Year 8 Science | Victorian Curriculum 2.0
Plate tectonics
Topic 06 | Earth and space sciences | Answer key

Year 8 answers

Fluency

Structure and plates

    1. Crust, mantle, outer core, inner core.
    2. The crust plus the rigid upper part of the mantle.
    3. A few centimetres per year (roughly 2-10 cm/year).
    4. Any two of: Pacific, Eurasian, North American, South American, African, Indo-Australian, Antarctic, Nazca.
    5. Continents (up to ∼70\sim 70 km); oceanic crust is only ∼5\sim 5-10 km thick.
Fluency

Boundaries and features

    1. Divergent, convergent, transform.
    2. Divergent.
    3. Transform.
    4. A mountain range (e.g. the Himalayas).
    5. Subduction is when one plate (usually a denser oceanic plate) dives beneath another plate into the mantle at a convergent boundary.
Reasoning

Explain

    1. At boundaries the plates either push into, pull away from, or slide past each other. All three movements generate stress in the rocks or let magma reach the surface, causing earthquakes and/or volcanic eruptions. Mid-plate regions are relatively stable.
    2. Australia is near the middle of the Indo-Australian Plate, far from the grinding boundaries. New Zealand sits directly on the boundary between the Pacific and Indo-Australian plates, so it gets frequent earthquakes and volcanic activity.
    3. A hotspot is a fixed column of hot mantle that pokes through the moving plate above. As the plate drifts, the hotspot punches new volcanoes through it in a line, leaving older volcanoes behind as a chain.
    4. New seafloor forms at the Mid-Atlantic Ridge (a divergent boundary). As new crust forms and spreads, the ocean floor either side gets pushed apart, widening the Atlantic by ∼2\sim 2 cm/year.
Problem solving

Applied contexts

    1. 1010 cm/year ×106\times 10^6 years =107= 10^7 cm =105= 10^5 m =100= 100 km.
    2. Frequent earthquakes (plate grinding), tsunamis (seafloor earthquakes displace water), and explosive volcanoes (magma rising from the subducting plate).
    3. Active volcanoes, hot springs, geysers, frequent small earthquakes, and new land forming as lava cools.
    4. The species could not cross today’s Atlantic Ocean, so the two continents must have been joined 200 million years ago and have since drifted apart — direct evidence of continental drift.
Reasoning

Challenge

    1. In the 1950s and 60s, mapping of the ocean floor revealed mid-ocean ridges, and palaeomagnetic stripes showed symmetric patterns of reversed/normal magnetism on either side of the ridges. This new evidence only made sense if the seafloor was forming and spreading. Science progresses when decisive new data arrive — theories are revised or accepted when the evidence is strong enough, not just because the idea is old.
    2. Transform boundaries involve plates sliding past each other close to the surface, so fault movement (and the earthquakes it causes) occurs at shallow depth. Convergent boundaries involve one plate diving deep into the mantle, so earthquakes along the subducting slab can be hundreds of km deep.
    3. The oceanic Nazca Plate subducts beneath the South American Plate. As the plate dives, water and minerals in it release, melting the mantle above. Magma rises and breaks through the crust well inland from the trench, forming the volcanoes of the Andes.
    4. The Pacific Plate will continue moving northwest, carrying the Big Island away from the hotspot. The Big Island’s volcanoes will go extinct and erosion will shrink it, while a new volcano rising over the hotspot (Loihi, already building south-east) will eventually breach the surface as a new island.
Year 8 Science study companion | Answer key