What is Geologic history on the Earth Science Regents?

Geologic history is a Geologic History topic on the Earth Science Regents. RegentsLab has 53 practice questions on it, each with a full worked solution.

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Geologic history: Earth Science Regents Practice

Practice Geologic history for the Earth Science Regents with real-style questions and full worked solutions. Miss one? Read the solution — the exam reuses the same ideas every year.

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Question 66 · 2 pts

Carbon-14 dating would be the most appropriate method for determining the age of which sample?

(A) The buried wood of a tree that lived about $8{,}000$ years ago ✓
(B) A bed of volcanic ash about $300$ million years old
(C) A stony meteorite about $4.5$ billion years old
(D) A granite intrusion estimated to be $1.1$ billion years old

Worked solution

$t_{1/2}(\text{C-14}) = 5.7 \times 10^{3} \text{ yr}$ Short half-life listed in the ESRT.

$\text{usable range} \approx 10 \text{ half-lives} \approx 5 \times 10^{4} \text{ yr}$ Beyond this, too little C-14 remains to measure.

$8{,}000 \text{ yr (once-living wood)} \ll 5 \times 10^{4} \text{ yr}$ Only this sample is both organic and young enough.

Answer: The buried wood of a tree that lived about $8{,}000$ years ago

Carbon-14 dating works only on the remains of once-living things and only for ages up to roughly $50{,}000$ years because of its short $5.7 \times 10^3$-year half-life.

Question 67 · 2 pts

Which characteristic makes an organism most useful as an index fossil for correlating rock layers?

(A) It existed for a very long time and lived in only one small area
(B) It existed for a geologically short time and was widespread over a large area ✓
(C) It is found preserved only within igneous rock
(D) It had a large body and a thick, hard shell

Worked solution

$\text{short time range} \Rightarrow \text{narrow, precise age for the layer}$ Pinpoints a specific interval of geologic time.

$\text{wide geographic range} \Rightarrow \text{same fossil correlates distant outcrops}$ Lets separated rock layers be matched.

$\text{best index fossil} = \text{short time} + \text{wide area}$

Answer: It existed for a geologically short time and was widespread over a large area

A good index fossil pins a rock layer to a narrow slice of geologic time yet appears across a broad region, allowing widely separated layers to be correlated.

Question 75 · 2 pts

A scientist wants to accurately determine the age of a buried wooden tool estimated to be about $5 \times 10^{4}$ (50,000) years old. According to the ESRT, which radioactive isotope would be most useful?

(A) Carbon-14 ✓
(B) Rubidium-87
(C) Potassium-40
(D) Uranium-238

Worked solution

$\text{Useful isotope: } T_{1/2} \sim \text{age being measured}$

$C\text{-}14:\ T_{1/2}=5.7\times10^{3}\ \text{yr},\quad \frac{5\times10^{4}}{5.7\times10^{3}}\approx 9 \text{ half-lives}$ Plenty of measurable decay.

$K\text{-}40,\ U\text{-}238,\ Rb\text{-}87:\ T_{1/2}\ge 1.3\times10^{9}\ \text{yr} \Rightarrow \text{negligible decay in } 5\times10^{4}\ \text{yr}$

Answer: Carbon-14

A radioactive isotope is most useful for dating when its half-life is similar in magnitude to the age being measured; C-14's short half-life suits recent, once-living material.

Question 80 · 2 pts

Earth is about 4600 million years old, and Precambrian time ended about 542 million years ago. Approximately what percentage of Earth's history occurred during Precambrian time?

(A) About 50%
(B) About 88% ✓
(C) About 12%
(D) About 99%

Worked solution

$\text{Precambrian duration} = 4600 - 542 = 4058\ \text{my}$

$\frac{4058}{4600} \approx 0.88 = 88\%$

Answer: About 88%

Precambrian time spans from Earth's formation (~4600 mya) to ~542 mya, roughly 4 billion years, which is about 88% of all geologic time.

