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 gets younger on both sides
- Older on one side, younger on the other
- It stays the same age
- 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?
- B, in the middle
- A, at the ridge
- C, farthest from 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)?
- Seawater soaking into the rock
- Magnetite grains that lock in the field's direction as the rock cools
- Sand grains laid down by currents
- Grains that keep turning to match today's field
What do alternating stripes of normal and reversed polarity show, by Luz's reading (USGS)?
- Changes in the field's strength, not its direction
- Random patches of iron
- Earth's magnetic field has reversed many times
- The seasons of the year
How are the stripes arranged around a ridge, by Milo's reading (USGS)?
- In matching, mirror-image bands on both sides
- Crossing the ridge at right angles
- At random, with no pattern
- 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 4,600 years
- About 180 million years
- About as old as Earth itself
- About 4 billion years
Which is older on average, ocean crust or continental crust, by Ren's reading (USGS)?
- Ocean crust, by about 2 billion years
- Ocean crust, by far
- Continental crust, by far
- They are about equal
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
- New crust keeps being added, so Earth grows larger over time
- Ocean crust is recycled at convergent boundaries
- The oceans formed long after the continents
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
- Earth is slowly expanding
- The extra crust drifts off into space
- Old crust sinks at trenches at about the same rate