Seafloor spreading and why ocean crust is young
Grade 9 Science Worksheet - Building Confidence
Circle the best answer.
Read this first
Use this
Ridge cross-section with stripes.
Where is the youngest ocean-floor rock, by Ari's reading (USGS)?
- In the middle of the widest ocean plain
- At the edges of the continents
- At the crest of a mid-ocean ridge
- Deep in ocean trenches
Moving away from a ridge on either side, how does rock age change, by Bo's reading (USGS)?
- It stays the same age
- It gets younger on both sides
- Older on one side, younger on the other
- It gets older on both sides
Rock samples (model data)
Sample Distance from ridge A 0 km B 100 km C 200 km Samples A, B and C sit at different distances from a ridge. Which is oldest, by Kira's reading?
- C, farthest from the ridge
- B, in the middle
- A, at the ridge
- They are the same age
Use this
Ridge cross-section with stripes.
What records the magnetic stripes in ocean-floor basalt, by Jun's reading (USGS)?
- Sand grains laid down by currents
- Grains that keep turning to match today's field
- Seawater soaking into the rock
- Magnetite grains that lock in the field's direction as the rock cools
What do alternating stripes of normal and reversed polarity show, by Luz's reading (USGS)?
- The seasons of the year
- Changes in the field's strength, not its direction
- Earth's magnetic field has reversed many times
- Random patches of iron
How are the stripes arranged around a ridge, by Milo's reading (USGS)?
- Crossing the ridge at right angles
- At random, with no pattern
- In matching, mirror-image bands on both sides
- On one side of the ridge
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 About how old is the oldest ocean floor, by Noah's reading (USGS)?
- About 4 billion years
- About 4,600 years
- About as old as Earth itself
- About 180 million years
About how old is the oldest known continental rock, by Pia's reading (USGS)?
- About 4 billion years
- About 180 million years, like the ocean floor
- About 4 million years
- About 55 million years
Use this
Conveyor from a ridge to a trench.
Why is ocean crust so much younger than continental crust, by Tala's reading (USGS)?
- Ocean crust wears away in waves
- Ocean crust is recycled at convergent boundaries
- The oceans formed long after the continents
- New crust keeps being added, so Earth grows larger over time
Where does old ocean crust return to the mantle, by Esme's reading (USGS)?
- At mid-ocean ridges
- At hotspots
- Nowhere; it keeps building up on the ocean floor
- At subduction zones under ocean trenches