Question 10 · 2 pts

In an undisturbed sequence of sedimentary rock layers that has not been overturned or faulted, the oldest rock layer is normally found

(A) it cannot be determined without radioactive dating
(B) at the top of the sequence
(C) in the middle of the sequence
(D) at the bottom of the sequence ✓

Worked solution

$Sediment is deposited in layers over time, with each new layer settling on top of previously deposited layers.$

$In an undisturbed sequence, this means the layer at the very bottom was deposited first and is therefore the oldest.$

$This relationship is known as the law of superposition.$

Answer: at the bottom of the sequence

The law of superposition states that in an undisturbed sequence of sedimentary rock layers, the oldest layer lies at the bottom and layers become progressively younger toward the top.

Question 24 · 2 pts

Which era of geologic time is often called the "Age of Reptiles" because dinosaurs were the dominant life-form on Earth?

(A) Cenozoic Era
(B) Precambrian time
(C) Paleozoic Era
(D) Mesozoic Era ✓

Worked solution

$The geologic time scale divides Earth's history into eras based on dominant life-forms and major fossil evidence.$

$The Mesozoic Era followed the Paleozoic Era and is marked by the rise, dominance, and extinction of the dinosaurs.$

$Because reptiles, especially dinosaurs, dominated life on land during this era, it is commonly called the Age of Reptiles.$

Answer: Mesozoic Era

The Mesozoic Era is known as the Age of Reptiles because dinosaurs and other large reptiles were the dominant terrestrial life-forms, distinguishing it from the Paleozoic (Age of Fishes/early life) and Cenozoic (Age of Mammals).

Question 37 · 2 pts

In an undisturbed sequence of sedimentary rock layers, the oldest layer lies at the bottom and the youngest layer lies at the top. This relationship is best described by the law of:

(A) cross-cutting relationships
(B) faunal succession
(C) original horizontality
(D) superposition ✓

Worked solution

$Sediment layers are deposited one on top of another over time.$

$In an undisturbed sequence, each new layer is deposited on top of previously existing layers.$

$Therefore the bottom layer must have been deposited first (oldest) and the top layer most recently (youngest); this is the law of superposition.$

Answer: superposition

The law of superposition states that in an undisturbed sequence of sedimentary rock, the oldest layers are at the bottom and the youngest are at the top, since each layer is deposited above the ones that came before it.

Question 49 · 2 pts

A road cut exposes an undisturbed sequence of horizontal sedimentary rock layers that have not been folded, overturned, or faulted. According to the law of superposition, which layer in this sequence is the oldest?

(A) Age cannot be determined without radiometric dating
(B) The middle layer
(C) The top layer
(D) The bottom layer ✓

Worked solution

$\text{Sediments are deposited in horizontal layers over time}$

$\text{Each new layer is deposited on top of the previous one}$

$\text{In an undisturbed sequence, bottom} = \text{oldest, top} = \text{youngest}$

Answer: The bottom layer

The law of superposition states that in an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and each successive layer above it is younger, because sediment accumulates from the bottom up over time.

Question 9 · 2 pts

Which characteristic makes a fossil most useful as an index fossil for dating and correlating rock layers?

(A) The organism was geographically widespread but existed for a short span of geologic time ✓
(B) The organism existed for a very long span of geologic time
(C) The organism was rare and found only in one small location
(D) The organism is still alive today

Worked solution

$\text{A useful index fossil must be found in many different locations to allow correlation between distant rock layers.}$

$\text{It must also have existed for a short, well-defined interval of time so it pinpoints a narrow age range.}$

$\text{Long-lived, rare, or still-living organisms fail one or both requirements.}$

Answer: The organism was geographically widespread but existed for a short span of geologic time

Index fossils are most useful when the species was widespread (allowing correlation across regions) but existed only briefly in geologic time (allowing precise dating of the rock layer in which it is found).

Question 35 · 2 pts

According to the geologic time scale, Earth's most recent epoch, the Holocene, began about 11,700 years ago at the end of the last ice age. Which epoch of the Cenozoic Era immediately preceded the Holocene Epoch?

