- In a road cut, a geologist observes a sequence of parallel sedimentary layers. The lowest layer contains fossils of trilobites from the Ordovician period. The layer directly above it, with no evidence of an irregular contact, contains fossils of fish from the Devonian period. Fossils from the Silurian period, which is between the Ordovician and Devonian, are absent. This boundary represents a(n):
Answer: disconformity, representing a period of erosion or non-deposition without tilting.
A disconformity is a type of unconformity where the sedimentary layers above and below the erosional surface are parallel. The key evidence is the missing time in the fossil record (the entire Silurian period is absent). This indicates that either deposition stopped for a long period, or layers were deposited and then later eroded away before Devonian deposition began. Since the layers are parallel, it is a disconformity, not an angular unconformity. It is not a nonconformity because the underlying rock is sedimentary, not igneous/metamorphic.
- In the cross-section below, a fault has displaced sedimentary layers A, B, and C, but does not cut through layer D. An igneous intrusion, E, cuts through layers A and B, but is truncated by the unconformity below layer C. What is the relative age of the fault?
Answer: The fault is younger than layer C but older than layer D.
Let's trace the events. Layers A and B were deposited. Intrusion E cut through A and B. Then an unconformity formed, eroding the top of A, B, and E. Layer C was deposited on this unconformity. After C was deposited, the fault occurred, as it cuts through A, B, and C. Finally, layer D was deposited on top of everything, including the fault, because layer D is not cut by the fault. This means the faulting event happened after the deposition of layer C but before the deposition of layer D.
- Relative dating principles are fundamental to interpreting geologic history. However, which of the following questions about the history of a rock sequence CANNOT be answered using relative dating principles alone?
Answer: How many millions of years passed during the time represented by the disconformity?
Relative dating principles (superposition, cross-cutting, inclusions) allow geologists to determine the sequence of events (what happened before what), but they do not provide numerical ages. To determine the duration of a time gap, such as an unconformity, in millions of years, absolute dating methods (like radiometric dating) of layers above and below the gap are required. The other questions can all be answered by applying relative dating principles to the geometric relationships between rock units.
- An igneous sill is identified between a lower layer of shale and an upper layer of sandstone. A geologist observes contact metamorphism (baking) on the top surface of the shale and on the bottom surface of the sandstone. What does this specific evidence imply about the formation of the sill?
Answer: The sill is younger than both the shale and the sandstone because it intruded between them after they were both present.
Contact metamorphism occurs when hot magma comes into contact with existing rock. The presence of a 'baked' zone on the shale below and the sandstone above indicates that both layers were already in place when the molten rock intruded. This makes the sill younger than both layers it is between, according to the principle of cross-cutting relationships. If it were a lava flow (older than the sandstone), there would be no contact metamorphism on the bottom of the sandstone.
- The diagram shows a rock outcrop with a basaltic sill and a basaltic dike. Both are of similar composition. Based on their relationships with the surrounding sedimentary layers, what can be concluded?
Answer: The dike is younger than the sill because the dike cuts across the sill.
The principle of cross-cutting relationships is key here. Layer 1 is oldest, followed by 2 and 3 by superposition. The sill intruded parallel to the layers, so it is younger than layers 1 and 2. The dike is a vertical intrusion that cuts through layers 1, 2, 3, and the sill. Because the dike cuts across the sill, the dike must be younger than the sill.
- The geologic cross-section below shows several rock units and geologic features. Which statement provides the most accurate relative age determination for Fault M and the Dike N?
Answer: Fault M is younger than Dike N because the dike is offset by the fault.
The principle of cross-cutting relationships states that a geologic feature which cuts another is the younger of the two. In the diagram, Dike N is clearly cut and displaced by Fault M. Therefore, Dike N must have existed first for the fault to offset it. This makes Fault M younger than Dike N.
- A sandstone layer contains inclusions of granite. The sandstone layer is cut by a basalt dike. The granite is not seen in the outcrop, but is known to exist deep in the region. What is the correct sequence of formation, from oldest to youngest?
