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Unit 6: Sound and space

Tectonics — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 14 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Tectonics — HIA INTERNATIONAL notes: Explain how convection currents move tectonic plates and the evidence for plate movement.Core heat — the inner core is over 5000 °C because of thermal energy left from the formation of the Earth, friction and reactions in rocks.Mantle — the in…

  • Slow movement
  • Hypothesis
  • Fossil record
  • Mesosaurus
  • Glossopteris
  • Magnetic crystals

Section 1 of 9Why plates move

Core heat
  • Core heat the inner core is over 5000 °C because of thermal energy left from the formation of the Earth, friction and reactions in rocks.
Mantle
  • Mantle the inner part of the mantle receives thermal energy from the core.
Convection current
  • Convection current heated mantle fluid expands, becomes less dense, rises towards the crust, cools and sinks again.
Slow movement
  • Slow movement the mantle is very thick and flows slowly.
Dragging
  • Dragging the currents pull the tectonic plates along at 0.6 to 10 cm a year.

Section 2 of 9Evidence: the jigsaw of continents

Fit
  • Fit the coasts of Africa, South America, Antarctica, India and Australia fit together like a jigsaw.
Hypothesis
  • Hypothesis there was once one large continent that separated; its pieces became some of the tectonic plates.
Pangaea
  • Pangaea the supercontinent that existed between 335 and 175 million years ago and started to break apart 175 million years ago.

Section 3 of 9Evidence: the fossil record

Fossil record
Fossil record the collection of thousands of fossils that provide information about times before humans.
Mesosaurus
  • Mesosaurus fossils of this reptile, about 275 million years old, are found in both Africa and South America,
    • and it could not have crossed the 5000 km Atlantic.
Glossopteris
  • 300 million-year-old fossils of this plant are found in Antarctica, India, Australia, Africa and South America.

Section 4 of 9Evidence: magnetism in rocks

Reversals
  • Reversals the Earth’s magnetic poles have swapped almost 200 times in the last 100 million years.
Magnetic crystals
  • Magnetic crystals in molten rock they line up with the Earth’s magnetic field, and keep that alignment when the rock solidifies.
Mid-oceanic ridges
  • Mid-oceanic ridges new rock forms from rising magma and its crystals point north today.
Older rocks
  • Older rocks rocks further from the ridges have the opposite alignment, showing they are older.
Sea-floor spreading
  • Sea-floor spreading new rock pushes continents apart.

Section 5 of 9Evidence: earthquakes and volcanoes

Location
  • Location most volcanoes and earthquakes are close to plate boundaries, as predicted.
Pacific Ring of Fire
  • Pacific Ring of Fire the best-known zone of volcanoes and earthquakes.
Not only boundaries
  • Not only boundaries some occur away from boundaries, so “always” is not correct.

Section 6 of 9Boundaries

BoundaryPlate movementResult
Divergent (constructive)Move apartMagma rises and forms new crust, e.g. Mid-Atlantic Ridge
Convergent (destructive)Move togetherSubduction, volcanoes, earthquakes or fold mountains such as the Himalayas
Transform (conservative)Slide past each otherEarthquakes, e.g. San Andreas Fault

Section 7 of 9Wegener and acceptance

Wegener (1912)
  • Wegener (1912) proposed continental drift, rejected at first because he could not say what moved the continents.
1960s
  • 1960s evidence from magnetic stripes in ocean rocks led to acceptance.
Speed
  • Speed at 2.5 cm a year a plate moves 25 km in a million years.

Section 8 of 9Key terms

TermMeaning
Tectonic plateA large piece of the Earth’s crust that moves slowly.
Convection currentThe circulation of heated fluid in the mantle.
Fossil recordThe collection of fossils giving information about the past.
Mid-oceanic ridgeA place where magma rises and forms new rock on the ocean floor.
AlignmentLining up in a particular direction.
PangaeaThe supercontinent that broke apart.

Section 9 of 9Exam technique

1
Plates float on the core — convection currents in the mantle move them.
2
Plates move quickly — a few centimetres a year.
3
Earthquakes and volcanoes occur only at boundaries — most do, but not all.
4
Wegener was rejected because he was wrong about the evidence — he could not explain the cause.

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