Seafloor spreading and why ocean crust is young
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
Read this first
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 A classmate says the ocean floor must be as old as the continents. Which reply fits USGS, by Esme's reading?
- Correct; both are about 4 billion years old
- No; the ocean floor is older than the continents
- Correct; the oceans formed first
- No; ocean floor reaches about 180 million years, continental rock about 4 billion
Conveyor from a ridge to a trench. A student says new crust at ridges makes Earth grow larger. Which reply fits USGS, by Jun's reading?
- Correct; Earth grows a little each year
- No; subduction recycles crust at about the same rate
- Correct; that growth is why the ocean sediment is so thin
- No; ridges do not make new crust
Ridge cross-section with stripes. A student says the stripes formed because the field grew stronger and weaker. Which reply fits USGS, by Kira's reading?
- Correct; changes in the field's strength made the ocean stripes
- No; the stripes come from seawater
- No; Vine, Matthews and Morley tied them to field reversals
- Correct; the stripes formed at random
A stripe boundary 75 km from a ridge marks a reversal 3 million years ago (model data). What is that side's spreading rate, by Luz's math?
- About 2.5 cm a year
- About 7.5 cm a year
- About 25 cm a year
- About 225 cm a year
USGS: the East Pacific Rise spreads more than 15 cm a year and the Arctic Ridge less than 2.5. How much crust does each add in 1 million years, by Milo's math?
Use the picture in question 4.
- More than 150 km versus less than 25 km
- More than 1,500 km versus less than 250 km
- About the same width for both
- More than 15 km versus less than 2.5 km
Four evidence cards, labeled A to D. Which evidence most strongly supports seafloor spreading, by Pia's reading (USGS)?
- Mirror-image stripes and rock ages that rise with distance from the ridge
- Thick sediment piled at the ridge crest
- Ocean floor as old as the continents
- Stripes on one side of the ridge
Model data: for the same span of reversals, stripes are 10 km wide at ridge P and 50 km wide at ridge Q. Which spreads faster, by Ren's reading?
- Ridge P, since narrow stripes mean faster spreading
- Neither; stripes say nothing about rate
- Ridge Q, since wider stripes mean faster spreading
- They spread at the same rate
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 Why do continents keep rock as old as 4 billion years while the ocean floor does not, by Tala's reading (USGS)?
- The continents formed later than the oceans
- Ocean crust is lighter and drifts away
- Light continental crust resists sinking; denser ocean crust subducts
- Ocean crust grows older until it melts back at the ridges where it formed
Conveyor from a ridge to a trench. Which summary of seafloor spreading fits USGS, by Zee's reading?
- Crust forms at ridges, and Earth grows larger
- New crust forms at ridges, ages outward, records reversals, and sinks at trenches
- Crust forms at ridges, and stripes record field strength
- Crust forms at trenches and sinks at ridges
Which summary of how spreading rates are found fits USGS, by Bo's reading?
- Divide a stripe's distance by the dated age of its reversal
- Count the stripes and call that cm a year
- Measure the color of the sediment
- Multiply the stripe's distance from the ridge by its dated age