WorksheetPremium

Two Kinds of Trilobites: Clues to a Lost Ocean

Grade 7 earth science: trilobites, volcanic rocks and folded layers from three field trip stops, plus Atlantic floor ages, show how plates moved.

Grade 7ScienceInteractive · Printable

Aligned toMS-ESS2-3

Buy this worksheet: $1.50

Or unlock every Premium worksheet with Premium

PrintUnlock to print

Answer keyUnlock to see answers

Assign to classUnlock to assign with a class code

What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
10 questions
Classroom-ready

Details

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 fossil and rock data from several sites as evidence that landmasses were once separated by an ocean.
Students use ocean-floor ages to estimate how fast the Atlantic has widened.
Students build an argument that plate motions closed one ocean and opened another.

How to use

Have students mark the three stops on a quick west-to-east line before reading Table 2; pairs can talk through the model question before the written argument.

Curriculum

NGSS standard covered

MS-ESS2-3

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

Preview

OwlSheets.com

Name Date

Two Kinds of Trilobites: Clues to a Lost Ocean

Use fossils and rocks from a geology field trip to argue that an ancient ocean closed and the Atlantic opened.

Three stops, two kinds of trilobites

Read the passage and study Tables 1 and 2. Then answer the questions.

The Field Trip That Did Not Add Up

1Ms. Beatrix Halvorsen's seventh graders spent two days on a geology field trip, collecting rocks and fossils at three stops along a 150 km road that runs west to east toward the Atlantic coast of North America. Dr. Ilse Varga, a geologist with the state geological survey, guided them. The students wore safety glasses and collected only loose pieces from piles of broken rock, labeling each bag with its stop. Table 1 shows what they found.

2Trilobites were sea animals with hard, jointed shells, and they died out long before the first dinosaurs. The kinds the class found lived and fed on shallow sea floors, and experts think they could not cross a deep, wide ocean. At Stop 1, the trilobites in the limestone belong to a group Dr. Varga called Type W. At Stop 3, only 150 km away, rock of nearly the same age holds a very different group, Type E. Wren Abernathy asked how two places so close together could have such different sea animals at the same time.

3At Stop 2, between the other two, the rock layers are bent into tight folds, and granite about 400 million years old cuts across the folds. The stop also has a belt of volcanic rock about 450 million years old. Dr. Varga said it looks like rock that forms today in chains of volcanic islands beside deep ocean trenches, where old ocean floor sinks back into the mantle.

4Back at school, the class looked up where else each trilobite group has been found (Table 2). They also read that the floor of the North Atlantic is youngest at the Mid-Atlantic Ridge, an underwater mountain chain down the middle of the ocean, and gets older toward both coasts. Its oldest parts, next to the continents, are about 180 million years old. Here the ocean is about 5,400 km wide.