Earth Science Quiz: Geologic Maps And Cross Sections

What this quiz covers

This quiz focuses on Geologic Maps And Cross Sections, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The apparent dip will be greater than 50°.
2. The apparent dip will be equal to 50°.
3. The apparent dip will be less than 50°. (correct answer)
4. The apparent dip could be greater or less than 50°, depending on the cross-section's orientation.

Explanation: True dip is the maximum angle of inclination of a geologic plane, measured in a vertical plane that is perpendicular to the strike. Any vertical cross-section taken at an angle oblique to the strike will intersect the plane at a shallower angle. This measured angle is the apparent dip, which is always less than or equal to the true dip. It is only equal if the cross-section is perpendicular to strike.

Question 2

1. A structural basin, with beds dipping away from the center.
2. A structural dome, with beds dipping away from the center. (correct answer)
3. A structural basin, with beds dipping toward the center.
4. A structural dome, with beds dipping toward the center.

Explanation: The key to identifying domes and basins is the age of the rocks at the center. In this case, the oldest rocks (Jurassic) are in the center, and the rocks get progressively younger outwards (Cretaceous, then Paleogene). This age pattern defines a structural dome. In a dome, the rock layers dip away from the central point of uplift.

Question 3

1. The layers were deposited in a cyclothem, with shale and sandstone naturally repeating.
2. The drill hole passed through the axis of an overturned isoclinal fold.
3. The drill hole passed through a thrust fault that placed older rocks on top of younger rocks. (correct answer)
4. An igneous sill of sandstone composition was intruded into the shale layer.

Explanation: Thrust faulting involves a low-angle reverse fault pushing an older sheet of rock (the hanging wall) over a younger sheet (the footwall). When a well is drilled through the top sheet, it encounters a certain sequence of rocks. After passing through the fault plane into the sheet below, it may encounter the same rock units again, but as part of the younger, lower sequence. This repetition of stratigraphy is a classic sign of drilling through a thrust fault.

Question 4

1. It dips upstream (north) at any angle.
2. It is vertical.
3. It dips downstream (south) at an angle STEEPER than the river valley gradient.
4. It dips downstream (south) at an angle GENTLER than the river valley gradient. (correct answer)

Explanation: This is a more complex application of the 'Rule of Vs.' While a contact dipping opposite to the stream flow creates a V pointing downstream, a V can also point downstream if the contact dips in the SAME direction as the stream. This happens only when the dip of the contact is less steep (gentler) than the gradient of the valley floor. In this case, the topography is incising downward faster than the geologic layer is dipping, creating a V that points downstream.

Question 5

1. 100 m (correct answer)
2. 115 m
3. 173 m
4. 400 m

Explanation: The relationship between outcrop width (W), true thickness (T), and dip angle (θ) on flat ground is given by the trigonometric formula T = W * sin(θ). Here, W = 200 meters and θ = 30°. Therefore, T = 200 m * sin(30°). Since sin(30°) = 0.5, the true thickness is 200 m * 0.5 = 100 m.

Question 6

1. The contact is vertical.
2. The contact is horizontal. (correct answer)
3. The contact dips steeply to the east.
4. The orientation cannot be determined from this information.

Explanation: A topographic contour line connects points of equal elevation. A geologic contact represents a surface. If the trace of a geologic surface on a map is parallel to contour lines, it means that the surface itself has a constant elevation. A geologic surface with a constant elevation is, by definition, horizontal.

Question 7

1. It dips upstream (north) at any angle.
2. It is vertical.
3. It dips downstream (south) at an angle STEEPER than the river valley gradient.
4. It dips downstream (south) at an angle GENTLER than the river valley gradient. (correct answer)

Explanation: This is a more complex application of the 'Rule of Vs.' While a contact dipping opposite to the stream flow creates a V pointing downstream, a V can also point downstream if the contact dips in the SAME direction as the stream. This happens only when the dip of the contact is less steep (gentler) than the gradient of the valley floor. In this case, the topography is incising downward faster than the geologic layer is dipping, creating a V that points downstream.

Question 8

1. 63°
2. 45°
3. 30°
4. 27° (correct answer)

Explanation: The dip angle can be calculated using trigonometry. We have a right triangle where the horizontal distance perpendicular to strike is the adjacent side (100 m) and the vertical change in elevation is the opposite side (400 m - 350 m = 50 m). The tangent of the dip angle (θ) is the ratio of the opposite side to the adjacent side. So, tan(θ) = 50 m / 100 m = 0.5. To find the angle, we take the arctangent: θ = arctan(0.5) ≈ 26.6°, which is approximately 27°.

Question 9

1. A structural basin, with beds dipping away from the center.
2. A structural dome, with beds dipping away from the center. (correct answer)
3. A structural basin, with beds dipping toward the center.
4. A structural dome, with beds dipping toward the center.

Explanation: The key to identifying domes and basins is the age of the rocks at the center. In this case, the oldest rocks (Jurassic) are in the center, and the rocks get progressively younger outwards (Cretaceous, then Paleogene). This age pattern defines a structural dome. In a dome, the rock layers dip away from the central point of uplift.

Question 10

1. The contact is vertical.
2. The contact is horizontal. (correct answer)
3. The contact dips steeply to the east.
4. The orientation cannot be determined from this information.

Explanation: A topographic contour line connects points of equal elevation. A geologic contact represents a surface. If the trace of a geologic surface on a map is parallel to contour lines, it means that the surface itself has a constant elevation. A geologic surface with a constant elevation is, by definition, horizontal.