Answer: Granite, sandstone, basalt dike
When you encounter geological formations with different rock types and structures, you need to apply the principles of relative dating to determine the sequence of events. The key is recognizing that certain geological relationships tell us about timing.
Let's work through this step by step. Since the sandstone contains inclusions of granite, the granite must have existed first and been weathered to provide fragments for the sedimentary rock. You can't include pieces of something that doesn't exist yet. Additionally, since the basalt dike cuts through the sandstone layer, the dike must be younger than the sandstone it intrudes. Igneous intrusions always occur after the rocks they cut through.
This gives us the sequence: granite (oldest) → sandstone → basalt dike (youngest), which is answer choice A.
Answer choice B incorrectly places sandstone before granite, but this is impossible since the sandstone contains granite fragments. Choice C puts the basalt dike first, which contradicts the fact that dikes intrude into pre-existing rocks. Choice D places the basalt dike before the sandstone, again ignoring the intrusive relationship where the dike cuts through the sandstone.
Remember the fundamental rule: inclusions are older than the rock containing them, and intrusions are younger than the rock they cut through. When you see inclusions and cross-cutting relationships in geology problems, immediately think about what had to exist first for these relationships to form. This logical approach will help you sequence geological events correctly every time.
- The contact between the shale and the sandstone in the diagram is a disconformity. What is the most significant geological implication of this feature?
Answer: A significant interval of geologic time is not represented by rock in this sequence.
Any unconformity, including a disconformity, represents a gap in the geologic record. This gap is often called 'missing time'. It means that for a period of time, either no sediment was deposited, or sediment was deposited and then later eroded away before the next sequence of deposition began. The primary implication is that the rock record at this location is incomplete. While a change in environment (A) might occur, the defining feature is the time gap. (C) describes an angular unconformity, not a disconformity. (D) is the opposite of what is implied by a non-depositional or erosional gap.
- A geologist is trying to distinguish between a buried lava flow and an intrusive sill. Which piece of evidence would uniquely identify the feature as a buried lava flow?
Answer: A fine-grained, vesicular (bubbly) texture in the upper portion of the igneous body.
When distinguishing between buried lava flows and intrusive sills, you need to focus on evidence that reveals how and where the magma cooled. Both are tabular igneous bodies, but lava flows formed at Earth's surface before being buried, while sills formed underground when magma intruded between existing rock layers.
The key evidence is a fine-grained, vesicular texture in the upper portion (A). This bubbly texture forms when gas bubbles are trapped as lava cools rapidly at the surface. The fine-grained upper portion indicates quick cooling in contact with air or water. Intrusive sills cool slowly underground and develop coarse-grained textures without vesicles, since the overlying pressure prevents gas bubble formation.
Contact metamorphism below the igneous body (B) occurs with both buried lava flows and sills, since both involve hot igneous material in contact with existing rocks. This evidence doesn't distinguish between them.
Inclusions of overlying sedimentary rock (C) would actually suggest an intrusive sill, where magma incorporated pieces of the rock it intruded through. A lava flow wouldn't contain inclusions from rocks that were deposited on top of it later.
Identical chemical composition to a nearby volcano (D) could apply to both features, since sills can have the same magma source as surface eruptions.
Remember this pattern: vesicular textures are your smoking gun for surface volcanic activity. The combination of vesicles plus fine-grained texture in the upper portion uniquely indicates rapid cooling at Earth's surface—the hallmark of a lava flow.
- A geologist maps a contact where a layer of sandstone rests directly on a large body of gneiss, a metamorphic rock. The contact is an uneven, erosional surface. Which relative dating principle best explains the relationship and the significant time gap at this contact?
Answer: The formation of a nonconformity, representing deposition on an eroded surface of older crystalline rock.
