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; ocean floor reaches about 180 million years, continental rock about 4 billion
- No; the ocean floor is older than the continents
- 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; that growth is why the ocean sediment is so thin
- Correct; Earth grows a little each year
- 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?
- 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
- No; the stripes come from seawater
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 225 cm a year
- About 7.5 cm a year
- About 2.5 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 375 km
- About 3.75 km
- About 37,500 km
- About 3,750 km
Four evidence cards, labeled A to D. Which evidence most strongly supports seafloor spreading, by Pia's reading (USGS)?
- Thick sediment piled at the ridge crest
- 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
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?
- Neither; stripes say nothing about rate
- Ridge P, since narrow stripes mean faster spreading
- They spread at the same rate
- Ridge Q, since wider stripes mean faster spreading
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
- The continents formed later than the oceans
- Ocean crust grows older until it melts back at the ridges where it formed
- 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 trenches and sinks at ridges
- Crust forms at ridges, and stripes record field strength
- New crust forms at ridges, ages outward, records reversals, and sinks at trenches
- Crust forms at ridges, and Earth grows larger
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
- Both up to about 4 billion years
- Both up to about 180 million years
- Ocean floor up to about 180 million years; continental rock up to about 4 billion