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

Earthquakes and Volcanoes

Students explore the causes and effects of earthquakes and volcanic eruptions, relating them to plate boundaries.

• Common Core State Standards
• MS-ESS2-2

About This Topic

Earthquakes occur when rocks under stress along a fault fracture suddenly, releasing stored elastic energy as seismic waves. The focus (hypocenter) is the underground point of rupture; the epicenter is the point directly above it on the surface. Seismic waves propagate outward as P-waves (compressional, travel fastest, arrive first) and S-waves (shear, slower, arrive second). Because P and S waves travel at known speeds, the difference in their arrival times at a seismic station gives the distance to the epicenter. With data from three stations, a unique epicenter location can be determined by triangulation. The MS-ESS2-2 standard asks students to construct an explanation based on evidence for how geoscience processes have changed Earth's surface.

Volcanic eruptions occur where magma reaches Earth's surface. Eruption style depends primarily on magma viscosity, which is controlled by silica content. Low-viscosity basaltic magma (common at divergent boundaries and hotspots) produces effusive lava flows. High-viscosity, silica-rich magma (common at subduction zones) traps gases until pressure builds to an explosive release. This explains why Mauna Loa and Mount St. Helens erupt so differently.

Active learning tasks that ask students to locate an earthquake epicenter from seismic data model the actual method used in geophysics, making the technique both meaningful and memorable.

Key Questions

1. Explain the relationship between plate boundaries and the occurrence of earthquakes and volcanoes.
2. Analyze the different types of seismic waves and their impact.
3. Predict the potential hazards associated with different types of volcanic eruptions.

Learning Objectives

• Classify seismic waves (P-waves, S-waves) based on their motion and arrival times.
• Construct an explanation for how plate tectonic movement causes earthquakes and volcanic activity at plate boundaries.
• Compare and contrast the eruption styles of different volcanoes based on magma composition and viscosity.
• Analyze seismic data from three stations to triangulate and determine the location of an earthquake epicenter.
• Evaluate the potential hazards associated with different types of volcanic eruptions, such as ash fall, pyroclastic flows, and lava flows.

Before You Start

Earth's Layers and Structure

Why: Students need to understand the basic composition and structure of Earth's interior to comprehend how seismic waves travel and where magma originates.

Introduction to Plate Tectonics

Why: A foundational understanding of tectonic plates, their movement, and the concept of plate boundaries is essential for explaining the causes of earthquakes and volcanoes.

Key Vocabulary

Plate Boundary	The region where two tectonic plates meet, characterized by geological activity like earthquakes and volcanoes.
Seismic Waves	Vibrations that travel through Earth's layers, produced by earthquakes or other disturbances.
Epicenter	The point on Earth's surface directly above the focus, or origin, of an earthquake.
Magma Viscosity	A measure of a magma's resistance to flow, which influences the explosivity of volcanic eruptions.
Subduction Zone	An area where one tectonic plate slides beneath another, often leading to volcanic activity and deep earthquakes.

Watch Out for These Misconceptions

Common Misconception

Earthquakes and volcanoes are random and unpredictable events.

What to Teach Instead

While the exact timing of individual events cannot be precisely predicted, their locations are highly predictable: both cluster along plate boundaries. Plotting global earthquake and volcano data on a world map shows their clear alignment with boundaries, demonstrating that location -- if not timing -- is a well-understood product of plate tectonics.

Common Misconception

All volcanic eruptions are similar -- lava just flows out.

What to Teach Instead

Volcanic eruptions range from the gentle, continuous lava flows of Hawaiian shield volcanoes to the catastrophic explosive eruptions of stratovolcanoes like Mount St. Helens. The key variable is magma viscosity, which depends on silica content and gas content. Comparing video footage of both eruption types makes the difference immediate and concrete.

Active Learning Ideas

Inquiry Circle: Locating an Earthquake Epicenter

Groups receive seismograms showing P-wave and S-wave arrival times from three monitoring stations, plus a scaled map. Students calculate the distance to the epicenter from each station using the P-S wave time difference, then draw circles of the appropriate radius on the map. The intersection of the three circles locates the epicenter. Groups compare their results and discuss sources of error.

50 min·Small Groups

Think-Pair-Share: Eruption Style Prediction

Present data cards for five volcanoes including silica content, plate boundary type, and recent eruption history. Students individually predict whether each eruption would be effusive or explosive and explain their reasoning. Partners compare predictions and construct a shared explanation that links magma composition to eruption style.

25 min·Pairs

Gallery Walk: Types of Volcanoes

Post three stations covering shield volcanoes, composite volcanoes (stratovolcanoes), and cinder cones, each with cross-section diagrams, photographs, and data on eruption frequency and intensity. Groups annotate each station with the likely plate setting, example locations in the US, and the specific hazards each type poses to nearby communities.

35 min·Small Groups

Real-World Connections

• Geophysicists use seismographs to detect and locate earthquakes worldwide, providing critical data for hazard assessment and early warning systems in seismically active regions like California and Japan.
• Volcanologists monitor active volcanoes such as Kilauea in Hawaii and Mount Vesuvius in Italy, studying eruption patterns and magma composition to predict future events and protect nearby communities.
• Engineers design earthquake-resistant structures in cities prone to seismic activity, incorporating principles of wave propagation and ground motion to ensure building safety.

Assessment Ideas

Quick Check

Provide students with a diagram showing P-wave and S-wave arrival times at three different seismic stations. Ask them to calculate the time difference between P and S waves at each station and use this to estimate the distance to the epicenter for each station. Then, ask them to explain how they would use this information to locate the epicenter.

Discussion Prompt

Pose the question: 'Why do volcanoes at subduction zones tend to erupt more explosively than volcanoes at divergent boundaries?' Guide students to discuss magma viscosity, silica content, and gas pressure, referencing specific examples like Mount St. Helens versus Mauna Loa.

Exit Ticket

On an index card, have students draw a simple diagram illustrating one type of plate boundary (divergent, convergent, or transform). Ask them to label the boundary type, indicate the direction of plate movement, and draw symbols for where earthquakes and volcanoes are likely to occur.

Frequently Asked Questions

What causes earthquakes?

Earthquakes are caused by the sudden release of stress that builds up along faults as tectonic plates move. When friction holding rocks together is overcome, the rocks fracture suddenly and release energy as seismic waves. The type of plate boundary determines the style of faulting: transform boundaries produce horizontal shearing, convergent boundaries produce thrust faults, and divergent boundaries produce normal faults.

How do scientists measure the strength of an earthquake?

The moment magnitude scale (Mw) measures the total energy released based on the area of the fault that ruptured and how much it slipped. It is logarithmic: each whole number increase represents about 32 times more energy released. The older Richter scale measured ground motion at a single seismometer type and is no longer used by seismologists for significant events.

What are the different types of volcanic eruptions?

The two main eruption styles are effusive (steady lava flows from low-viscosity basaltic magma, typical of shield volcanoes at hotspots and rift zones) and explosive (violent pyroclastic eruptions from high-viscosity, gas-rich silicic magma, typical of composite volcanoes at subduction zones). Most eruptions fall somewhere between these extremes.

How does active learning help students understand earthquakes and volcanoes?

The epicenter triangulation activity converts a procedural skill into a genuine model of how seismologists work. When students use realistic wave arrival data to pinpoint a location on a map, they understand why three stations are required and what each contributes to the solution. This is more meaningful than simply being told that scientists use seismographs to locate earthquakes.