(A) Miocene
(B) Paleocene
(C) Pliocene
(D) Pleistocene ✓

Worked solution

$The Cenozoic Era is divided into a sequence of epochs based on the geologic time scale.$

$The epoch directly before the Holocene, marked by repeated glaciations (ice ages), is the Pleistocene.$

$The Holocene began when the Pleistocene's last glacial period ended about 11,700 years ago.$

Answer: Pleistocene

The Pleistocene Epoch, characterized by widespread glaciation, immediately precedes the Holocene, the current epoch, on the geologic time scale.

Question 45 · 2 pts

A fossil is most useful as an index fossil for correlating rock layers between widely separated locations if the organism it represents was

(A) preserved only in igneous rock
(B) present on Earth for a very long span of geologic time
(C) geographically widespread but existed for only a short span of geologic time ✓
(D) found in only one small geographic region

Worked solution

$Correlation requires matching rock layers of the same age across distant locations.$

$An organism that lived over a wide geographic area could be deposited in rock layers in many different places.$

$An organism that existed for only a short time span pinpoints a narrow window of geologic time, making the correlation precise.$

$A long-lived or geographically limited organism, or one found only in igneous rock (which does not preserve fossils), would not allow precise, widespread correlation.$

Answer: geographically widespread but existed for only a short span of geologic time

Ideal index fossils combine wide geographic distribution with a short geologic time range, allowing geologists to match and date rock layers precisely across distant locations.

Question 38 · 2 pts

In an undisturbed sequence of sedimentary rock layers, a horizontal layer of shale is cut through by a vertical igneous dike. Which statement correctly compares the relative ages of the shale and the dike?

(A) The relative ages cannot be determined from this information
(B) The shale and the dike formed at exactly the same time
(C) The shale is older than the dike ✓
(D) The dike is older than the shale

Worked solution

$According to the law of superposition, the shale layer, being an undisturbed sedimentary rock, formed before any feature that cuts through it.$

$The principle of cross-cutting relationships states that any rock feature (such as an igneous dike) that cuts across another rock unit must have formed after that rock unit.$

$Since the dike cuts through the shale, the dike must be younger, meaning the shale is older.$

Answer: The shale is older than the dike

The principle of cross-cutting relationships states that a feature which cuts through existing rock must have formed after that rock was already in place, so the igneous dike, which cuts the shale, must be younger, making the shale the older unit.

Question 40 · 1 pt

Carbon-14 has a half-life of about $5{,}700$ years. After $11{,}400$ years, what fraction of an original sample of carbon-14 remains?

(A) $\tfrac{1}{8}$
(B) $\tfrac{1}{4}$ ✓
(C) $0$
(D) $\tfrac{1}{2}$

Worked solution

$11400 / 5700 = 2 \text{ half-lives}$ Count the half-lives.

$\tfrac12 \to \tfrac14$ Halve twice.

Answer: \tfrac{1}{4}

Each half-life halves the remaining amount. Two half-lives leave ½ × ½ = ¼.

Question 68 · 2 pts

According to the Geologic History of New York State in the Earth Science Reference Tables, the boundary between the Mesozoic and Cenozoic Eras, marked by a mass extinction that included the dinosaurs and ammonoids, occurred approximately how many years ago?

(A) $541$ million years ago
(B) $65.5$ million years ago ✓
(C) $1.8$ million years ago
(D) $251$ million years ago

Worked solution

$\text{Mesozoic} \to \text{Cenozoic boundary} = 65.5 \text{ million years ago}$ Read directly from the time scale on the chart.

$\text{extinction of dinosaurs and ammonoids marks this boundary}$ End of the Cretaceous Period.

Answer: $65.5$ million years ago

The ESRT places the Mesozoic-Cenozoic boundary at about $65.5$ million years ago, coinciding with the mass extinction that ended the dinosaurs and ammonoids.

Question 69 · 2 pts

A sample of igneous rock contains equal amounts of radioactive Uranium-238 and its stable decay product Lead-206. Using the Radioactive Decay Data table, what is the approximate age of the rock?

(A) $4.5 \times 10^{9}$ years ✓
(B) $9.0 \times 10^{9}$ years
(C) $5.7 \times 10^{3}$ years
(D) $2.25 \times 10^{9}$ years

Worked solution

$\text{equal U-238 and Pb-206} \Rightarrow \tfrac{1}{2} \text{ of original U-238 remains}$ Half the parent has converted to daughter.

