonstrates that the outer core is liquid. P-waves also refract at this boundary, producing a P-wave shadow zone.

Question 33Earth's Structure and Internal Processes

At a mid-ocean ridge, new oceanic lithosphere forms as plates diverge. Which process best explains why ridges sit higher than surrounding abyssal plains?

  1. Ridge lithosphere is hot and thermally buoyant Correct answer.
  2. Ridge rock is more felsic than abyssal rock
  3. Ridge rock is older and therefore denser
  4. Sediment thickens over the ridge axis

Explanation

Young lithosphere near a ridge is hot and low in density, so it floats higher on the asthenosphere. As it moves away and cools, it contracts, thickens, and subsides, forming the sloping ridge flanks.

Question 34Earth's Structure and Internal Processes

A geologist observes magnetic stripe patterns on the seafloor that are symmetric about a mid-ocean ridge. What do these stripes record?

  1. Periods of increased meteorite impacts
  2. Variations in sediment deposition rate
  3. Reversals of Earth's magnetic field during crust formation Correct answer.
  4. Seasonal changes in ocean chemistry

Explanation

As basaltic magma cools at a ridge, magnetic minerals lock in the current field direction. Alternating normal and reversed polarity stripes on either side of the ridge record geomagnetic reversals and support seafloor spreading.

Question 35Earth's Structure and Internal Processes

At a subduction zone, which feature typically forms on the overriding plate above the descending slab?

  1. A transform fault valley
  2. An abyssal plain
  3. A volcanic arc Correct answer.
  4. A continental rift

Explanation

Water released from the descending slab lowers the melting point of mantle rock, generating magma that rises to form a volcanic arc on the overriding plate. Examples include the Andes and the Cascades.

Question 36Earth's Structure and Internal Processes

A deep-ocean trench is most directly associated with which plate boundary setting?

  1. Transform boundary
  2. Divergent boundary at a mid-ocean ridge
  3. Continental collision
  4. Ocean-continent or ocean-ocean convergence Correct answer.

Explanation

Trenches form where dense oceanic lithosphere bends and descends into the mantle at a convergent boundary. Continental collisions and ridges do not produce deep trenches.

Question 37Earth's Structure and Internal Processes

Which statement best describes the mechanism of elastic rebound during an earthquake?

  1. Faults slip continuously without storing strain
  2. Strain accumulates in rock until it ruptures and snaps back Correct answer.
  3. Magma pressure lifts the crust and triggers shaking
  4. Rock melts along a fault and then refreezes

Explanation

Rocks on either side of a locked fault deform elastically as stress builds. When friction is overcome, the rock ruptures and rebounds toward an undeformed shape, releasing stored energy as seismic waves.

Question 38Earth's Structure and Internal Processes

A seismograph station records P-wave arrival at 10:00:00 and S-wave arrival at 10:00:40. Using a time difference of 40 seconds, what can the station determine?

  1. The magnitude of the earthquake
  2. The distance to the epicenter Correct answer.
  3. The exact depth of the earthquake focus
  4. The direction the fault slipped

Explanation

The S-minus-P time difference grows with distance from the epicenter and can be converted to distance using a travel-time curve. Three stations are needed to triangulate the epicenter location.

Question 39Earth's Structure and Internal Processes

Which type of plate boundary is characterized by plates sliding horizontally past one another, producing shallow earthquakes but little volcanism?

  1. Transform Correct answer.
  2. Subduction
  3. Divergent
  4. Convergent

Explanation

Transform boundaries accommodate lateral motion between plates, such as the San Andreas Fault. Crust is neither created nor destroyed, so volcanism is generally absent and earthquakes are shallow.

Question 40Earth's Structure and Internal Processes

Which evidence most strongly supports the theory of continental drift proposed by Wegener?

  1. Similar atmospheric pressure worldwide
  2. The presence of liquid water on all continents
  3. Matching fossil and rock sequences across continents now separated by oceans Correct answer.
  4. Uniform global rainfall patterns

Explanation

Wegener cited matching fossils such as Glossopteris and aligned rock formations across South America, Africa, and Antarctica. These correlations imply the continents were once joined.

Question 41Earth's Structure and Internal Processes

The mantle convects slowly over geologic time. Which combination of properties makes mantle convection possible?

