AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/4/4-1)
Unit 4 · Topic 4.1
4.1 Plate Tectonics
Earth's rigid outer shell is broken into plates that move a few centimeters a year. What happens at the edges, whether plates collide, pull apart or slide past each other, explains where mountains, volcanoes, trenches and earthquakes are found.
Key terms
- convergent boundary
- divergent boundary
- transform boundary
- subduction
- seafloor spreading
Earth's layers and why plates move
Earth has a solid inner core, a liquid outer core, a thick mantle and a thin crust. The lithosphere is the rigid outer layer made of the crust plus the very top of the mantle. It's broken into large pieces called tectonic plates. Below it is the asthenosphere, a hot, soft part of the upper mantle that can flow very slowly.
Heat from Earth's interior drives slow convection currents in the mantle: hot material rises, cools and sinks. Together with gravity pulling old, dense plate edges down into the mantle, this moves the plates a few centimeters per year, about as fast as your fingernails grow.
There are two kinds of crust. Oceanic crust is thinner and denser (made largely of basalt). Continental crust is thicker and less dense (largely granite). This difference decides what happens when plates meet.
Three types of plate boundaries
Subduction is when one plate dives beneath another into the mantle. As the sinking plate heats up, water released from it helps melt the mantle above, and the magma rises to form volcanoes. Earthquakes happen on faults, cracks where blocks of rock move past each other. When a fault is locked, its rough sides stick while stress keeps building. When the stress finally overcomes the friction, the rock suddenly slips and releases the stored energy as an earthquake.
| Boundary | Plate motion | What forms | Example |
|---|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, seafloor spreading, rift valleys, volcanoes, shallow earthquakes | Mid-Atlantic Ridge and Iceland; East African Rift |
| Convergent (oceanic–continental) | Denser oceanic plate subducts (sinks) under continental plate | Deep ocean trench, coastal volcanic mountains, strong earthquakes | Nazca Plate under South America, forming the Andes |
| Convergent (oceanic–oceanic) | One oceanic plate subducts under another | Trench and an arc of volcanic islands | Mariana Trench; Japan; the Aleutian Islands |
| Convergent (continental–continental) | Neither plate subducts easily; crust crumples | Tall mountain ranges, earthquakes | India colliding with Asia, forming the Himalayas |
| Transform | Plates slide past each other sideways | Earthquakes (little volcanic activity) | San Andreas Fault in California |
Seafloor spreading and hot spots
At mid-ocean ridges, magma rises, cools and forms new oceanic crust, pushing older crust away on both sides. That means seafloor rock is youngest at the ridge and gets older with distance from it, in a mirror-image pattern on both sides.
Not all volcanoes are at plate boundaries. A hot spot is a plume of unusually hot mantle that melts through the plate above it. As the plate moves over the stationary hot spot, it leaves a chain of volcanoes, oldest farthest away. The Hawaiian Islands formed this way, and Yellowstone sits over a hot spot under North America.
Reading a plate boundary map
Most earthquakes and volcanoes occur along plate boundaries. The Ring of Fire, a horseshoe of boundaries around the Pacific Ocean, has most of the world's active volcanoes and earthquakes, because it's ringed by subduction zones.
On a map, look for arrows. Arrows pointing toward each other mean convergent; pointing away mean divergent; pointing in opposite directions along the boundary mean transform. Knowing the boundary type tells you which hazards to expect, which matters for where people live and build.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Identifying the boundary
Identify the boundary type and describe the landform: (a) two plates carrying oceanic crust move apart in the middle of the Atlantic; (b) an oceanic plate moves toward a continent along the west coast of South America; (c) two plates slide past each other along a fault in California.
Show the solutionHide the solution
- Step 1: (a) Plates moving apart: divergent. Magma rises to form new crust, creating a mid-ocean ridge with seafloor spreading.
- Step 2: (b) Plates moving together: convergent. Dense oceanic crust subducts under continental crust, forming a deep trench offshore and a chain of volcanic mountains (the Andes).
- Step 3: (c) Plates sliding past: transform. Stress builds and releases as earthquakes; few volcanoes form.
Answer: (a) Divergent, mid-ocean ridge; (b) convergent (subduction), trench and volcanic mountains; (c) transform, earthquake-prone fault.
- Example 2Calculator allowed
Calculating a spreading rate
Rock on the seafloor 250 km from the Mid-Atlantic Ridge is 10 million years old. Assuming steady spreading, how fast has this side of the plate moved away from the ridge, in cm per year?
Show the solutionHide the solution
- Step 1: Rate = distance ÷ time.
- Step 2: Convert distance to centimeters: 250 km × 1,000 m/km × 100 cm/m = 25,000,000 cm, or 2.5 × 10⁷ cm.
- Step 3: Time = 10,000,000 years, or 1.0 × 10⁷ years.
- Step 4: Rate = 2.5 × 10⁷ cm ÷ 1.0 × 10⁷ yr = 2.5 cm/yr.
Answer: 2.5 cm per year.
Common mistakes
- Saying continental plates subduct under oceanic plates. Denser oceanic crust is the one that sinks.
- Expecting volcanoes at transform boundaries. Transform boundaries mainly produce earthquakes.
- Thinking the newest seafloor is near continents. It's youngest at the mid-ocean ridge.
On the exam
- Questions often describe a map with plate boundaries and ask what hazard or landform to expect in a location. Name the boundary type first, then the result.
- Unit conversions like km to cm come up in spreading-rate calculations. Show the conversion.
Connected topics
Videos
Check yourself
4 questions on 4.1 Plate Tectonics. Pick an answer to see if you got it, and why.
A curved chain of volcanic islands with frequent earthquakes forms where one oceanic plate sinks beneath another oceanic plate. This setting is best described as
| Distance from mid-ocean ridge (km) | Age of seafloor rock (millions of years) |
|---|---|
| 0 | 0 |
| 100 | 4 |
| 200 | 8 |
| 300 | 12 |
Hypothetical measurements on one side of a mid-ocean ridge
The pattern in the data is best explained by which process?
At what average rate has the seafloor moved away from the ridge? (1 km = 100,000 cm)
Along the San Andreas Fault in California, the Pacific Plate and the North American Plate slide past each other horizontally. Which hazard is most closely associated with this kind of boundary?
0 of 4 answered