Seafloor spreading and why ocean crust is young

Grade 9 Science Worksheet - Building Confidence

Circle the best answer.

1. Read this first

   Use this

   Ridge cross-section with stripes.

2. 1.

   Where is the youngest ocean-floor rock, by Ari's reading (USGS)?

   • In the middle of the widest ocean plain
   • Deep in ocean trenches
   • At the crest of a mid-ocean ridge
   • At the edges of the continents
3. 2.

   Moving away from a ridge on either side, how does rock age change, by Bo's reading (USGS)?

   • Older on one side, younger on the other
   • It gets older on both sides
   • It stays the same age
   • It gets younger on both sides

4. Rock samples (model data)

   Sample	Distance from ridge
   A	0 km
   B	100 km
   C	200 km

5. 3.

   Samples A, B and C sit at different distances from a ridge. Which is oldest, by Kira's reading?

   • They are the same age
   • B, in the middle
   • A, at the ridge
   • C, farthest from the ridge

6. Use this

   Ridge cross-section with stripes.

7. 4.

   What records the magnetic stripes in ocean-floor basalt, by Jun's reading (USGS)?

   • Seawater soaking into the rock
   • Grains that keep turning to match today's field
   • Sand grains laid down by currents
   • Magnetite grains that lock in the field's direction as the rock cools
8. 5.

   What do alternating stripes of normal and reversed polarity show, by Luz's reading (USGS)?

   • The seasons of the year
   • Random patches of iron
   • Changes in the field's strength, not its direction
   • Earth's magnetic field has reversed many times
9. 6.

   How are the stripes arranged around a ridge, by Milo's reading (USGS)?

   • At random, with no pattern
   • Crossing the ridge at right angles
   • In matching, mirror-image bands on both sides
   • On one side of the ridge

10. Crust ages (USGS)

    Crust	Age
    average ocean crust	about 55 million years
    average continental crust	about 2.3 billion years
    oldest ocean floor	about 180 million years
    oldest continental rock	about 4.03 billion years

11. 7.

    About how old is the oldest known continental rock, by Pia's reading (USGS)?

    • About 55 million years
    • About 180 million years, like the ocean floor
    • About 4 million years
    • About 4 billion years

12. Use this

    Conveyor from a ridge to a trench.

13. 8.

    Why is ocean crust so much younger than continental crust, by Tala's reading (USGS)?

    • The oceans formed long after the continents
    • New crust keeps being added, so Earth grows larger over time
    • Ocean crust is recycled at convergent boundaries
    • Ocean crust wears away in waves
14. 9.

    New crust forms at ridges every year. Why doesn't Earth get bigger, by Zee's reading (USGS)?

    • New crust is not really being made
    • Old crust sinks at trenches at about the same rate
    • The extra crust drifts off into space
    • Earth is slowly expanding
15. 10.

    Where does old ocean crust return to the mantle, by Esme's reading (USGS)?

    • At subduction zones under ocean trenches
    • Nowhere; it keeps building up on the ocean floor
    • At hotspots
    • At mid-ocean ridges