ESS 101 B EXAMINATION 1 ACTUAL EXAM 2026/2027 |
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DOMAIN 1: EARTH'S STRUCTURE AND COMPOSITION (12 Questions)
Q1: The Mohorovičić discontinuity (Moho) represents the boundary between which two
Earth layers?
A. The outer core and inner core
B. The lithosphere and asthenosphere
C. The crust and the mantle [CORRECT]
D. The mantle and the outer core
Correct Answer: C
Rationale: The Moho is the seismic discontinuity that marks the boundary between
Earth's crust and the underlying mantle. It was discovered by Croatian seismologist
Andrija Mohorovičić in 1909, who observed that seismic waves increase in velocity at
this boundary due to the change in composition from felsic/mafic crustal rocks to
ultramafic mantle peridotite. Option A is incorrect—this boundary is defined by the
Lehmann discontinuity and changes in seismic wave behavior (liquid outer core to solid
inner core). Option B confuses the Moho with the lithosphere-asthenosphere boundary,
which is a rheological (physical state) boundary in the upper mantle, not a
,compositional boundary. Option D describes the core-mantle boundary (Gutenberg
discontinuity), located approximately 2,900 km depth.
Q2: Seismic wave velocities change dramatically at approximately 2,900 km depth,
where P-waves drop significantly and S-waves disappear entirely. What does this
indicate?
A. A transition from solid mantle to liquid outer core [CORRECT]
B. A change from felsic to mafic composition in the mantle
C. The boundary between crust and mantle
D. Evidence for a completely solid core throughout
Correct Answer: A
Rationale: The disappearance of S-waves (shear waves cannot travel through liquids)
and the dramatic decrease in P-wave velocity at 2,900 km depth defines the Gutenberg
discontinuity, marking the core-mantle boundary. This seismic evidence proves the
outer core is liquid. Option B is incorrect—compositional changes within the solid
mantle do not eliminate S-waves. Option C describes the Moho at 5-70 km depth.
Option D contradicts the seismic evidence; S-waves reappear in the inner core,
indicating it is solid.
Q3: Which statement accurately describes the relationship between the lithosphere and
asthenosphere?
A. The lithosphere is entirely located within the crust
,B. The asthenosphere is mechanically rigid and drives plate motion through active
pushing
C. The lithosphere includes the crust and uppermost rigid mantle, riding atop the ductile
asthenosphere [CORRECT]
D. The asthenosphere is composed of liquid rock that flows rapidly
Correct Answer: C
Rationale: The lithosphere is Earth's rigid outer layer, comprising the crust and
uppermost mantle (to ~100 km depth), broken into tectonic plates. It overlies the
asthenosphere, a mechanically weak, ductile layer of the upper mantle (100-660 km)
that deforms plastically under stress, allowing plate movement. Option A is
incorrect—the lithosphere extends below the crust into the mantle. Option B reverses
the properties; the lithosphere is rigid, while the asthenosphere is ductile. Option D is
incorrect—the asthenosphere is solid but ductile (flows slowly over geologic time), not
liquid.
Q4: Earth's magnetic field is generated primarily by:
A. The permanent magnetization of crustal rocks
B. Convection of liquid iron-nickel alloy in the outer core, creating a self-sustaining
geodynamo [CORRECT]
C. Solar wind interactions with the solid inner core
D. Remnant magnetism from Earth's formation 4.6 billion years ago
Correct Answer: B
, Rationale: Earth's magnetic field is generated by the geodynamo effect: convection of
electrically conductive liquid iron-nickel in the outer core, combined with Earth's rotation
(Coriolis effect), creates complex fluid motions that generate and sustain the magnetic
field. This is confirmed by the presence of magnetic field reversals recorded in seafloor
basalts. Option A describes remanent magnetism in rocks, not the planetary field.
Option C confuses the magnetosphere (protective bubble) generation with field
generation. Option D is incorrect—permanent magnetism cannot explain field reversals
or the field's dynamic nature.
Q5: [Image Identification] A cross-sectional diagram shows concentric layers labeled A
through D from surface to center. Layer B shows intermediate seismic velocities
between the slow surface layer and the very fast deep layer. Layer B extends from ~10
km to ~660 km depth. Identify Layer B.
A. Outer core
B. Inner core
C. Mantle [CORRECT]
D. Crust
Correct Answer: C
Rationale: The mantle extends from the Moho (~10 km depth in oceans, ~70 km under
continents) to the core-mantle boundary at 2,900 km. It displays intermediate seismic
velocities (P-waves ~8-14 km/s) between the slow crust (P-waves ~6-7 km/s) and the
fast core (P-waves ~8-10 km/s in outer core, ~11 km/s in inner core). The 660 km depth
marks the base of the upper mantle transition zone. The outer core (A) begins at 2,900
km. The inner core (B) is the central sphere. The crust (D) is the thin outermost layer.