$\frac{1}{2} = \left(\frac{1}{2}\right)^{1} \Rightarrow 1 \text{ half-life}$

$t = 1 \times (4.5 \times 10^{9} \text{ yr}) = 4.5 \times 10^{9} \text{ yr}$ U-238 half-life from the ESRT.

Answer: $4.5 \times 10^{9}$ years

When a radioactive parent and its decay product are present in equal amounts, exactly one half-life has passed; for Uranium-238 that is $4.5 \times 10^9$ years.

Question 70 · 2 pts

A fault cuts across several layers of sedimentary rock. According to the principle of cross-cutting relationships, the fault must be:

(A) the same age as the bottom layer it cuts
(B) younger than all of the layers it cuts through ✓
(C) older than all of the layers it cuts through
(D) older than the top layer but younger than the bottom layer

Worked solution

$\text{a rock must exist before it can be faulted}$ The cause must precede the effect.

$\text{fault cuts all the layers} \Rightarrow \text{fault formed after all of them}$ Principle of cross-cutting relationships.

$\therefore \text{the fault is the youngest feature shown}$

Answer: younger than all of the layers it cuts through

The principle of cross-cutting relationships states that any feature that cuts across rock, such as a fault or intrusion, must be younger than the rock it disrupts.

Question 71 · 2 pts

A thin layer of volcanic ash is found within sedimentary rock sequences at many widely separated locations. Why is such an ash layer especially useful as a time marker for correlating these rock sequences?

(A) The ash accumulates slowly and continuously over millions of years
(B) The ash from one eruption is deposited over a wide area during a very short interval of time ✓
(C) The ash layer always contains abundant index fossils
(D) The ash is found only at the very bottom of every rock sequence

Worked solution

$\text{single eruption} \Rightarrow \text{ash falls over a wide region almost instantly}$ Geologically, the deposit is essentially the same age everywhere.

$\text{same-age layer in many places} \Rightarrow \text{ideal time marker for correlation}$ Lines up distant rock sequences.

Answer: The ash from one eruption is deposited over a wide area during a very short interval of time

A volcanic ash bed records a single brief eruption spread over a large area, so it has essentially the same age everywhere and serves as an excellent time marker for correlation.

Question 74 · 2 pts

A mineral originally contained only radioactive potassium-40. After $3.9 \times 10^{9}$ years, what fraction of the original potassium-40 remains? (Use the half-life from the ESRT.)

(A) $\tfrac{1}{4}$
(B) $\tfrac{1}{3}$
(C) $\tfrac{1}{16}$
(D) $\tfrac{1}{8}$ ✓

Worked solution

$n = \frac{3.9\times10^{9}\ \text{yr}}{1.3\times10^{9}\ \text{yr}} = 3 \text{ half-lives}$ K-40 half-life from ESRT is 1.3 billion years.

$\text{fraction remaining} = \left(\tfrac{1}{2}\right)^{3} = \tfrac{1}{8}$

Answer: $\tfrac{1}{8}$

After each half-life, half of the remaining radioactive material decays, so after $n$ half-lives the fraction left is $(1/2)^n$; 3.9 billion years equals three K-40 half-lives.

Question 78 · 2 pts

Two rock outcrops located 200 kilometers apart each contain one thin layer of the same volcanic ash. What is the best interpretation of this evidence?

(A) The ash layers formed at very different times because the outcrops are far apart.
(B) The rock at the ash layer is the same age at both outcrops, because volcanic ash settles over a wide area in a geologically short time. ✓
(C) The two outcrops cannot be correlated because they are separated by 200 km.
(D) The ash layer must be the oldest layer at both outcrops.

Worked solution

$\text{Volcanic ash blankets a wide area in a geologically brief time}$

$\text{Same ash bed at both sites} \Rightarrow \text{same-age time marker for correlation}$

Answer: The rock at the ash layer is the same age at both outcrops.

A widespread volcanic ash bed is deposited almost instantly across a large area, making it an excellent time marker for correlating rocks of the same age at distant locations.

Question 79 · 2 pts

The ESRT geologic history chart shows a major extinction of many marine organisms at about 251 million years ago. This extinction event marks the end of which era?

