Science · 7th Grade · Earth's Changing Surface · Weeks 28-36

Continental Drift and Seafloor Spreading

Students investigate the historical development of the theory of plate tectonics, starting with continental drift and seafloor spreading.

• Common Core State Standards
• MS-ESS2-3

About This Topic

The theory of plate tectonics grew from two ideas developed decades apart. In 1912, Alfred Wegener proposed that Earth's continents were once joined in a single landmass called Pangea and had since drifted apart. His evidence was compelling: matching shapes of continental coastlines, identical fossil species on opposite sides of the Atlantic (including the freshwater reptile Mesosaurus and the plant Glossopteris), and continuous mountain ranges and rock types that align across continents. The MS-ESS2-3 standard asks students to analyze data on fossil distribution, rock types, continental shapes, and seafloor structures as evidence for plate tectonics.

Despite strong evidence, Wegener's theory was rejected for decades because he could not identify a mechanism powerful enough to move continents. The missing piece came in the 1950s and 1960s through ocean floor mapping. Scientists found a global system of mid-ocean ridges where new seafloor is continuously created. Seafloor rock ages systematically from youngest at the ridges to oldest at the margins, and magnetic striping patterns on either side of ridges record Earth's periodic magnetic reversals -- the most quantitative confirmation of seafloor spreading.

Active learning tasks that reproduce Wegener's fossil-and-rock-distribution reasoning put students in the position of building the argument themselves, which is far more instructive than being told the conclusion.

Key Questions

1. Analyze the evidence Alfred Wegener used to support continental drift.
2. Explain the process of seafloor spreading and its role in plate movement.
3. Critique early objections to the theory of continental drift.

Learning Objectives

• Analyze the fossil, geological, and paleoclimatic evidence Alfred Wegener presented to support his continental drift hypothesis.
• Explain the mechanism of seafloor spreading, including the role of mid-ocean ridges and magnetic striping, in driving plate tectonics.
• Evaluate the scientific objections to Wegener's continental drift theory and explain how later discoveries addressed these criticisms.
• Compare and contrast the evidence for continental drift with the evidence for seafloor spreading.

Before You Start

Earth's Layers and Composition

Why: Understanding the basic structure of Earth's interior, including the crust and mantle, is foundational for comprehending plate movement.

Fossil Formation and Interpretation

Why: Students need to know how fossils form and how they can be used to infer past environments and locations of landmasses.

Key Vocabulary

Continental Drift	The hypothesis that Earth's continents were once joined together in a single landmass and have since moved apart over geologic time.
Pangea	The supercontinent proposed by Alfred Wegener that existed during the late Paleozoic and early Mesozoic eras, comprising all the landmasses of the present continents.
Seafloor Spreading	The process by which new oceanic crust is formed at mid-ocean ridges and then moves away from the ridges towards the continental margins.
Mid-Ocean Ridge	An underwater mountain range, formed by plate tectonics, where new oceanic crust is generated as the plates move apart.
Magnetic Reversals	Periodic flips in Earth's magnetic field, where the north magnetic pole becomes the south magnetic pole and vice versa, recorded in the magnetic minerals of rocks.

Watch Out for These Misconceptions

Common Misconception

Wegener's theory was rejected because his evidence was weak.

What to Teach Instead

The evidence was actually compelling -- the problem was the absence of a plausible mechanism. Scientists could not explain what force was capable of moving continents through the solid ocean floor. This makes continental drift a valuable case study in how scientific acceptance depends on both evidence quality and mechanistic explanation, not just on observations.

Common Misconception

Seafloor spreading means the ocean floor keeps getting larger over time.

What to Teach Instead

As new seafloor forms at mid-ocean ridges, old seafloor sinks back into the mantle at subduction zones, so the total area of ocean floor stays roughly constant. Tracing the seafloor age gradient on a map and locating the deep trenches at plate margins makes this balance between creation and destruction concrete.

Active Learning Ideas

Inquiry Circle: Wegener's Evidence Assembly

Groups receive a set of data cards: printable continent shape maps for physical fitting, distribution maps for Mesosaurus and Glossopteris fossils, rock type and age distributions, and matching mountain ranges. Without being told what conclusion to reach, students arrange the evidence to argue for or against continental drift, then compare their reconstructions with other groups and identify where they agree and disagree.

45 min·Small Groups

Think-Pair-Share: Why Was Wegener Rejected?

After reviewing Wegener's evidence, students individually write the strongest objection they can think of to his theory. Partners share objections and together identify the most serious one -- the absence of a mechanism. The class discusses what additional evidence would have been needed to convince the scientific community in 1912.

20 min·Pairs

Gallery Walk: Seafloor Spreading Evidence

Post stations showing seafloor age maps, magnetic striping profiles, ocean depth profiles of mid-ocean ridges, and Harry Hess's seafloor spreading hypothesis. Groups annotate each station with what it shows, what question it answers, and what question it raises. End with a class discussion of how the evidence accumulated into a coherent theory in the 1960s.

35 min·Small Groups

Real-World Connections

• Geophysicists use data from seafloor spreading, including magnetic anomalies and seismic activity, to map tectonic plate boundaries, which is crucial for understanding earthquake and volcanic hazards in regions like the Pacific Ring of Fire.
• Paleontologists analyze the distribution of ancient fossils, such as Mesosaurus and Glossopteris, found on different continents to reconstruct past continental arrangements and understand evolutionary pathways.

Assessment Ideas

Quick Check

Provide students with a map showing the distribution of Glossopteris fossils and Mesosaurus fossils across South America, Africa, India, Antarctica, and Australia. Ask them to write two sentences explaining how this distribution supports continental drift.

Discussion Prompt

Pose the question: 'Why was Wegener's theory initially rejected, and what evidence eventually convinced scientists?' Facilitate a class discussion where students identify the lack of a mechanism and the later discovery of seafloor spreading as key points.

Exit Ticket

Ask students to draw a simple diagram illustrating seafloor spreading at a mid-ocean ridge. They should label the ridge, the direction of plate movement, and indicate where the oldest and youngest rocks are located.

Frequently Asked Questions

What evidence did Alfred Wegener use to support continental drift?

Wegener used four lines of evidence: the geometric fit of continental coastlines (especially South America and Africa), identical fossil species found on continents now separated by oceans, matching rock types and mountain ranges that align across continents, and evidence of ancient glaciation in areas now near the equator, indicating those regions were once near the poles.

What is seafloor spreading?

Seafloor spreading is the process by which new oceanic crust is created at mid-ocean ridges as magma rises from the mantle, cools, and adds to the plates on either side. The two plates move away from the ridge at rates of 2-18 cm per year, carrying the new crust with them. This is the mechanism Wegener was missing.

Why was continental drift rejected when Wegener first proposed it?

The main objection was that Wegener could not identify a mechanism powerful enough to move entire continents. Most geologists believed the ocean floor was too rigid for continents to plow through. Without a plausible driving force, even strong observational evidence was not sufficient to overturn the prevailing view.

How does active learning help students understand continental drift?

Physically fitting continent cutouts together and matching fossil distribution maps puts students in Wegener's position, reasoning from data to a conclusion. This is far more compelling than being told the continents once fit together. It also builds the skill of constructing an argument from multiple independent lines of evidence, which is central to the MS-ESS2-3 performance expectation.