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)?
- Deep in ocean trenches
- In the middle of the widest ocean plain
- At the edges of the continents
- At the crest of a mid-ocean ridge
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)?
- Random patches of iron
- Changes in the field's strength, not its direction
- The seasons of the year
- Earth's magnetic field has reversed many times
How are the stripes arranged around a ridge, by Milo's reading (USGS)?
- On one side of the ridge
- At random, with no pattern
- Crossing the ridge at right angles
- In matching, mirror-image bands on both sides
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 as old as Earth itself
- About 180 million years
- About 4,600 years
About how old is the oldest known continental rock, by Pia's reading (USGS)?
- About 4 million years
- About 55 million years
- About 180 million years, like the ocean floor
- 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
- The oceans formed long after the continents
- Ocean crust is recycled at convergent boundaries
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
Where does old ocean crust return to the mantle, by Esme's reading (USGS)?
- Nowhere; it keeps building up on the ocean floor
- At hotspots
- At mid-ocean ridges
- At subduction zones under ocean trenches