  1. A heat source and the ability of rock to deform ductilely Correct answer.
  2. Rapid rotation of Earth's core
  3. High rigidity and low temperature
  4. Complete melting of the entire mantle

Explanation

Convection requires a temperature gradient to drive buoyancy and a material that can flow over long timescales. Hot mantle rock rises, cools, and sinks, deforming ductilely despite being solid.

Question 42Earth's Structure and Internal Processes

Which heat-transfer process is primarily responsible for moving thermal energy through the solid interior of Earth over long distances?

  1. Conduction alone
  2. Convection in the mantle Correct answer.
  3. Radiation through opaque rock
  4. Latent heat release at the surface

Explanation

Although conduction transfers heat locally, large-scale heat transport in the mantle occurs by convection. Buoyant hot rock rises and cooler rock sinks, carrying heat efficiently over geologic time.

Question 43Earth's Structure and Internal Processes

Which characteristic distinguishes the asthenosphere from the lithosphere?

  1. The asthenosphere is rigid and brittle
  2. The asthenosphere is weaker and deforms more readily Correct answer.
  3. The lithosphere extends to the core-mantle boundary
  4. The lithosphere has no crustal component

Explanation

The asthenosphere is partially molten and ductile, allowing it to flow slowly and enabling plate motion. The lithosphere above it is cooler, rigid, and includes both crust and uppermost mantle.

Question 44Earth's Structure and Internal Processes

A geologist measures the age of seafloor basalt at increasing distances from a ridge axis. What pattern is expected if seafloor spreading is occurring?

  1. Age decreases away from the ridge
  2. Age is the same everywhere
  3. Age increases symmetrically away from the ridge Correct answer.
  4. Age increases only on one side of the ridge

Explanation

New crust forms at the ridge and moves outward, so crust gets progressively older with distance on both sides. This symmetric age pattern is a key confirmation of seafloor spreading.

Question 45Earth's Structure and Internal Processes

Which feature forms where two continental plates collide and neither is subducted?

  1. A broad mountain belt with thickened crust Correct answer.
  2. A mid-ocean ridge
  3. A deep-sea trench
  4. An oceanic plateau

Explanation

Continental collision, such as the India-Eurasia convergence, crumples and thickens buoyant continental crust into high mountains. Because both plates are buoyant, neither readily subducts.

Question 46Earth's Structure and Internal Processes

Which layer of Earth is inferred to be liquid based on the behavior of seismic waves?

  1. The lower mantle
  2. The inner core
  3. The outer core Correct answer.
  4. The lithosphere

Explanation

The outer core blocks S-waves and refracts P-waves, indicating a liquid metallic layer. The inner core is solid, and the mantle and lithosphere are solid rock.

Question 47Earth's Structure and Internal Processes

A researcher finds pillow basalts and sheeted dikes in an ancient rock sequence. What tectonic setting does this assemblage most likely represent?

  1. A continental collision zone
  2. A deep-sea trench
  3. A transform fault
  4. A former mid-ocean ridge Correct answer.

Explanation

Pillow basalts and sheeted dikes are characteristic of ophiolite sequences formed at spreading ridges. They record submarine eruption and intrusion of basaltic magma.

Question 48Earth's Structure and Internal Processes

The heat that drives plate tectonics comes primarily from which sources?

  1. Chemical reactions in the crust
  2. Meteorite impacts
  3. Radioactive decay and residual heat from Earth's formation Correct answer.
  4. Solar radiation and tidal friction

Explanation

Earth's internal heat budget is dominated by decay of radioactive isotopes and leftover heat from accretion and core formation. This heat drives mantle convection and plate motion.

Question 49Earth's Structure and Internal Processes

A fault offsets a lava flow dated at 5 million years and is overlain by sediments dated at 2 million years. What can be concluded about the fault?

  1. It formed before 5 Ma
  2. It formed after 5 Ma and before 2 Ma Correct answer.
  3. It formed after 2 Ma
  4. It formed exactly 5 Ma

Explanation

Cross-cutting relationships show the fault is younger than the flow it offsets and older than the sediments that cover it. The fault therefore formed between 5 and 2 million years ago.

Question 50Earth's Structure and Internal Processes

Which factor most directly controls whether a subducting slab generates a deep earthquake?

  1. The depth to which the cold slab remains brittle Correct answer.
  2. The wind speed above the ocean
  3. The presence of coral reefs
  4. The color of the oceanic sediment

Explanation

Deep earthquakes occur within cold subducting slabs where rock remains brittle and can rupture under stress. They trace the descending slab to depths of hundreds of kilometers.