A nonconformity is a specific type of unconformity where sedimentary rocks are deposited directly on top of older, eroded igneous or metamorphic rocks (crystalline rocks). The gneiss is a metamorphic rock, and the sandstone is sedimentary. The erosional surface between them represents a significant time gap during which the gneiss was formed, uplifted, and eroded before the sand was deposited. While superposition also applies (sandstone is younger), the term 'nonconformity' specifically describes this entire relationship and the major time gap it implies.
- A geologist's field notes describe a sequence of events. Based on the cross-section, what event occurred immediately before the erosion that formed the angular unconformity (Surface X)?
Answer: Movement along the fault (Fault F).
To find the event immediately preceding the erosion of Surface X, we must determine the sequence of events affecting the rocks below Surface X. The layers (shale, limestone) were deposited, then folded. The granite pluton (G) cuts the folds, so it is younger than the folding. The fault (F) cuts the folded layers and the pluton, so it is the youngest event before the erosion. The erosion that created Surface X then cut across all of these features (the folded layers, the pluton, and the fault). Therefore, movement along Fault F was the last event before the period of erosion began.
- A xenolith is a piece of pre-existing rock that becomes enclosed within an igneous intrusion as it cools. Observing a xenolith of sandstone within a granite pluton would allow a geologist to apply which relative dating principle to determine their ages?
Answer: The principle of inclusions, indicating the sandstone is older than the granite.
A xenolith is a type of inclusion. The principle of inclusions states that the included rock (the xenolith) must be older than the rock that encloses it (the igneous intrusion). For the sandstone xenolith to be inside the granite, the sandstone must have existed first, been broken off, and then incorporated into the magma before it solidified. Therefore, the sandstone is older than the granite.
- A geologist examines a conglomerate layer that is situated directly on top of a large granite pluton. The conglomerate contains numerous rounded pebbles of the same granite. The contact between the two rock bodies is irregular and shows signs of significant weathering on the granite's surface. What is the most logical conclusion about the relative ages of the granite and the conglomerate?
Answer: The conglomerate is younger because it contains inclusions of the granite, which had to exist and be eroded first.
The principle of inclusions states that any rock fragments (clasts or inclusions) found within another rock must be older than the rock in which they are contained. Here, the conglomerate contains pebbles of the granite. This means the granite had to form, be uplifted, weathered, and eroded to produce the pebbles before the sediment that formed the conglomerate could be deposited. Therefore, the conglomerate is younger than the granite pluton.
- The sequence of events listed is: 1. Deposition of limestone. 2. Deposition of shale. 3. Intrusion of a granite dike. 4. Faulting. 5. Erosion to a flat surface. 6. Deposition of sandstone. Which diagram accurately represents this geologic history?
Answer: A diagram where a fault cuts through limestone, shale, and a granite dike, with a horizontal layer of sandstone resting on the eroded, faulted surface.
This question requires visualizing the sequence. 1 & 2: Limestone is below shale (superposition). 3: The granite dike must cut through both limestone and shale (cross-cutting). 4: The fault must cut through the limestone, shale, and the dike (cross-cutting). 5 & 6: An erosional surface must cut across all the previously mentioned features, and the sandstone must be deposited horizontally on top of this surface. Choice C is the only one that shows the fault cutting all three older units and the sandstone lying on top of the resulting unconformity.
- Examine the provided geologic cross-section. A geologist concludes that the limestone layer is younger than the shale but older than the fault. Which two principles of relative dating, in order, are required to reach this conclusion?
Answer: Superposition, then Cross-cutting relationships.
To determine that the limestone is younger than the shale, one must assume the layers were deposited sequentially in an undisturbed sequence. The limestone lies on top of the shale, so the principle of superposition is used first. To determine that the limestone is older than the fault, one must observe that the fault cuts through the limestone layer. The principle of cross-cutting relationships states that the feature that is cut is older than the feature that does the cutting. Therefore, superposition is used to date the layers relative to each other, and then cross-cutting relationships are used to date the fault relative to the layers.
