The Unlabeled Map: Find the Hidden Plate Boundaries

Investigation question: Using only the locations, depths and magnitudes of 40 earthquakes and the positions of 11 active volcanoes, where are the plate boundaries on this map, what kind is each one, and which towns face which hazards?

Scientific purpose

The regional planning office of a fictional coastal region has an earthquake catalog and a volcano list but no map of plate boundaries. Students plot every event on a grid, color earthquakes by depth and size them by magnitude, draw a depth cross-section across the most active zone, and reason from the patterns to the hidden boundaries and their types. They finish by forecasting which of four towns faces the greatest earthquake hazard and which faces volcanic hazards, and by recommending mitigation. The lab turns the lesson's Key Science Concept (which settings produce eruptions, earthquakes or both) into something students discover from data rather than read.

Why this helps students with this chapter

Reading that 'earthquakes happen at plate boundaries' is easy to memorize and easy to misapply. Plotting 51 points by hand makes the belts appear, makes the exceptions stand out, and forces students to use several kinds of evidence at once: depth, magnitude and the presence or absence of volcanoes. The cross-section shows, with no extra explanation, that the deepest earthquakes trace a sinking plate and that the largest earthquake is shallow, which corrects the textbook's 'convergent quakes are deep'. Ending with a town ranking keeps the focus of MS-ESS3-2: patterns in hazard data are used to forecast and to plan.

Standards connections

MS-ESS3-2 (assesses, the forecasting half: using the mapped locations, depths and magnitudes of past events to forecast where future earthquakes and eruptions are likely and to suggest mitigation): Students analyze simulated hazard data (locations, magnitudes, depths) to rank four towns by earthquake hazard, identify which face volcanic hazards, and recommend mitigation (q8, q9, q12). This produces direct evidence toward the forecasting part of the PE; Lab 3 adds the frequency data and the technology choice.

MS-ESS2-3 (supports, earthquake and volcano locations revealing ridges, trenches and boundaries as evidence of plate motion): The plotted belts reveal a spreading ridge, a trench with a sinking plate and a transform boundary, and the cross-section shows the direction one plate is moving. This supports the plate-motion evidence assessed in Lesson 16 but is not the main target here.

Roles (rotate each session)

• North plotter: Plots every earthquake and volcano with y = 8 or more on the group map, using the depth colors and magnitude sizes in the key.
• South plotter: Plots every earthquake and volcano with y = 7 or less, then checks five of the North plotter's points against the table.
• Depth analyst: Builds the cross-section from the earthquakes in the north band (y = 8 or more, x = 10 or more) and leads q3 and q4.
• Skeptic and recorder: Fills the classification table, asks 'what is the evidence?' for every interpretation, and records the group's town ranking with its reasons.

Materials

Estimated cost: 2 to 4 per class (estimate) (an estimate; Printing about 30 double-sided copies; colored pencils, rulers and highlighters are classroom stock.)

Safety

• Paper and pencil only; there are no physical hazards. If you use the floor-sized masking-tape grid option, keep walkways clear and have students step, not run, between points.
• The towns and region are fictional. If students ask about hazards where they live, point them to the official USGS and California Geological Survey hazard maps rather than drawing conclusions from this teaching dataset.

Teacher preparation and setup

1. Print the recording sheet double-sided. Plot the full dataset yourself once (about 15 minutes) so you can spot misplotted points quickly; the most common error is swapping x and y.
2. Make an answer overlay on a transparency or tracing paper: the ridge along x = 3, the trench along x = 10 from y = 8 to y = 16, the transform along x = 11 from y = 0 to y = 7, the hot-spot cluster near (6, 3)-(6, 4), the cluster inside the continent near (18, 4)-(19, 5), and the small volcanic field near (16, 3)-(17, 2).
3. Decide whether to tell students before they start that the map is loosely modeled on a real region. The debrief works best if they discover the resemblance to western North America afterward.
4. Post the plotting key on the board: depth by color, magnitude by dot size (small below 5.0, medium 5.0 to 6.9, large 7.0 and above), volcanoes as triangles.

Investigation design

Student procedure

1. Read the scenario and the plotting key together. Agree on the grid: x counts squares east, y counts squares north, and each square is 50 km. Ocean lies west of x = 11; land lies to the east.
2. Split the plotting: the North plotter takes every point with y = 8 or more and the South plotter every point with y = 7 or less. Plot each earthquake as a dot colored by depth and sized by magnitude, and each volcano as a small triangle. Write the ID beside every point.
3. Swap and check: each plotter checks five of the other's points against the table. Mark the four towns with a square and their initials.
4. Answer q1 and q2 as a group. Then look for lines and clusters of points. Lightly sketch where you think each boundary runs, but do not label it yet.
5. Depth analyst: plot the cross-section using every earthquake with y = 8 or more and x = 10 or more (distance east on the horizontal axis, depth on the vertical axis, deeper lower). The group answers q3 and q4.
6. Classify each line or cluster in the classification table: where it is, the earthquake depths, the largest magnitude, whether volcanoes are present, your interpretation, and the key evidence. Answer q5 and q6.
7. Highlight the boundaries you are confident about and leave dashed lines where you are unsure. Answer the analysis questions individually: the magnitude comparison (q7), the town forecast (q8) and the remaining questions.
8. Compare maps with one other group. Where your interpretations differ, decide which evidence would settle the disagreement.

