Tectonics
🎯 In this topic you will
- Discover how convection currents cause the movement of tectonic plates
- Learn about the evidence we have for tectonic plates
🧠 Key Words
- jigsaw
- continental coasts
- fossil record
- alignment
Show Definitions
- jigsaw: A puzzle made of interlocking pieces, used as an analogy for how continents appear to fit together.
- continental coasts: The edges of continents that show matching shapes, suggesting they were once joined.
- fossil record: The collection of preserved remains or traces of ancient organisms found in rock layers.
- alignment: The orientation of magnetic crystals in rocks, which reveals the direction of Earth’s magnetic field when the rock formed.
🌍 Convection and Plate Movement
You may recall from Stage 7 that the outer layers of the Earth are the solid, rocky crust, which rests on the more fluid mantle. The mantle is heated from the innermost part of the Earth: the inner core. The inner core is estimated to be at a temperature of over 5000 ℃! The high temperature of the inner core is due to thermal energy left over from the formation of the Earth, friction inside the Earth and the type of reactions that happen in the rocks.
♨️ Thermal Convection in the Mantle
You may also recall from Unit 3 that thermal energy is transferred through fluids by convection and that convection currents occur in fluids. The inner part of the mantle gets thermal energy from the core. The fluid in the mantle then expands when heated and becomes less dense than the fluid surrounding it. This hotter, less dense fluid in the mantle rises towards the crust, cools and sinks again, resulting in a convection current.
🧪 Did you know?
Some scientists estimate that convection currents in Earth’s mantle can take up to 500 million years to complete a single cycle — slowly reshaping our planet’s surface over geological time.
🐢 Slow Movement of the Mantle
The mantle is a very thick fluid and does not flow easily like water, so the convection currents move very slowly. As the convection currents in the mantle move across underneath the crust, the tectonic plates that make up the crust are pulled along. Just as the convection currents are slow, the movement of the tectonic plates is also slow, varying between 0.6 and 10 cm per year.
🧩 Continents as a Jigsaw
When you look at a world map, it looks like the continents could fit together like pieces of a giant jigsaw puzzle. A jigsaw puzzle is a picture that is cut into small interlocking pieces. The object of the puzzle is to put the pieces back together to form the original picture.
🌐 Matching Coasts and the Continental Jigsaw
Scientists have done this with the continents on Earth. The continental coasts can fit together as shown in the diagram. Scientists say that this continental jigsaw appearance is evidence for tectonic plates. There is a hypothesis that there was once only one large continent that eventually separated. The separate parts became some of the tectonic plates, and the convection currents from the mantle drove their movement.
🦴 Fossils as Plate Evidence
The fossil record provides more evidence for tectonic plates and their movement. The fossil record is the name given to the collection of thousands of fossils that provide us with information about the time before humans were on Earth. Fossils are the remains of dead animals and plants that have turned to stone over millions of years.
🦎 Mesosaurus Fossils Across Continents
Fossils of an extinct reptile called Mesosaurus have been found in the parts of Africa and South America that would fit together in the jigsaw model of the continents. Mesosaurus lived about 275 million years ago and was similar to a crocodile. These animals lived at the coast and in shallow water. It is not likely that they would have crossed the 5000 km-wide Atlantic Ocean that now separates these continents.
🌿 Glossopteris Fossils Worldwide
300 million-year-old fossils of the plant Glossopteris have been found in Antarctica, India, Australia, Africa and South America. This gives more evidence for the hypothesis that these continents were once joined.
🧭 Earth's Magnetic Field and Poles
You may recall from Stage 8 that the Earth has a magnetic field. Today, the needle of a magnetic compass will point towards north, but that was not always the case. Around 780,000 years ago, the same compass needle would have pointed south! Scientists now know that the Earth’s magnetic poles have swapped positions almost 200 times in the last 100 million years.
🧲 Magnetic Alignment in Rocks
Magnetic crystals in molten rocks line up to point north in the same way as a compass needle. When the rock solidifies, scientists can use these crystals to tell the direction of the Earth’s magnetic field at the time when the rock became solid. The direction that the crystals are pointing is called alignment. The word alignment means to line up in a particular way.
🌊 Seafloor Spreading and Magnetic Clues
The magnetic crystals in rocks found in the middle of the Earth’s oceans have been studied. In these locations, called mid-oceanic ridges, magma is coming up from the mantle and solidifying to form new rocks. This action pushes the continents away from each other. The magnetic crystals in the mid-oceanic ridges always have an alignment to north because the Earth’s magnetic field is currently in that direction. Rocks further away from these ridges contain magnetic crystals with the opposite alignment. This suggests that these rocks are much older.
📌 Important Concept
Evidence for Plate Tectonics: The distribution of identical fossils across continents and the symmetrical alignment of magnetic minerals in oceanic crust both provide strong evidence that Earth's surface is broken into moving tectonic plates.
🌋 Earthquake and Volcano Patterns
If the hypothesis of moving tectonic plates was correct, then we would expect there to be more earthquakes and volcanoes at the tectonic plate boundaries. This is indeed the case, as shown on the map. Most of the volcanoes and earthquakes that have been recorded occur close to the tectonic plate boundaries.
⚠️ Common Mistake
Be careful not to assume that all earthquakes and volcanoes happen only at plate boundaries — some occur within plates due to hotspots or crustal weaknesses.
❓ QUESTIONS
1. Describe what makes convection currents in the Earth’s mantle.
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2. Describe what is meant by the term tectonic plate.
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3. The continents of Earth appear to fit together like a jigsaw. Explain how this provides evidence for tectonic plates.
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4a. Describe how the fossil record provides evidence for the movement of tectonic plates.
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4b. Describe how the alignment of magnetic materials in rocks provides evidence for the movement of tectonic plates.
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5. Scientists think that the Earth’s magnetic field has reversed 183 times in 83 million years. Calculate the average time between reversal events.
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6. Use the map shown above to discuss the statement ‘Earthquakes and volcanoes always occur at tectonic plate boundaries.’