(A) Cenozoic Era
(B) Mesozoic Era
(C) Precambrian
(D) Paleozoic Era ✓

Worked solution

$\text{Major extinction at } \approx 251\ \text{mya}$

$251\ \text{mya} = \text{Permian},\ \text{the Paleozoic--Mesozoic boundary} \Rightarrow \text{end of Paleozoic}$

Answer: Paleozoic Era

The largest known mass extinction occurred about 251 million years ago and defines the boundary between the Paleozoic and Mesozoic eras.

Question 9 · 2 pts

Rock outcrops located hundreds of kilometers apart both contain the same species of index fossil within one of their rock layers. Geologists use this evidence to conclude that

(A) the fossil organism existed throughout the entire geologic time scale
(B) those two rock layers are made of the same type of rock
(C) those two rock layers formed during the same time period ✓
(D) the two outcrops must have originally been in the exact same location

Worked solution

$A useful index fossil comes from an organism that was geographically widespread but existed for only a relatively short span of geologic time.$ This makes the fossil a reliable time marker.

$Finding the same index fossil in rock layers at different locations means those layers formed during the same narrow time interval, regardless of distance apart.$

$This method of matching rock layers by their contained fossils is called correlation.$

Answer: those two rock layers formed during the same time period

Index fossils are widespread, short-lived species used to correlate, or match up, rock layers of the same age at locations that may be far apart from one another.

Question 17 · 2 pts

Which characteristic makes a fossil organism most useful as an index fossil for correlating rock layers between distant locations?

(A) It had a wide geographic distribution and existed for a short geologic time span ✓
(B) It had a narrow geographic distribution and existed for a long geologic time span
(C) It was resistant to weathering and erosion
(D) It had an unusually large body size compared to other organisms

Worked solution

$Correlating rock layers between distant locations requires a fossil that can be found over a large area.$

$The fossil must also represent a narrow window of geologic time so that layers containing it can be dated precisely.$

$An organism with widespread distribution and a short existence on Earth best meets both requirements, making it an ideal index fossil.$

Answer: It had a wide geographic distribution and existed for a short geologic time span

Index fossils are useful for correlation because they are geographically widespread (found in many locations) yet existed for only a brief span of geologic time, allowing precise matching of rock layers across regions.

Question 31 · 2 pts

Geologists use certain fossil organisms to correlate, or match up, rock layers exposed in widely separated locations. Which characteristic makes a fossil organism most useful for this purpose (an index fossil)?

(A) The organism existed for a very long geologic time span and lived in only one small area
(B) The organism is extremely rare and has been found at only one location
(C) The organism is found only within igneous rock layers
(D) The organism existed for a relatively short geologic time span but was widespread geographically ✓

Worked solution

$A useful index fossil must pinpoint a narrow time interval, so the species should have existed briefly.$

$It must also appear in rock layers from many different locations to allow correlation between them.$

$Therefore the best index fossils are short-lived but geographically widespread organisms.$

Answer: The organism existed for a relatively short geologic time span but was widespread geographically

Index fossils are used to correlate rock layers because they represent organisms that lived briefly (narrowing the time period) but spread over a large geographic area (allowing comparison between distant outcrops).

Question 43 · 2 pts

Geologists rely on index fossils to correlate rock layers between distant locations. Which combination of characteristics makes an organism's fossil most useful as an index fossil?

(A) The organism existed for a short span of geologic time and was found only in one small region
(B) The organism existed for a long span of geologic time and was widespread geographically
(C) The organism existed for a short span of geologic time and was widespread geographically ✓
(D) The organism existed for a long span of geologic time and was found only in one small region

Worked solution

$\text{Short time range} \Rightarrow \text{pinpoints a narrow age for the rock layer}$

$\text{Widespread distribution} \Rightarrow \text{allows correlation between distant outcrops}$

Answer: The organism existed for a short span of geologic time and was widespread geographically

A useful index fossil must have lived over a narrow, well-defined interval of geologic time (so its presence precisely dates the rock) and must have been geographically widespread (so it can be found in many locations and used to match up rock layers across a large region).

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