Question 11

1. 63°
2. 45°
3. 30°
4. 27° (correct answer)

Explanation: The dip angle can be calculated using trigonometry. We have a right triangle where the horizontal distance perpendicular to strike is the adjacent side (100 m) and the vertical change in elevation is the opposite side (400 m - 350 m = 50 m). The tangent of the dip angle (θ) is the ratio of the opposite side to the adjacent side. So, tan(θ) = 50 m / 100 m = 0.5. To find the angle, we take the arctangent: θ = arctan(0.5) ≈ 26.6°, which is approximately 27°.

Question 12

1. 100 m (correct answer)
2. 115 m
3. 173 m
4. 400 m

Explanation: The relationship between outcrop width (W), true thickness (T), and dip angle (θ) on flat ground is given by the trigonometric formula T = W * sin(θ). Here, W = 200 meters and θ = 30°. Therefore, T = 200 m * sin(30°). Since sin(30°) = 0.5, the true thickness is 200 m * 0.5 = 100 m.

Question 13

1. The layers were deposited in a cyclothem, with shale and sandstone naturally repeating.
2. The drill hole passed through the axis of an overturned isoclinal fold.
3. The drill hole passed through a thrust fault that placed older rocks on top of younger rocks. (correct answer)
4. An igneous sill of sandstone composition was intruded into the shale layer.

Explanation: Thrust faulting involves a low-angle reverse fault pushing an older sheet of rock (the hanging wall) over a younger sheet (the footwall). When a well is drilled through the top sheet, it encounters a certain sequence of rocks. After passing through the fault plane into the sheet below, it may encounter the same rock units again, but as part of the younger, lower sequence. This repetition of stratigraphy is a classic sign of drilling through a thrust fault.

Question 14

1. The apparent dip will be greater than 50°.
2. The apparent dip will be equal to 50°.
3. The apparent dip will be less than 50°. (correct answer)
4. The apparent dip could be greater or less than 50°, depending on the cross-section's orientation.

Explanation: True dip is the maximum angle of inclination of a geologic plane, measured in a vertical plane that is perpendicular to the strike. Any vertical cross-section taken at an angle oblique to the strike will intersect the plane at a shallower angle. This measured angle is the apparent dip, which is always less than or equal to the true dip. It is only equal if the cross-section is perpendicular to strike.

Question 15

1. Deposition of upper layers, tilting, erosion, deposition of lower layers.
2. Deposition of lower layers, erosion, tilting, deposition of upper layers.
3. Deposition of lower layers, tilting and uplift, erosion, subsidence and deposition of upper layers. (correct answer)
4. Deposition of all layers, tilting of the entire sequence, then erosion of the top surface.

Explanation: The formation of an angular unconformity requires a specific sequence. First, the lower sedimentary layers were deposited horizontally (Principle of Original Horizontality). Second, these layers were subjected to tectonic forces that caused them to tilt, and were uplifted. Third, the tilted layers were eroded, creating a flat surface. Fourth, the area subsided, and the upper horizontal layers were deposited on top of the erosion surface.

Question 16

1. A structural dome, with beds dipping away from a central point.
2. A west-plunging anticline.
3. A north-south trending monocline with beds dipping to the east.
4. A west-plunging syncline. (correct answer)

Explanation: The strike and dip symbols indicate the three-dimensional orientation of the rock layers. In the northern part of the map, the beds strike east-west and dip to the south. In the southern part, the beds strike east-west and dip to the north. This indicates the beds are dipping toward a central east-west axis, which defines a syncline. At the western edge, the strike lines curve around and the dip is to the east. This indicates the fold axis is plunging to the west.

Question 17

1. An intrusive contact
2. A fault contact
3. A conformable contact
4. A nonconformity (correct answer)

Explanation: Location Z is on the boundary between the underlying granite (an intrusive igneous rock) and the overlying sandstone (a sedimentary rock). This specific type of unconformity, where sedimentary rocks are deposited on top of eroded igneous or metamorphic rocks, is called a nonconformity. An intrusive contact would show the granite cutting into the sandstone, meaning the granite would be younger.

Question 18

1. Compressional stress, which causes shortening of the crust.
2. Tensional stress, which causes extension of the crust. (correct answer)
3. Shear stress, which causes lateral tearing of the crust.
4. Lithostatic stress, which causes compaction of sediments.

Explanation: The fault shown, with the hanging wall moving down relative to the footwall, is a normal fault. Normal faults accommodate stretching or extension of the Earth's crust. This extension is caused by a tensional stress regime, where forces pull the crust apart.

Question 19

1. A right-lateral strike-slip fault.
2. A left-lateral strike-slip fault. (correct answer)
3. A normal fault, with the east side down.
4. A reverse fault, with the west side up.

Explanation: To determine the motion on a strike-slip fault, imagine standing on one side of the fault and looking across to the other. In this map, if you stand on the western block and look east, the sandstone layer on the eastern block is displaced to the left. Conversely, standing on the eastern block and looking west, the sandstone on the western block is displaced to the left. This indicates left-lateral motion.

Question 20

1. The contact represents a horizontal rock layer.
2. The contact represents a vertical rock layer.
3. The contact is dipping to the south (downstream). (correct answer)
4. The contact is dipping to the north (upstream).

Explanation: The 'Rule of Vs' states that when a dipping planar feature (like a geologic contact) crosses a valley, it forms a 'V' shape on the map. The 'V' points in the direction of dip, unless the bed is dipping upstream, in which case the V points downstream. In the common case where the V points upstream (as described here), the layer must be dipping downstream. Horizontal layers would create a V parallel to topographic contours, and vertical layers would form a straight line across the topography.