Worksheet

Deep-Sea Rock on a Mountain Road: Plate Clues in Thornbury

Grade 7 earth science: deep-sea chert in landslide scars, earthquake depths and ocean floor ages show how plates moved at a mountain town near a trench.

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11 questions
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Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Matching, Circle the answer, Short answer

About this worksheet

What students practice

Students use rock and fossil evidence to infer that ocean floor rock was carried to a continent.
Students use ocean floor ages, plate speed and earthquake depths to describe past and present plate motion.
Students argue from evidence that the ocean floor and coast have not always been where they are now.

How to use

Read the passage together and sketch the ridge, trench and town on the board before students begin. Students can check the rate questions with a partner.

Curriculum

NGSS standard covered

MS-ESS2-3

Use fossil, rock and seafloor data as evidence of past plate motions.

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Deep-Sea Rock on a Mountain Road: Plate Clues in Thornbury

Use landslide rocks, earthquake depths and ocean floor ages as evidence that plates have moved.

Rock in the landslide scars

Read the passage and study Table 1. Then answer the questions.

What the Landslides Uncovered

1Thornbury is a small town high in a coastal mountain range, about 60 km inland from a deep ocean trench. Small earthquakes rattle the town most months, and every few winters heavy rain sets off landslides along the mountain road. Last winter, after three days of storms, four slides tore the soil and forest off the slopes above the road and showed bare rock that is usually hidden. In March, Mr. Desmond Ekwueme's seventh-grade class walked the road with Dr. Yuki Lindahl, a geologist from the state survey. The class stayed on the pavement while she pointed out the rock in each scar (Table 1).

2Dr. Lindahl explained what each rock can tell them. Chert is a hard rock made mostly of the tiny glassy shells of radiolarians, plankton that drift in the open ocean. Today, thick layers of these shells build up mainly on the deep ocean floor far from land, where little mud from rivers reaches, and they build up very slowly, a few millimeters every thousand years. Pillow basalt forms when lava erupts underwater and cools into rounded blobs; much of the ocean floor is basalt that formed this way at mid-ocean ridges. Mudstone forms from mud, and sandstone from sand, often in rivers or near a shore.

3Offshore, a mid-ocean ridge makes new ocean floor. Drill cores show that the ocean floor is youngest at the ridge and about 10 million years old at the trench. GPS stations on the ocean floor show the plate moving toward the coast at about 4 cm per year. At the trench, the ocean plate sinks under the continent. As it slides down, some of its rock and mud can be scraped off and pressed against the edge of the continent, the way snow piles up in front of a plow.

4Earthquake records from the past 20 years show how deep the rock breaks at different distances inland (Table 2). About 200 km inland from the trench stands a line of volcanoes, where magma forms above the sinking plate. The oldest lava in that line of volcanoes is about 30 million years old.

Table 1. Rock in the four landslide scars
Scar	Rock	Fossils	Age (million years)
1	Red chert in thin layers	Radiolarians	about 150
2	Pillow basalt	None	about 160
3	Broken mudstone, chert and basalt blocks	Radiolarians in the chert blocks	about 50 (mudstone)
4	Sandstone	Leaves of land plants	about 20

Scar 1 is about 900 m above sea level. Which explanation of its radiolarian chert fits paragraphs 2 and 3 best?

A. It formed on deep ocean floor far from land and was later scraped onto the continent.
B. A tsunami washed ocean shells up the slope, where they hardened into rock.
C. Last winter's storms dissolved glass from the basalt and left it behind as chert.
D. It formed in a mountain lake, where plankton shells slowly piled up in calm water.

Circle the three observations that are evidence that rock formed on the ocean floor is now part of the mountains.

Thin layers of radiolarian chert in Scar 1Three days of storms before the landslidesPillow basalt in Scar 2Leaves of land plants in Scar 4Blocks of chert and basalt in the Scar 3 mudstoneSmall earthquakes in Thornbury most months

Use the ages in Table 1 and the passage. Which order of events fits the evidence best?

A. River sand covers the area → basalt erupts on top of it → radiolarian shells pile up → trench mud forms
B. Trench mud forms → radiolarian shells pile up in it → basalt erupts at the trench → river sand forms
C. Radiolarian shells pile up → basalt erupts beneath them → river sand forms → trench mud covers the sand
D. Basalt erupts at a ridge → radiolarian shells pile up on it → it is scraped into trench mud → river sand forms

The ocean plate offshore

Use Table 2 and paragraphs 3 and 4. Show your work on scrap paper.

Table 2. Earthquakes under the region, 20 years of records
Inland from trench (km)	Typical quake depth (km)
0	10
50	30
100	50
150	70
200	90

Which statement best describes Table 2?

A. The earthquakes are deepest at the trench and under the town of Thornbury.
B. The earthquakes stay about 10 km deep from the trench to the volcanoes.
C. The earthquakes get deeper farther inland, about 20 km deeper for each 50 km.
D. The earthquakes get shallower farther inland, about 20 km for each 50 km.

Which model of the ocean plate fits Table 2 and paragraph 3 best?

A. The continent slides down under the ocean plate and moves out toward the ridge.
B. The ocean plate slopes down under the continent, and rock breaks along it as it sinks.
C. The ocean plate stops at the trench and piles up there into a tall wall of rock.
D. The ocean plate melts at the trench, and the melt flows inland along the surface.

The ocean floor at the trench formed at the ridge about 10 million years ago. Suppose the plate has always moved at 4 cm per year. (1 km = 100,000 cm)

About how many kilometers is the ridge from the trench? km

The mudstone in Scar 3 formed from trench mud about 50 million years ago. Suppose ocean floor has moved into the trench at 4 cm per year ever since. (1 km = 100,000 cm)

About how many kilometers of ocean floor have sunk at the trench in that time? km

The chert in Scar 1 is about 150 million years old, but the oldest ocean floor offshore today is only about 10 million years old. Which explanation fits the evidence best?

A. The drill cores missed older rock, which lies under the young ocean floor offshore.
B. The ocean floor that carried the chert has sunk, and the chert was scraped off.
C. The chert formed in the last few years and was quickly buried under deep mud.
D. The chert grew on land near Thornbury, where radiolarians live in mountain streams.

Use paragraph 4. What does the age of the oldest lava in the line of volcanoes suggest?

A. An ocean plate has been sinking under this coast for at least 30 million years.
B. The volcanoes formed from chert that melted when last winter's landslides fell.
C. The ocean plate began to sink here within the last 10 million years.
D. The volcanoes formed as the mid-ocean ridge first opened, then went quiet.

Putting the evidence together

Use the whole passage and both tables.

10.

Match each observation to the idea it best supports.

Radiolarian chert in a landslide scar 900 m above the sea
Earthquakes that are deeper the farther inland they are
Ocean floor that is youngest at the ridge and older toward the trench
Lava about 30 million years old in the line of volcanoes

A. New ocean floor forms at the ridge and moves toward the coast.
B. Rock from the deep ocean floor became part of the continent.
C. The plate began sinking here long ago, not recently.
D. The sinking plate slopes downward beneath the land.

11.

A visitor says, "A giant wave must have washed that deep-sea rock up the mountain. The ocean floor and this coast have always been where they are now." Do you agree? Make a claim, use evidence from Table 1, Table 2 and the passage, and explain your reasoning about how the plates have moved.