Heat inside Earth and mantle convection

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Card (example data)

   The blob starts at 3.30 g/cm³ in rock of 3.40 g/cm³. Near the plates it cools to 3.45 g/cm³ and the rock around it is 3.40 g/cm³.

2. 1.

   Bo follows a blob of hot mantle rock using the card. What path does the blob take, and why?

   • It stays put, since 3.30 and 3.45 are both near 3.40 and nothing pushes it
   • It rises while lighter than its surroundings, then sinks once cooling makes it denser
   • It sinks first because it holds more heat, then rises once it reaches the plates
   • It rises the whole way, since heat adds buoyancy and cooling cannot remove it

3. Card

   USGS says convection cannot take place without a source of heat. In the model, temperatures slowly even out once heating stops.

4. 2.

   In a lab model, Esme turns off the heater under a fluid layer that is cooled at the top. Using USGS, what happens to the flow over time?

   • It reverses, with cold fluid rising
   • It keeps going, since density differences last forever
   • It speeds up as the fluid cools
   • It slows and stops as temperatures even out

5. Card (example data)

   New sea floor forms at a ridge and moves 4 cm a year to a trench 6,000 km away. 1 km = 100,000 cm.

6. 3.

   Kira models a convection cell carrying sea floor from a ridge to a trench. About how long does the trip take?

   • About 150 million years
   • About 1,500 million years
   • About 15 million years
   • About 1,500 years

7. Sea-floor age (example data)

   Distance from ridge	Age
   0 km	0 million years
   500 km	20 million years
   1,500 km	60 million years

8. 4.

   The table shows sea-floor ages at two spots. Luz finds the rate, then predicts the age 3,000 km from the ridge. What are the rate and age? (1 km = 100,000 cm.)

   • 2.5 cm a year; about 80 million years old
   • 2.5 cm a year; about 120 million years old
   • 25 cm a year; about 12 million years old
   • 0.25 cm a year; about 1,200 million years old

9. Card

   Seismic waves travel faster through cold rock than hot rock. A scan shows a fast zone running from a trench down to about 700 km.

10. 5.

    Milo reads a seismic scan. What does the fast zone most likely show?

    • A pocket of liquid rock under the trench
    • A plume of hot rock rising toward the plates
    • A cold, dense slab sinking into the mantle
    • A place where convection has stopped moving rock

11. Plate speeds (example data)

    Plate	Subducting edge	Speed
    Plate P	12,000 km	8 cm a year
    Plate Q	5,000 km	5 cm a year
    Plate R	0 km	2 cm a year

12. 6.

    Noah compares plates in the table. Which claim does the pattern support best?

    • More subducting edge goes with faster plates, which fits slab pull
    • Speed is unrelated to the edge, since Plate R still moves
    • The data show slab pull is the sole force on plates
    • Longer ridges speed plates up, which shows ridge push is the main force

13. Card (example data)

    Plate Z has no sinking slab at any edge and is bordered by ridges. It still moves about 1 cm a year.

14. 7.

    Pia reads the card about Plate Z. What does it show about the forces on plates?

    • Convection cannot exist under Plate Z, so nothing can move it
    • Plate Z is not really moving, since 1 cm a year is too small to count as motion
    • Slab pull still moves it, through a hidden slab on one of its far edges
    • Slab pull cannot be moving Plate Z, so ridge push or other forces must act

15. Card

    Model 1 has cells only above about 700 km. Model 2 has cells through the whole mantle. USGS: the number and structure of cells are unanswered.

16. 8.

    Which observation would best help Ren decide between Model 1 and Model 2?

    • Measurements of surface plate speeds, which both of the models can match
    • Direct drilling into the lower mantle to see the cells
    • Seismic images showing whether cold slabs sink below 700 km
    • The age of the oldest rocks found on any of the continents
17. 9.

    Each side of a ridge moves away from it at 2.5 cm a year. 1 km = 100,000 cm. At a ridge, each side moves away at 2.5 cm a year. By how much does the ocean widen in 20 million years, by Tala's math?

    • About 1,000 km
    • About 10,000 km
    • About 500 km
    • About 100 km
18. 10.

    a plate moves 25 mm a year. 1 km = 1,000,000 mm. A plate moves 25 mm a year. How far does it go in 40 million years, by Ari's math?

    • About 1,000 km
    • About 100 km
    • About 1.6 km
    • About 10,000 km