Plate tectonics and continental evidence

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Lesson notes

What is Plate Tectonics?

**Plate tectonics** is the scientific theory that Earth's **lithosphere** is broken into large, moving plates.
The lithosphere includes the **crust** and **upper mantle** and is rigid and cool.
Below the lithosphere is the **asthenosphere**, which is hotter and flows like a slow-moving liquid.
Plates move because of **convection currents** in the mantle and the pull of sinking plates at subduction zones.

Types of Crust

**Oceanic crust** is thin (about 7 km), dense, and made of basalt (rich in silicon and magnesium).
**Continental crust** is thick (about 30–50 km), less dense, and made of granite (rich in silicon and aluminum).
Oceanic crust is denser, so it sits lower, forming ocean basins; continental crust floats higher, forming land.

Plate Boundaries

**Divergent boundaries** – plates move apart; new crust forms at mid-ocean ridges (e.g., Mid-Atlantic Ridge).
**Convergent boundaries** – plates collide; one plate subducts under the other, forming trenches, volcanoes, and mountains.
**Transform boundaries** – plates slide past each other horizontally, causing earthquakes (e.g., San Andreas Fault).

Evidence for Plate Tectonics

**Continental fit** – The shapes of continents like South America and Africa fit together like puzzle pieces.
**Fossil evidence** – Identical fossils (e.g.,×Mesosaurus×) are found on continents now separated by oceans.
**Rock and mountain evidence** – Similar rock types and mountain ranges (e.g., Appalachians) match across continents.
**Seafloor spreading** – New oceanic crust forms at mid-ocean ridges and spreads outward, recorded by magnetic stripes.

Seafloor Spreading

At mid-ocean ridges, magma rises, cools, and forms new oceanic crust, pushing older crust away.
The age of oceanic crust increases with distance from the ridge; the youngest crust is at the ridge.
Magnetic stripes on the seafloor record reversals of Earth's magnetic field, confirming seafloor spreading.

Driving Forces of Plate Motion

**Slab pull** – The main force: cold, dense oceanic crust sinks at subduction zones, pulling the plate behind it.
**Ridge push** – Gravity pushes plates away from the elevated mid-ocean ridges.
**Mantle convection** – Hot mantle rises at ridges, cools, and sinks at subduction zones, driving plate motion.

Plate Motion Rates

Plates move at speeds of **1–10 cm per year** (about as fast as fingernails grow).
The Nazca Plate moves fastest (~16 cm/yr), while the Eurasian Plate moves slowly (~1 cm/yr).

Geologic Features at Plate Boundaries

**Divergent** – Mid-ocean ridges, rift valleys, shallow earthquakes, volcanic activity.
**Convergent (oceanic-continental)** – Deep ocean trenches, volcanic mountain ranges (e.g., Andes).
**Convergent (oceanic-oceanic)** – Island arcs (e.g., Japan).
**Convergent (continental-continental)** – Mountain ranges (e.g., Himalayas).
**Transform** – Faults, earthquakes (no volcanoes).

Three types of plate boundaries: divergent (plates move apart), convergent (plates collide), and transform (plates slide past).

The same Mesosaurus fossil is found in South America and Africa, supporting that they were once joined.

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Practice questions

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1.What is the scientific theory that Earth's lithosphere is divided into large plates that move slowly over time?
Easy
- AContinental drift
- BPlate tectonics
- CSeafloor spreading
- DSubduction
2.Which layer of Earth is rigid and broken into tectonic plates?
Easy
- AAsthenosphere
- BLithosphere
- CMantle
- DOuter core
3.Which type of crust is denser and generally lies below sea level?
Easy
- AContinental crust
- BOceanic crust
- CLithospheric crust
- DSial
4.At what type of plate boundary does subduction occur?
Medium
- ADivergent
- BConvergent
- CTransform
- DAll boundaries
5.What process creates new oceanic crust at mid-ocean ridges?
Medium
- ASubduction
- BSeafloor spreading
- CContinental drift
- DConvection
6.The movement of tectonic plates is primarily driven by which process in the mantle?
Medium
- AConduction
- BConvection
- CRadiation
- DSubduction
7.Why does oceanic lithosphere become thicker as it moves away from a mid-ocean ridge?
Hard
- AIt accumulates sediment
- BIt cools and more mantle material attaches to its base
- CIt is compressed by convergent boundaries
- DIt absorbs water and expands
8.Which of the following is NOT evidence used to support plate tectonics?
Hard
- AFossil distribution across continents
- BMatching rock types on different continents
- CThe shape of continents fitting together
- DThe constant temperature of Earth's interior

