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 older on both sides
- It gets younger on both sides
- Older on one side, younger on the other
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?
- They are the same age
- A, at the ridge
- C, farthest from the ridge
- B, in the middle
Use this
Ridge cross-section with stripes.
What do alternating stripes of normal and reversed polarity show, by Luz's reading (USGS)?
- Random patches of iron
- Earth's magnetic field has reversed many times
- The seasons of the year
- Changes in the field's strength, not its direction
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 180 million years
- About as old as Earth itself
About how old is the oldest known continental rock, by Pia's reading (USGS)?
- About 4 million years
- About 180 million years, like the ocean floor
- About 4 billion 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)?
- The oceans formed long after the continents
- Ocean crust is recycled at convergent boundaries
- New crust keeps being added, so Earth grows larger over time
- Ocean crust wears away in waves
New crust forms at ridges every year. Why doesn't Earth get bigger, by Zee's reading (USGS)?
- The extra crust drifts off into space
- New crust is not really being made
- Old crust sinks at trenches at about the same rate
- Earth is slowly expanding
Where does old ocean crust return to the mantle, by Esme's reading (USGS)?
- At hotspots
- Nowhere; it keeps building up on the ocean floor
- At mid-ocean ridges
- At subduction zones under ocean trenches