Download Lesson 2 Worksheet - OG

Lesson 2 Worksheet
Seafloor Spreading
What is seafloor spreading?
1. What forms basalt?
Lava that cools and solidifies
2. Which type of rock forms oceanic crust?
Basalt
3. What happens to old oceanic crust as new
oceanic crust forms?
New crust pushes old crust away from the ridge
4. What happens to the density of rock as it
cools
It becomes more dense
5. Where does the crust move as it becomes
denser?
Away from the mid-ocean ridge
6. Which force draws the cooler, denser crust
downward and away from the ridge?
Gravity
7. What is formed when lava cools and
crystallizes on top of the oceanic crust?
Basalt (sediment)
8. Where is seafloor sediment thickest?
Far away from mid-ocean ridge
9. What shape does the seafloor take where
sediment is the thickest?
Smooth plain
10. What is this area of the seafloor called?
Abyssal plain
11. What is the ocean crust always doing?
Spreading
12. What happens as the ocean crust spreads?
Continents move!
13. What does the crust record as it cools?
Direction of Earth’s magnetic field
14. What does basalt contain that makes this
possible?
Iron-rich minerals
15. What forms when Earth’s magnetic field
changes direction?
Magnetic stripes on ocean floor
16. What do magnetic stripes confirm?
Ocean crust is made at mid-ocean ridge and is
carried away from ridges as magma flows
upward and out as lava
What evidence is used to support
seafloor spreading?
1. What does normal polarity mean?
It means that magnets point north.
2. What does reversed polarity mean?
Changes direction and magnets point south
3. How can scientists tell when changes occur in
Earth’s magnetic field?
Minerals give a magnetic signature that will
point either north or south.
4. How often do magnetic reversals occur?
Every few hundred thousand (100,000) years to
a few million (1,000,000) years
5. What do Earth’s magnetic stripes indicate
about ocean crust?
New ocean crust is found at mid-ocean ridges
and older crust is further away.
6. How do Earth’s magnetic signatures support
the idea that continents move?
It shows that crust is moving and the seafloor is
spreading.