Worksheet

Cordero's Water Line: Matching Rocks Across the Linares Fault

Grade 7 earth science: a city's water line crosses a plate boundary; offset rocks, fossils and a gulf's seafloor ages show how far the plates have moved.

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1 NGSS standard
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12 questions
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Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students use matching rock bodies and fossils cut by a fault to infer the direction and rate of plate motion.
Students use seafloor ages and a coastline fit to explain how a gulf opened.
Students argue from rock and seafloor evidence that the land on two plates has moved.

How to use

Read the planning notes together and have students mark the four features in Table 1 on a sketch of the fault before they answer. Section 2 works well as pair work.

Curriculum

NGSS standard covered

MS-ESS2-3

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

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Cordero's Water Line: Matching Rocks Across the Linares Fault

Use rocks cut in two by a fault and a gulf's seafloor to trace how two plates have moved.

Rocks cut by the Linares Fault

Read the planning notes and Table 1, then answer the questions.

Planning Notes: Where the Water Line Crosses the Fault

1Cordero is a growing city of about 120,000 people in a dry valley in the western United States. The city plans a new water line from a reservoir in the eastern hills and a new power line from a solar farm. Both routes must cross the Linares Fault, a long break in Earth's crust that runs north to south through the valley. At a public meeting, city geologist Dr. Paloma Arriaga explained what the rocks near the fault show. Rafferty Osei, a seventh grader on the city's youth council, took these notes.

2The Linares Fault is a boundary between two plates. The land west of the fault sits on one plate, and the land east of it sits on another. Along this kind of boundary, the plates slide past each other sideways. Most of the time the fault is stuck. Then it slips suddenly during an earthquake, often by a few meters at a time, and over long periods the slips add up.

3Geologists have found rock bodies that were cut in two by the fault. For each one, the two halves have the same kind of rock and the same age, and the sandstone halves hold the same kinds of fossil scallop shells. Yet the halves now lie far apart along the fault. Table 1 lists four of them. In every case, the half on the west side of the fault lies to the north of its match on the east side. GPS stations near the fault show that the motion is still going on today.

4Far to the south, the fault ends at a long, narrow gulf of the ocean. A peninsula lies on the west side of the gulf, and the mainland lies on the east side. Down the middle of the gulf, short ridges on the seafloor are spreading apart, and lava cools there into new basalt. Rocks on the two coasts of the gulf are listed in Table 2.

5Dr. Arriaga's team used a computer map to slide the peninsula back toward the southeast. When it had moved about 180 km, the edge of its continental shelf fit snugly against the mainland's shelf, and the matching rocks on the two coasts lined up.

Table 1. Rock bodies cut in two by the Linares Fault
Feature	Age	Halves apart (km)
Dry stream channel	about 10,000 years	0.3
Lava flow	2 million years	60
Sandstone with fossil scallops	5 million years	150
Field of volcanic rock	12 million years	360

Which explanation of the halves in Table 1 fits the planning notes best?

A. Each body formed as one piece, and the plates sliding along the fault carried its halves apart.
B. The fault opened into a wide gap, and each body split as the two sides moved away from each other.
C. Streams carried broken pieces of each rock body north along the valley floor.
D. Two separate lava flows of the same age happened to form 60 km apart, one on each side.

Which statement best describes the pattern in Table 1?

A. The distances jump around, with no link to the age of the feature.
B. The halves of the lava flow are farther apart than the sandstone halves.
C. The younger the feature, the farther apart its two halves are.
D. The older the feature, the farther apart its two halves are.

Use the lava flow row. 60 km is 6,000,000 cm.

On average, the two halves of the lava flow moved apart by about cm per year.

Use paragraph 3. What does the position of the west halves tell you about how the west plate has moved compared with the east plate?

A. It has moved south along the fault, toward the gulf.
B. It has moved west, away from the fault and the east plate.
C. It has moved north along the fault, past the east plate.
D. It has sunk down and slid underneath the east plate.

The new water line should last 100 years. Suppose the plates keep moving at the rate you found in question 3.

Over 100 years, the part of the pipe west of the fault would be carried about cm north of the part east of it.

Circle the three observations that are evidence that the two plates have moved past each other.

Sandstone with the same fossil scallops lies 150 km apart on the two sides.The fault runs north to south through a dry valley.A dry stream channel is broken and shifted 0.3 km where it crosses the fault.The reservoir for the water line sits in the eastern hills.GPS stations west of the fault keep moving north compared with stations east of it.

The gulf and the peninsula

Use Table 2 and paragraphs 4 and 5.

Table 2. Rocks around the gulf
Place	What is found	Age (million years)
Peninsula's east coast	Granite with an unusual mix of minerals	90
Mainland's west coast	Granite with the same unusual mix	90
Both coasts	Matching layers of one lava flow	9
Seafloor near the gulf's edges	Basalt	6
Seafloor in the middle of the gulf	New basalt	0 to 1

Lava flows spread over land and cool where they stop; one cannot flow across 180 km of ocean. What does the 9-million-year-old lava flow on both coasts suggest?

A. The ridges in the middle of the gulf poured the lava out onto both coasts.
B. The peninsula and the mainland were one piece of land when the lava flowed.
C. Ocean currents carried pieces of the cooled lava from one coast to the other.
D. The gulf was already about 180 km wide when the lava erupted on both coasts.

Which model fits the seafloor ages in Table 2 best?

A. The whole gulf floor formed at one time, about 9 million years ago.
B. New seafloor forms in the middle and moves outward, so the gulf has widened for about 6 million years.
C. The oldest seafloor formed in the middle, and newer rock was added later along the coasts.
D. The gulf is closing, so seafloor from the coasts is being squeezed into the middle.

Use paragraph 5 and the oldest seafloor in Table 2. Suppose the peninsula moved about 180 km away from the mainland at a steady rate since the gulf began to open. 180 km is 18,000,000 cm.

On average, the peninsula moved about cm per year.

10.

Compare your answers to questions 3 and 9. What does the comparison suggest?

A. The rates match, so the gulf and the fault likely record the same plate motion.
B. The rates differ by a lot, so the peninsula moved about ten times faster than the fault.
C. The shelf fit is a coincidence, since a coastline changes shape too quickly to compare.
D. The rates match, so the peninsula likely stopped moving about 6 million years ago.

11.

The oldest seafloor in the gulf is about 6 million years old. Sort each rock or feature by when it formed.

The 9-million-year-old lava flow on both coastsThe new basalt in the middle of the gulfThe granite with the unusual mix of mineralsThe lava flow in Table 1 that is now cut in twoThe field of volcanic rock in Table 1The dry stream channel shifted by the fault

Before the gulf began to open	After the gulf began to open

12.

A council member says, "The land on both sides of the fault has always been where it is now. The matching rocks are just a coincidence, so the water line does not need joints that can slide." Do you agree? Make a claim, use evidence from both tables, and explain your reasoning about how the plates have moved.