Study Guide for Sea-floor Spreading / Plate Tectonics:
SEA-FLOOR SPREADING is the theory that developed after continental drift. It helped
explain what Wegener couldn’t explain. If you remember, Wegener’s theory was really good,
but he couldn’t explain how Pangaea broke apart and drifted away, so nobody believed his
theory. Sea-floor spreading supplied the missing answers.
 Scientists discovered underwater volcanoes, when using sonar to “map” the ocean floor.
 Then scientists used a magnetometer to discover that there are “magnetic stripes”
on the ocean floor.
 These “stripes” are found in pairs, one on each side of the mid-ocean ridge. Each
pair of stripes is the same thickness and the same distance from the mid-ocean ridge.
 Scientists collected drilling samples from the ocean floor and noticed that the ocean floor
rock is newer near the mid ocean ridge, and gets older if you move away, no matter which
side of the ocean you are on.
 If the ocean floor was all made at the same time, then you would expect the age to be the
same and the magnetic “charge” to be the same throughout the whole ocean floor. The
fact that the ocean floor has different ages and has different charges suggests that the
ocean floor was made at different times, supporting sea-floor spreading.
 Finally, they sent remote controlled submarines down to these volcanoes and they noticed
lava coming out and almost instantly forming new rock.
QUESTIONS TO CONSIDER:
- How does sonar work? What did it show us?
- How are the magnetic stripes formed? How do they help explain sea-floor spreading?
- How do the drilling samples help explain sea-floor spreading?
- What did scientists discover when they sent the submarine down to the mid-ocean ridge?
RELATED WORDS:
sonar
mid-ocean ridge
magma
convection currents
magnetic reversal drilling samples
divergent boundary
sea-floor spreading
acronym
rift valley
magnetometer
A.L.V.I.N.
CONVECTION CURRENTS: Scientists still needed to answer the question… “what force is
moving the ocean floor apart?”
 By doing observations of other types of liquids and gases, they concluded that clay-like rock
in the asthenosphere was creating a “conveyor belt” under the crust.
QUESTIONS TO CONSIDER:
 How does solid, clay-like rock create a “conveyor belt”? Describe the steps involved.
 How is the Earth like a check stand at the grocery store?
 What do the “wheels” turning in the check stand conveyor represent in the Earth?
 What does the “rubber belt” of the check stand represent in the Earth? The “groceries”?
 What makes these “wheels” spin inside the Earth? Describe this process?
 How does density play a part in this process?
 Why do hot things become less dense?
 How do convection currents help move the continents?
 In what layers of the Earth are these processes taking place?
 Where does the heat come from that drives these forces?
 Describe the two other processes that also help convection currents move the plates?
 What role does gravity play in moving the plates?
 How is slab pull like a long train?
RELATED WORDS:
asthenosphere
lithosphere
conveyor belt
convection currents
quartz
granite
density
core
ridge push
slab pull
mesosphere
continental crust
iron
radioactive decay
gravity
tectonic plates
oceanic crust
basalt
uranium
BOUNDARIES: Once scientists began to look at places in the Earth’s crust that were
spreading apart, they soon realized that there were other places where the Earth’s crust was
moving in different directions.
 The Earth’s crust is broken into huge pieces called “plates”.
 The plates can move apart, they can come together and collide, or they can move
side by side horizontally. This creates three different types of boundaries.
- divergent boundary = sea-floor spreading occurs (@ mid-ocean ridge)
- convergent boundary = volcanoes are formed (on, or near, the edge of continents), or
mountains are formed, also on the edge of continents
- transform boundary = earthquakes formed (also at other two types, though not as often)
QUESTIONS TO CONSIDER:
- What is a boundary?
- Why are there three types of boundaries? What are they?
- What type of boundary produces mountains? volcanoes? earthquakes?
- Why are there three types of CONVERGENT boundaries, when transform and divergent
only have one each? Explain each one.
- How do scientists know where the edges of these plates are located?
RELATED WORDS:
boundary
divergent
earthquakes
ocean trench
Juan deFuca
mountains
slab pull
continental plate
sea-floor spreading
convergent
volcanoes
Mt. St. Helens
Himalayas
satellite GPS
oceanic plate
transforms
subduction
volcanic arc
island arc
ridge push
mid-ocean ridge
San Andreas Fault
density
Andes Mtns
Philippines/Japan
gravity
PLATE TECTONICS is the theory that combines sea-floor spreading with Pangaea, and the
idea that the Earth’s crust is divided into large pieces called plates. Convection currents inside
the crust help push and pull on the crust. Since all of the plates are attached to each other,
movement on one piece moves them all. Since it is only the edges of the plates that actually
interact with each other, the results of plate movement shows up on the edges (boundaries)...
 When they grind together, they produce earthquakes. All types of boundaries have
earthquakes. Usually, though, the earthquakes are smaller at divergent, medium to strong
at transform, and very large at convergent. A good example of a transform boundary is the
San Andreas Fault.
 When one plate goes under another plate, the sinking crust creates pressure and friction.
That creates magma that rises up and makes volcanoes, such as Japan, or Mt. St. Helens.
 Sometimes when they collide together, they push up and create mountains, like when India
and Asia collided and created the Himalayas.
QUESTIONS TO CONSIDER:
- What is the difference between the most recent theory called Plate Tectonics, and the older
theory called Sea-Floor Spreading?
- How come the Earth doesn’t get bigger and bigger if the ocean floor is spreading?
- What is subduction?
- Why are there NO volcanoes found in the middle of continents, only on the edges?
- How does a trench form? What makes it become a “V-shaped” valley?
- What is “slab pull” and how does it help sea-floor spreading?
- What is “ridge push”, and how does it help sea-floor spreading?
- Which piece sinks down into the mantle at a convergent boundary, and WHY?
- What is the difference between an “island arc” and a “volcanic arc”?
- When two continental plates collide, why aren’t there any volcanoes being made?
- How do scientists know where the plate boundaries are?
- When two pieces of oceanic plate collide, how come BOTH pieces don’t sink into the
mantle? What makes one piece “float” on the other?
- What is the difference between an “island arc” and a “volcanic arc”?
- Name / identify the major plates, as well as little Juan de Fuca.
- Identify types of boundaries in different parts of the world.
Please understand that these aren’t the only questions or words that we used. You are
responsible for ALL the material that we have covered:
 from the textbook…
 from the movies we watched…
 from class discussions…
 from the models that I showed you, or the maps and models you made…
 from the worksheets we corrected, discussed, and returned…
 from the computer projector / CD-ROM…
 from my awesome drawings on the overhead (yeah, right)