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)?
- At the crest of a mid-ocean ridge
- Deep in ocean trenches
- At the edges of the continents
- In the middle of the widest ocean plain
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
What records the magnetic stripes in ocean-floor basalt, by Jun's reading (USGS)?
- Grains that keep turning to match today's field
- Magnetite grains that lock in the field's direction as the rock cools
- Sand grains laid down by currents
- Seawater soaking into the rock
What do alternating stripes of normal and reversed polarity show, by Luz's reading (USGS)?
- The seasons of the year
- Random patches of iron
- Earth's magnetic field has reversed many times
- Changes in the field's strength, not its direction
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
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 as old as Earth itself
- About 180 million years
- About 4 billion years
About how old is the oldest known continental rock, by Pia's reading (USGS)?
- About 4 million years
- About 4 billion years
- About 55 million years
- About 180 million years, like the ocean floor
Which is older on average, ocean crust or continental crust, by Ren's reading (USGS)?
- Ocean crust, by far
- They are about equal
- Ocean crust, by about 2 billion years
- Continental crust, by far
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 wears away in waves
- New crust keeps being added, so Earth grows larger over time
- 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
- The extra crust drifts off into space
- Old crust sinks at trenches at about the same rate
- Earth is slowly expanding