Unit 2 of Earth Science: Plate Tectonics and Earthquakes.
This unit covers tectonic plates, earthquake waves, plate boundaries and seismology — essential concepts for Earth Science. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers tectonic plates, earthquake waves, plate boundaries and seismology — essential concepts for Earth Science. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Tectonic Plates
Earth's lithosphere is broken into large pieces called tectonic plates that float on the semi-fluid asthenosphere. Plates move due to convection currents in the mantle, which transfer heat from Earth's interior. Students must know the difference between oceanic and continental crust and how density drives plate interactions.
Key Points
- Oceanic crust is denser and thinner (~7 km) than continental crust (~35 km)
- Plates move 2–10 cm per year, driven by mantle convection and ridge push/slab pull
- The lithosphere includes the crust and upper mantle; the asthenosphere is the weak, flowing layer below
- When oceanic and continental plates collide, the denser oceanic plate subducts
A test question asks: 'Why does oceanic crust subduct beneath continental crust at a convergent boundary?' Choose the best answer: (A) Oceanic crust is older, (B) Oceanic crust is denser, (C) Continental crust moves faster, (D) Oceanic crust is thicker.
The correct answer is (B). Oceanic crust is composed mainly of basalt, giving it a density of ~3.0 g/cm³, while continental crust is granite-based at ~2.7 g/cm³. Because denser material sinks in the mantle, oceanic plates always subduct when colliding with continental plates.
2 Plate Boundaries
There are three types of plate boundaries — convergent, divergent, and transform — and each produces distinct landforms and geologic events. Students must be able to identify the boundary type from a description or diagram and name the associated features. The Ring of Fire is a key real-world example of convergent and transform boundaries.
Key Points
- Divergent boundaries: plates move apart, forming mid-ocean ridges or rift valleys (e.g., Mid-Atlantic Ridge)
- Convergent boundaries: plates collide, forming subduction zones, trenches, volcanic arcs, or mountain ranges (e.g., Himalayas, Andes)
- Transform boundaries: plates slide horizontally past each other, causing earthquakes with no volcanism (e.g., San Andreas Fault)
- Subduction zones produce the deepest earthquakes and most explosive volcanoes
The Mariana Trench is located where the Pacific Plate meets the Philippine Plate. What type of boundary is this, and what feature forms here?
This is a convergent boundary where two oceanic plates meet. The denser Pacific Plate subducts beneath the Philippine Plate, creating a deep ocean trench — the deepest in the world at ~11,000 m. The descending plate also generates a chain of volcanic islands called an island arc.
3 Earthquake Waves
Earthquakes release energy in the form of seismic waves that travel through and along Earth. Students must know the three main wave types — P, S, and surface waves — including their speed, motion, and which materials they can travel through. The P-S time difference is used to determine distance to the epicenter.
Key Points
- P-waves (Primary): fastest, compress and expand material, travel through solids AND liquids
- S-waves (Secondary): slower, move material side-to-side (shear), travel through solids ONLY — cannot pass through Earth's liquid outer core
- Surface waves: slowest, travel along Earth's surface, cause the most structural damage
- The greater the P-S wave arrival time difference, the farther the seismic station is from the epicenter
A seismic station records P-waves arriving at 10:00:00 AM and S-waves arriving at 10:01:30 AM. The P-S time difference is 90 seconds. Using a travel-time graph, this corresponds to a distance of approximately 900 km from the epicenter. How many stations are needed to locate the epicenter?
At least three seismic stations are required — a method called triangulation. Each station determines its distance from the epicenter and draws a circle with that radius on a map. The single point where all three circles intersect is the epicenter's location.
4 Seismology
Seismology is the scientific study of earthquakes and seismic waves, using instruments called seismographs to record ground motion. Students must understand how epicenter vs. focus (hypocenter) differ, how the Richter and Moment Magnitude scales work, and how data from multiple stations is used to locate an earthquake. Magnitude and intensity are different measurements and are frequently confused on exams.
Key Points
- Focus (hypocenter): the underground point where the earthquake originates; epicenter is the point directly above it on the surface
- Each whole-number increase on the Richter scale = 10× greater amplitude and ~31.6× more energy released
- Intensity (Modified Mercalli Scale) measures how shaking is felt at a location; magnitude is a fixed measurement of total energy
- Shadow zones — regions that receive no direct P or S waves — revealed that Earth has a liquid outer core
Earthquake A has a magnitude of 5.0 and Earthquake B has a magnitude of 7.0. How many times more energy does Earthquake B release than Earthquake A?
The magnitude difference is 7.0 − 5.0 = 2.0. Each whole-number step represents ~31.6× more energy, so for two steps the calculation is 31.6 × 31.6 ≈ 1,000. Earthquake B releases approximately 1,000 times more energy than Earthquake A, even though the magnitude numbers appear close together.
Questions, answered.
What is Plate Tectonics and Earthquakes?
Plate Tectonics and Earthquakes is Unit 2 of Earth Science, covering tectonic plates, earthquake waves, plate boundaries and seismology.
How to study for Earth Science Unit 2?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 28+ review questions across 5 different game modes.