Earth's Layers and Plate Tectonics
Grade 10 · Science · Study Guide
Explore Earth's internal structure and discover how moving tectonic plates shape the continents, create mountains, and cause earthquakes.
Earth is structured in concentric layers: the inner core (solid iron-nickel, ~5,000°C), outer core (liquid iron-nickel — its convective motion generates Earth's magnetic field), mantle (thick layer of semi-solid silicate rock that flows very slowly), and crust (thin outermost layer, either continental or oceanic). The lithosphere — the rigid outer layer of the crust and uppermost mantle — is broken into tectonic plates that move on the semi-fluid asthenosphere below.
The theory of plate tectonics explains how Earth's surface has changed over billions of years. Plates move 2–15 cm per year, driven by convection currents in the mantle and slab pull (the sinking of dense oceanic crust). The three types of plate boundaries determine what geologic events occur: convergent (plates collide — forms mountains and subduction zones), divergent (plates separate — forms rift valleys and mid-ocean ridges), and transform (plates slide past each other — causes earthquakes along fault lines).
Major evidence for plate tectonics includes matching fossil records across continents, the fit of continental coastlines (like Africa and South America), seafloor spreading, and the distribution of earthquakes and volcanoes along plate boundaries. Alfred Wegener proposed the idea of continental drift in 1912, and plate tectonic theory was fully established by the 1960s with the discovery of seafloor spreading.
| Layer | State | Composition | Approximate Temperature |
|---|---|---|---|
| Crust (oceanic) | Solid | Basalt | 0–300°C |
| Crust (continental) | Solid | Granite | 0–300°C |
| Mantle | Semi-solid (plastic) | Silicate rock (peridotite) | 300–3,000°C |
| Outer core | Liquid | Iron-nickel | ~3,000–5,000°C |
| Inner core | Solid | Iron-nickel | ~5,000–6,000°C |
Key terms
- Lithosphere
- The rigid outer layer of Earth, including the crust and upper mantle; broken into tectonic plates.
- Asthenosphere
- The semi-fluid upper portion of the mantle on which the tectonic plates float and move.
- Convergent boundary
- A plate boundary where two plates collide; can form mountains, trenches, and volcanic arcs.
- Divergent boundary
- A plate boundary where two plates move apart; forms mid-ocean ridges and rift valleys.
- Subduction
- The process where one tectonic plate slides beneath another into the mantle, often causing volcanoes.
- Seafloor spreading
- The process at mid-ocean ridges where new oceanic crust forms as plates separate; key evidence for plate tectonics.
Flashcards
- Tectonic plate
- A massive, rigid segment of Earth's lithosphere that moves on the asthenosphere.
- Continental drift
- Wegener's hypothesis that continents were once joined and have moved apart over millions of years.
- Transform boundary
- Plate boundary where plates slide horizontally past each other; causes earthquakes (e.g., San Andreas Fault).
- Convection current
- Circular flow in the mantle driven by heat; the main driving force of plate motion.
- Mid-ocean ridge
- Underwater mountain chain at divergent boundaries where new oceanic crust is continuously formed.
- Richter scale
- A logarithmic scale measuring earthquake magnitude; each whole number represents 10x more ground motion.
Practice questions
At which type of boundary do two plates move away from each other?
- Convergent
- Transform
- Divergent
- Subduction
Show answer
Answer: Divergent
At divergent boundaries, tectonic plates move apart. Magma wells up to fill the gap, creating new crust — forming mid-ocean ridges underwater and rift valleys on land.
Earth's outer core is liquid, and its convective motion generates Earth's magnetic field.
Show answer
Answer: True
The liquid iron-nickel outer core moves in convection currents. This moving electrical conductor generates Earth's protective magnetic field — the magnetosphere.
The process where a denser plate sinks beneath another at a convergent boundary is called ___.
Show answer
Answer: subduction
Subduction occurs when a denser oceanic plate dives beneath a less dense continental plate (or another oceanic plate) at a convergent boundary, often generating volcanoes and deep ocean trenches.
Which of the following is the best evidence for seafloor spreading?
- Volcanoes on continents
- Matching fossils on different continents
- Symmetric magnetic striping on the ocean floor
- Earthquakes near transform faults
Show answer
Answer: Symmetric magnetic striping on the ocean floor
As new crust forms at mid-ocean ridges, it records Earth's magnetic polarity. Symmetric patterns of magnetic striping on both sides of a ridge directly confirm that new crust is spreading outward.
Continental crust is denser and thinner than oceanic crust.
Show answer
Answer: False
Oceanic crust is denser and thinner (~7 km thick, basalt-based). Continental crust is less dense and much thicker (~35 km thick, granite-based). The greater density of oceanic crust is why it subducts under continental crust.
Fun facts
- The Himalayan mountains are still growing taller today — about 5 mm per year — as the Indian and Eurasian plates continue to collide.
- Earth's magnetic field, generated by the liquid outer core, flips polarity every 200,000–300,000 years on average — and we may be overdue for one.
- The Pacific Ocean is slowly shrinking as the Americas move westward. The Atlantic Ocean is getting wider by about 2.5 cm every year.
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