Geologic Time & Earth's History — Science

  1. Grade 9 — Geologic Time & the Rock Record
  2. Grade 9 — Fossils & Relative Dating

Apply it in a new setting

Problem 1

A cliff shows, from bottom to top: sandstone, then shale. A vertical volcanic dike (hardened magma) cuts up through both the sandstone and the shale but stops abruptly at a flat surface. Lying on top of that flat surface, uncut, is a layer of limestone. Which rock formed LAST?
  1. The sandstone, because it is a common rock
  2. The dike, because it cuts through layers
  3. The limestone, because it lies on top and the dike does not cut into it
  4. The shale, because it is in the middle
Hints

Use superposition (younger on top) together with cross-cutting (a cut feature is younger than what it cuts).

The dike cuts the sandstone and shale, so it is younger than both of them.

But the limestone lies on top of the eroded dike and is not cut — so it came after the dike.

Answer and explanation

The limestone, because it lies on top and the dike does not cut into it

By cross-cutting, the dike is younger than the sandstone and shale it slices through.

The limestone sits on a flat (eroded) surface on top and is not cut by the dike, so it was deposited after the dike was already there and worn flat.

Therefore the limestone formed last — C.

Problem 2

The same distinctive index fossil is found in a rock layer in Wales and in a rock layer in New York, on different continents. What is the best conclusion?
  1. The two rock layers formed at about the same time
  2. The two layers must be made of the exact same rock type
  3. The two continents have never moved
  4. The fossil animal swam across the Atlantic Ocean
Hints

An index fossil comes from a species that lived over a wide area but only for a short span of time.

Finding the same short-lived species in two places tells you about their relative timing.

This technique is called correlation.

Answer and explanation

The two rock layers formed at about the same time

An index fossil marks a narrow slice of geologic time but is spread over a wide area.

Finding the same index fossil in two distant layers means those layers formed during that same short time span — they correlate in age.

The best conclusion is A.

Problem 3

Which fossil species would make the BEST index fossil for dating rock layers?
  1. A species that lived for a very long time and only in one small area
  2. A species that was widespread across many continents but existed for only a short span of geologic time
  3. A species that lived for hundreds of millions of years worldwide
  4. A species known from only a single fossil ever found
Hints

An index fossil must be easy to find in many places AND pin down a narrow age.

Wide geographic range helps you find it far apart; short time range makes the age precise.

Which option has both a wide range and a short time span?

Answer and explanation

A species that was widespread across many continents but existed for only a short span of geologic time

A good index fossil needs a wide geographic range (so it turns up in many places) and a short time range (so its presence pins the age precisely).

Only a species that was widespread but short-lived satisfies both requirements.

So the best index fossil is B.

Problem 4

In an undisturbed sequence, a fossil-bearing bed is sandwiched between two volcanic ash layers. The ash layer BELOW the fossils is radiometrically dated at 90 million years, and the ash layer ABOVE them at 70 million years. What is the best estimate of the fossils' age?
  1. Exactly 90 million years
  2. Older than 90 million years
  3. Between 70 and 90 million years
  4. Younger than 70 million years
Hints

Use superposition: in undisturbed layers, lower = older, higher = younger.

The fossils sit above the 90-million-year ash and below the 70-million-year ash.

So their age is bracketed between the two dated layers.

Answer and explanation

Between 70 and 90 million years

By superposition, the fossils are younger than the ash beneath them (90 My) and older than the ash above them (70 My).

That brackets the fossils between the two radiometric dates.

So their age is between 70 and 90 million years — C.

Problem 5

A radioactive isotope used in dating has a half-life of 1.25 billion years. A rock contains an amount of this isotope showing it is 3.75 billion years old. What percent of the original isotope remains? (After each half-life, half of it is left.)
Hints

First find how many half-lives have passed: total age ÷ half-life.

After each half-life the remaining amount is halved: ½, then ¼, then …

Convert the final fraction to a percent.

Answer and explanation

12.5 %

Number of half-lives = 3.75 billion ÷ 1.25 billion = 3.

After 3 half-lives the fraction remaining is ½ × ½ × ½ = 1/8.

1/8 = 12.5% of the original isotope remains.

Problem 6

Earth is about 4.6 billion years old, and the Precambrian covers roughly its first 4.0 billion years. About what percent of Earth's history is the Precambrian? (Give just the number.)
Hints

Percent = (part ÷ whole) × 100.

Here the part is 4.0 billion years and the whole is 4.6 billion years.

Compute 4.0 ÷ 4.6, then multiply by 100.

Answer and explanation

87 %

Percent = (Precambrian ÷ total age) × 100 = (4.0 ÷ 4.6) × 100.

4.0 ÷ 4.6 ≈ 0.87.

That is about 87% of Earth's history (roughly the 88% often quoted).

Problem 7

A drill core passes down through rock layers. In the upper layers, fossils of large mammals dominate; just below a sharp boundary, dinosaur fossils appear and the mammal fossils vanish. This boundary most likely corresponds to which event?
  1. The Mesozoic-Cenozoic boundary about 66 million years ago, a mass extinction that ended the dinosaurs
  2. The formation of Earth 4.6 billion years ago
  3. Next week's weather
  4. The start of the Precambrian
Hints

A sudden change in which fossils appear marks a major turning point in life's history.

Dinosaurs below, mammals above — which era boundary separates the 'age of reptiles' from the 'age of mammals'?

Recall the roughly 66-million-year-ago mass extinction.

Answer and explanation

The Mesozoic-Cenozoic boundary about 66 million years ago, a mass extinction that ended the dinosaurs

A sharp switch from dinosaur-dominated layers (below) to mammal-dominated layers (above) marks a major boundary in the fossil record.

That is the Mesozoic-Cenozoic boundary about 66 million years ago, when a mass extinction ended the dinosaurs and mammals rose.

So the boundary corresponds to A.

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