Expected observations

• Thirty of the 40 earthquakes are shallow (red), 8 intermediate (blue) and 2 deep (black). All of the blue and black dots lie in the north-east band between x = 13 and x = 17.
• A straight north-south line of small, shallow earthquakes (magnitudes 4.1 to 5.2) with three volcanoes runs along x = 3.
• In the north, earthquakes deepen steadily eastward from about 20-25 km at x = 10 to 320-330 km at x = 17, about 43 km deeper per grid square; four volcanoes sit at x = 13 to 14, above the 90 to 140 km part of the slab.
• A line of shallow earthquakes (6 to 14 km) with magnitudes up to 7.4 and no volcanoes runs along x = 11 from y = 0 to y = 7.
• Isolated clusters appear in the ocean near (6, 3)-(6, 4) with two volcanoes and inside the continent near (18, 4)-(19, 5) with no volcanoes; a small volcanic field with tiny shallow earthquakes sits near (16, 3)-(17, 2).

Likely difficulties

• Swapping x and y while plotting. The cross-check step catches most of these; tell students to say 'east first, then north' aloud.
• Reading the cross-section's vertical axis upside down. Point out that 0 km is at the top and depth increases downward, like a real slice through Earth.
• Calling every line of earthquakes a subduction zone because 'that is where big earthquakes are'. Ask what the depths and the volcanoes say.
• Adding the magnitude factors instead of multiplying them in q7 (for example 3 x 32 = 96). Show the ladder 32, 1,024, 32,768.
• Treating the anomaly near (16, 3) as an error to ignore. Real data has exceptions; q10 asks students to reason about it.

Structured debrief

1. Lay one group's map under your overlay. Name the five settings the groups found: a spreading ridge along x = 3, a subduction zone with its trench along x = 10 in the north, a transform boundary along x = 11 in the south, a hot spot near (6, 3)-(6, 4), and a fault zone inside the continent near (18, 4)-(19, 5). Ask which evidence was most decisive for each: depth for the subduction zone, missing volcanoes for the transform, isolation for the hot spot.
2. Reveal the inspiration: the map is loosely modeled on western North America. The northern trench and its volcano chain resemble the Cascadia subduction zone and the Cascade volcanoes; the southern transform resembles the San Andreas Fault; the point where they meet resembles Cape Mendocino; the inland cluster resembles faults inside a plate such as the New Madrid zone; the hot spot stands in for Hawaii (which is really thousands of kilometers away). The small volcanic field near (16, 3) is a stand-in for Long Valley Caldera, which is not on a plate boundary and is linked to stretching of the crust east of the Sierra Nevada.
3. Use the cross-section to correct the textbook gently: the largest earthquake on the map, E14 (magnitude 8.7), was only 25 km deep, right at the trench, while the two deepest (E39 at 330 km and E29 at 320 km) were magnitude 6.6 and 5.0. Subduction zones produce both kinds.
4. Close on forecasting: the map tells you where hazards are likely and what kinds, not when. Ask what extra data a planner would want before building in Port Halden (how often big earthquakes have struck in the past, from geology; tsunami heights; soft ground). Lab 3 adds exactly that kind of data.

Recording sheet: what students fill in

Lab 1 recording sheet: The Unlabeled Map

The planning office of a fictional coastal region has lost its map of plate boundaries. It still has a sample of 40 earthquakes from its 100-year catalog and a list of the 11 volcanoes that have erupted in the last 10,000 years. Your job is to rebuild the boundary map from the data alone and then advise four towns. Every number on this sheet is a teaching dataset invented for this lab. On the grid, x counts squares east and y counts squares north; each square is 50 km. Ocean lies west of x = 11 and land lies east of it.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own copy of the plotted map (half plotted by them, half checked), the classification table, and individual answers to q7 through q12. The town ranking in q8 and the planner response in q9 are the strongest individual evidence for MS-ESS3-2 because each must cite specific earthquakes, depths, magnitudes or volcanoes.

Assessment criteria

Accessibility adaptations

• Provide an enlarged grid (A3 or 11 x 17 inches) and pre-cut sticky dots in three colors and three sizes for students with fine-motor or vision needs.
• Offer a version of the earthquake table sorted into the two plotting halves with the y = 8 dividing line drawn on the map.
• Provide sentence frames for the classification table: 'The quakes here are ___ and the largest is ___, and there are/are no volcanoes, so this is a ___ boundary.'
• Allow the town forecast (q8) to be given orally or recorded, with the student pointing to the evidence on the map.

Low-cost alternative

No materials beyond printing are needed. If colored pencils are short, use symbols instead of colors: an open circle for shallow, a filled circle for intermediate and a filled square for deep. A floor-sized grid made with masking tape and sticky notes turns the plotting into a whole-class activity.

Model limitations

This lab has not been physically tested with a class as written; timings are estimates. The region, towns and every number are simulated. Real boundaries are wider zones with many parallel faults, not single grid lines, and a real 100-year catalog for a region this size would hold thousands of earthquakes, not 40; the sample was chosen to make the patterns readable. Real subduction zones also have offshore earthquakes in the bending plate and inland crustal faults. The map is flat and ignores Earth's curvature. The hot spot is placed far closer to the subduction zone than Hawaii is to North America. Use the map to practice reasoning from patterns, not as a picture of any real place.

Teacher notes

All data are simulated for a fictional region; the earthquake and volcano IDs were shuffled so the table does not reveal the groups. Grouping for reference: ridge E10, E11, E19, E27, E34 and V3, V7, V11; subduction slab E2, E3, E4, E6, E7, E8, E9, E12, E14, E23, E28, E29, E30, E33, E35, E39, with the crustal E24 under volcanoes V2, V6, V8, V9; transform E13, E15, E17, E18, E21, E22, E36 plus E25 just east of the line; hot spot E26, E31, E40 and V1, V10; continental cluster E20, E32, E38; volcanic field E5, E37 and V4, V5; isolated events E1 and E16 (small earthquakes away from boundaries happen in real catalogs too). In the debrief, connect the map to California: Cascadia in the north, the San Andreas in the south, Cape Mendocino where they meet, and Long Valley as the inland volcanic exception.