Earth's Layers and Plate Tectonics
Grade 9 · Science · Study Guide
Explore Earth's internal structure and how the movement of tectonic plates shapes our planet's surface through earthquakes, volcanoes, and mountain building.
Earth is not a solid, uniform ball — it is layered like a hard-boiled egg. From the inside out: the inner core (solid iron-nickel, ~1,220 km radius, extremely hot ~5,000°C), the outer core (liquid iron-nickel, generates Earth's magnetic field), the mantle (thick semi-solid rock, ~2,900 km thick, where convection currents drive plate motion), and the crust (thin outermost layer — oceanic crust is denser and thinner ~5-10 km; continental crust is thicker ~30-70 km and less dense).
Plate tectonics is the theory that Earth's lithosphere (crust + upper mantle) is divided into about 15 major tectonic plates that move slowly (a few centimeters per year) driven by convection currents in the mantle. Plates interact at three types of boundaries. Convergent boundaries occur where plates move toward each other — oceanic plates can subduct under continental plates, forming trenches and volcanic mountains (like the Andes). Two continental plates colliding create mountain ranges (like the Himalayas). Divergent boundaries occur where plates move apart, allowing magma to rise and create new crust — the Mid-Atlantic Ridge and rift valleys are examples. Transform boundaries occur where plates slide horizontally past each other, causing earthquakes — the San Andreas Fault in California is a famous example.
Evidence for plate tectonics includes matching fossil records across continents, complementary coastline shapes, similar rock types on different continents, seafloor spreading, and the distribution of earthquakes and volcanoes along plate boundaries.
| Boundary Type | Plate Movement | Features Formed | Example |
|---|---|---|---|
| Convergent | Toward each other | Mountains, trenches, volcanoes | Himalayas, Mariana Trench |
| Divergent | Away from each other | Rift valleys, mid-ocean ridges | Mid-Atlantic Ridge |
| Transform | Sliding past each other | Fault lines, earthquakes | San Andreas Fault, CA |
Key terms
- Lithosphere
- Earth's rigid outer shell consisting of the crust and upper mantle; broken into tectonic plates.
- Tectonic Plates
- Large sections of Earth's lithosphere that move relative to each other due to mantle convection currents.
- Convergent Boundary
- A plate boundary where two plates move toward each other; can form mountains, volcanoes, or ocean trenches.
- Divergent Boundary
- A plate boundary where two plates move apart; magma wells up to form new crust (seafloor spreading).
- Transform Boundary
- A plate boundary where plates slide horizontally past each other; associated with earthquakes.
- Subduction
- The process where a denser oceanic plate sinks beneath a less dense plate at a convergent boundary.
- Convection Current
- Circular flow of heat in the mantle where hot material rises, cools, and sinks; drives plate movement.
Flashcards
- Tectonic Plates
- Large rigid pieces of Earth's lithosphere that move slowly over the mantle.
- Convergent Boundary
- Plates moving together; creates mountains, trenches, and volcanoes.
- Divergent Boundary
- Plates moving apart; creates new crust and rift valleys.
- Transform Boundary
- Plates sliding past each other; causes earthquakes.
- Subduction
- Dense oceanic plate sinks under less dense plate at convergent boundary.
- Convection Current
- Heat-driven circular flow in the mantle that moves tectonic plates.
Practice questions
Which layer of Earth is responsible for generating the planet's magnetic field?
- Inner core
- Outer core
- Mantle
- Crust
Show answer
Answer: Outer core
The liquid iron-nickel outer core flows as Earth rotates, generating electric currents that produce Earth's magnetic field — which protects us from harmful solar radiation.
Oceanic crust is thicker and less dense than continental crust.
Show answer
Answer: False
Oceanic crust is THINNER (5-10 km) and DENSER (mainly basalt) than continental crust (30-70 km thick, mainly granite). Its greater density causes it to subduct beneath continental crust at convergent boundaries.
At a ___ boundary, two tectonic plates move apart and new crust is formed.
Show answer
Answer: divergent
Divergent boundaries pull plates apart. Magma rises through the gap, cools, and solidifies as new oceanic crust — a process called seafloor spreading. The Mid-Atlantic Ridge grows about 2.5 cm per year.
The San Andreas Fault is an example of which type of plate boundary?
- Convergent
- Divergent
- Transform
- Subduction
Show answer
Answer: Transform
Transform boundaries are where plates slide horizontally past each other, building up stress that is released as earthquakes. The San Andreas Fault marks the boundary between the Pacific Plate and the North American Plate.
What drives the movement of tectonic plates?
- Gravity from the Moon
- Earth's rotation
- Convection currents in the mantle
- Magnetic field reversals
Show answer
Answer: Convection currents in the mantle
Heat from Earth's core warms the lower mantle, making rock less dense so it rises. It cools near the surface, becomes denser, and sinks — creating circular convection currents that drag the overlying plates.
Fun facts
- The Himalayas are still growing about 5 mm per year because the Indian Plate continues to push into the Eurasian Plate — they've been colliding for about 50 million years.
- About 90% of all earthquakes and 75% of all active volcanoes occur along the 'Ring of Fire' — a horseshoe-shaped zone around the Pacific Ocean at plate boundaries.
- Alfred Wegener proposed continental drift in 1912 but was ridiculed. It took decades and the discovery of seafloor spreading in the 1960s for the theory to be accepted.
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