Revision notes

## Plate Tectonics & Continental Evidence ### Key Idea Earth's lithosphere (crust + upper mantle) is broken into **tectonic plates** that move slowly over the **asthenosphere** (hot, flowing layer). This theory explains continental drift, seafloor spreading, and many geologic features. ### Evidence for Plate Motion - **Fossil distribution**: Same fossils (e.g., *Mesosaurus*) found on continents now far apart. - **Rock types & mountain belts**: Matching rock layers and mountain ranges across oceans (e.g., Appalachians in US and Caledonides in Scotland). - **Continental shapes**: Coastlines of South America and Africa fit like puzzle pieces. - **Seafloor structures**: Mid-ocean ridges, trenches, and magnetic stripes on ocean floor show spreading. ### Plate Boundaries - **Divergent**: Plates move apart; new crust forms (e.g., Mid-Atlantic Ridge). - **Convergent**: Plates collide; one subducts (oceanic under continental) or both crumple (continent-continent). Creates mountains, volcanoes, trenches. - **Transform**: Plates slide past each other; earthquakes occur (e.g., San Andreas Fault). ### How Plates Move - **Convection currents** in the mantle drive plate motion. - **Ridge push**: New crust at mid-ocean ridges pushes plates away. - **Slab pull**: Dense oceanic crust sinks at subduction zones, pulling the plate. ### Key Facts - Plates move **2–15 cm/year** (about as fast as fingernails grow). - Oceanic crust is **denser** (basalt) than continental crust (granite). - Subduction recycles crust; seafloor spreading creates new crust. ### Remember - **Lithosphere**: rigid outer layer (plates). - **Asthenosphere**: soft, convecting layer below. - **Continental drift** was proposed by Wegener; plate tectonics added the mechanism.

Plate tectonics: Scientific theory that Earth's lithosphere is divided into moving plates, explaining continental drift, seafloor spreading, and geologic activity.
Lithosphere: Rigid outer layer of Earth, including crust and upper mantle, broken into tectonic plates.
Asthenosphere: Hot, ductile layer beneath the lithosphere that flows slowly and allows plates to move.
Continental drift: Hypothesis that continents move over Earth's surface; part of plate tectonics.
Seafloor spreading: Process at mid-ocean ridges where new oceanic crust forms as plates diverge.
Subduction: Process where one plate sinks under another at a convergent boundary, recycling crust into the mantle.
Convergent boundary: Plate boundary where plates move toward each other, causing subduction or collision.
Divergent boundary: Plate boundary where plates move apart, creating new crust.
Transform boundary: Plate boundary where plates slide horizontally past each other, causing earthquakes.
Convection current: Circular motion in the mantle caused by heat, driving plate movement.

Flashcards

What is the scientific theory that Earth's lithosphere is divided into moving plates?

Plate tectonics.

What is the lithosphere?

The rigid outer shell of Earth, including the crust and upper mantle.

What is the asthenosphere?

The hot, ductile layer beneath the lithosphere that flows slowly and allows plates to move.

How fast do tectonic plates typically move?

From 0 to 10 cm per year, similar to the growth rate of fingernails or hair.

What are the three types of plate boundaries?

Convergent, divergent, and transform.

What happens at a divergent plate boundary?

Plates move apart, and new oceanic crust forms through seafloor spreading.

What happens at a convergent plate boundary?

Plates move toward each other; one plate may subduct beneath the other, forming trenches, volcanoes, and mountains.

What happens at a transform plate boundary?

Plates slide horizontally past each other, causing earthquakes.

What is subduction?

The process where one tectonic plate sinks under another into the mantle at a convergent boundary.

How does seafloor spreading balance subduction?

New oceanic crust forms at divergent boundaries, balancing the crust destroyed at subduction zones, keeping Earth's surface area constant.

What evidence from fossils supports plate tectonics?

Identical fossils of the same species are found on continents now separated by oceans, suggesting they were once joined.

How do continental shapes provide evidence for plate tectonics?

Continents like South America and Africa fit together like puzzle pieces, indicating they were once connected.

What rock evidence supports plate tectonics?

Matching rock types and mountain ranges on different continents suggest they were once part of the same landmass.

What is the difference between oceanic and continental crust?

Oceanic crust is denser, thinner, and made of basalt (sima); continental crust is less dense, thicker, and made of granite (sial).

Why does oceanic crust sink below continental crust at subduction zones?

Oceanic crust is denser than continental crust, so it subducts beneath the less dense continental crust.

What is the main driver of plate motion?

The sinking of cold, dense oceanic crust at subduction zones (slab pull) is the strongest driver.

What is the role of convection currents in plate tectonics?

Convection in the mantle moves heat and helps drive plate motion, but slab pull is the primary force.

What features are found at convergent boundaries?

Oceanic trenches, volcanic arcs, mountain ranges, and earthquakes.

What features are found at divergent boundaries?

Mid-ocean ridges, rift valleys, and new oceanic crust.

What features are found at transform boundaries?

Faults and earthquakes, but typically no volcanoes.

What is the Mid-Atlantic Ridge?

A divergent plate boundary where the Eurasian and North American plates are moving apart, creating new oceanic crust.

How does the age of oceanic crust vary with distance from a mid-ocean ridge?

Oceanic crust is youngest at the ridge and gets older as it moves away.

What is the lithosphere-asthenosphere boundary?

The boundary between the rigid lithosphere and the ductile asthenosphere, based on mechanical properties.

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