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; the oceans formed first
- No; the ocean floor is older than the continents
- Correct; both are about 4 billion years old
- 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?
- No; ridges do not make new crust
- No; subduction recycles crust at about the same rate
- Correct; Earth grows a little each year
- Correct; that growth is why the ocean sediment is so thin
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 25 cm a year
- About 225 cm a year
- About 7.5 cm a year
- About 2.5 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 3.
- About the same width for both
- More than 15 km versus less than 2.5 km
- More than 150 km versus less than 25 km
- More than 1,500 km versus less than 250 km
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 3.
- About 37,500 km
- About 3,750 km
- About 3.75 km
- About 375 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
- Ocean floor as old as the continents
- Stripes 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 Why do continents keep rock as old as 4 billion years while the ocean floor does not, by Tala's reading (USGS)?
- 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
- The continents formed later than the oceans
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
- Crust forms at trenches and sinks at ridges
- New crust forms at ridges, ages outward, records reversals, and sinks at trenches
- Crust forms at ridges, and stripes record field strength
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?
- Both up to about 180 million years
- Ocean floor up to 4 billion; continents up to 180 million
- Both up to about 4 billion years
- Ocean floor up to about 180 million years; continental rock up to about 4 billion
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
- Count the stripes and call that cm a year
- Multiply the stripe's distance from the ridge by its dated age