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?
- No; ocean floor reaches about 180 million years, continental rock about 4 billion
- No; the ocean floor is older than the continents
- Correct; both are about 4 billion years old
- Correct; the oceans formed first
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?
- No; subduction recycles crust at about the same rate
- Correct; Earth grows a little each year
- No; ridges do not make new crust
- Correct; that growth is why the ocean sediment is so thin
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?
- No; the stripes come from seawater
- No; Vine, Matthews and Morley tied them to field reversals
- Correct; changes in the field's strength made the ocean stripes
- 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 225 cm a year
- About 25 cm a year
USGS: the Atlantic grew over the past 100 to 200 million years. At an example 2.5 cm a year, how wide would 150 million years of spreading make it, by Noah's math?
Use the picture in question 4.
- About 3,750 km
- About 375 km
- About 3.75 km
- About 37,500 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
- Stripes on one side of the ridge
- Ocean floor as old as the continents
- Thick sediment piled at the ridge crest
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)?
- Light continental crust resists sinking; denser ocean crust subducts
- Ocean crust grows older until it melts back at the ridges where it formed
- The continents formed later than the oceans
- Ocean crust is lighter and drifts away
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
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 Which summary of crust ages fits USGS, by Ari's reading?
- Ocean floor up to 4 billion; continents up to 180 million
- Ocean floor up to about 180 million years; continental rock up to about 4 billion
- Both up to about 180 million years
- Both up to about 4 billion years
Which summary of how spreading rates are found fits USGS, by Bo's reading?
- Measure the color of the sediment
- Divide a stripe's distance by the dated age of its reversal
- Multiply the stripe's distance from the ridge by its dated age
- Count the stripes and call that cm a year