Already Graded A+ Questions and Answers with Detailed
Rationales | Geology Comprehensive Assessment | Pass
Guaranteed
DOMAIN 1: EARTH'S STRUCTURE AND COMPOSITION (12 Questions)
Q1: The Mohorovičić discontinuity (Moho) represents the boundary between which two
Earth layers?
A. The outer core and inner core
B. The lithosphere and asthenosphere
C. The crust and the mantle [CORRECT]
D. The mantle and the outer core
Correct Answer: C
Rationale: The Moho is the seismic discontinuity that marks the boundary between
Earth's crust and the underlying mantle. It was discovered by Croatian seismologist
Andrija Mohorovičić in 1909, who observed that seismic waves increase in velocity at
this boundary due to the change in composition from felsic/mafic crustal rocks to
ultramafic mantle peridotite. Option A is incorrect—this boundary is defined by the
Lehmann discontinuity and changes in seismic wave behavior (liquid outer core to solid
inner core). Option B confuses the Moho with the lithosphere-asthenosphere boundary,
which is a rheological (physical state) boundary in the upper mantle, not a
,compositional boundary. Option D describes the core-mantle boundary (Gutenberg
discontinuity), located approximately 2,900 km depth.
Q2: Seismic wave velocities change dramatically at approximately 2,900 km depth,
where P-waves drop significantly and S-waves disappear entirely. What does this
indicate?
A. A transition from solid mantle to liquid outer core [CORRECT]
B. A change from felsic to mafic composition in the mantle
C. The boundary between crust and mantle
D. Evidence for a completely solid core throughout
Correct Answer: A
Rationale: The disappearance of S-waves (shear waves cannot travel through liquids)
and the dramatic decrease in P-wave velocity at 2,900 km depth defines the Gutenberg
discontinuity, marking the core-mantle boundary. This seismic evidence proves the
outer core is liquid. Option B is incorrect—compositional changes within the solid
mantle do not eliminate S-waves. Option C describes the Moho at 5-70 km depth.
Option D contradicts the seismic evidence; S-waves reappear in the inner core,
indicating it is solid.
Q3: Which statement accurately describes the relationship between the lithosphere and
asthenosphere?
A. The lithosphere is entirely located within the crust
,B. The asthenosphere is mechanically rigid and drives plate motion through active
pushing
C. The lithosphere includes the crust and uppermost rigid mantle, riding atop the ductile
asthenosphere [CORRECT]
D. The asthenosphere is composed of liquid rock that flows rapidly
Correct Answer: C
Rationale: The lithosphere is Earth's rigid outer layer, comprising the crust and
uppermost mantle (to ~100 km depth), broken into tectonic plates. It overlies the
asthenosphere, a mechanically weak, ductile layer of the upper mantle (100-660 km)
that deforms plastically under stress, allowing plate movement. Option A is
incorrect—the lithosphere extends below the crust into the mantle. Option B reverses
the properties; the lithosphere is rigid, while the asthenosphere is ductile. Option D is
incorrect—the asthenosphere is solid but ductile (flows slowly over geologic time), not
liquid.
Q4: Earth's magnetic field is generated primarily by:
A. The permanent magnetization of crustal rocks
B. Convection of liquid iron-nickel alloy in the outer core, creating a self-sustaining
geodynamo [CORRECT]
C. Solar wind interactions with the solid inner core
D. Remnant magnetism from Earth's formation 4.6 billion years ago
Correct Answer: B
, Rationale: Earth's magnetic field is generated by the geodynamo effect: convection of
electrically conductive liquid iron-nickel in the outer core, combined with Earth's rotation
(Coriolis effect), creates complex fluid motions that generate and sustain the magnetic
field. This is confirmed by the presence of magnetic field reversals recorded in seafloor
basalts. Option A describes remanent magnetism in rocks, not the planetary field.
Option C confuses the magnetosphere (protective bubble) generation with field
generation. Option D is incorrect—permanent magnetism cannot explain field reversals
or the field's dynamic nature.
Q5: [Image Identification] A cross-sectional diagram shows concentric layers labeled A
through D from surface to center. Layer B shows intermediate seismic velocities
between the slow surface layer and the very fast deep layer. Layer B extends from ~10
km to ~660 km depth. Identify Layer B.
A. Outer core
B. Inner core
C. Mantle [CORRECT]
D. Crust
Correct Answer: C
Rationale: The mantle extends from the Moho (~10 km depth in oceans, ~70 km under
continents) to the core-mantle boundary at 2,900 km. It displays intermediate seismic
velocities (P-waves ~8-14 km/s) between the slow crust (P-waves ~6-7 km/s) and the
fast core (P-waves ~8-10 km/s in outer core, ~11 km/s in inner core). The 660 km depth
marks the base of the upper mantle transition zone. The outer core (A) begins at 2,900
km. The inner core (B) is the central sphere. The crust (D) is the thin outermost layer.
