Earth’s Interior and Plate Tectonics
Honors Physical Science
1. You are responsible for the following terms: crust, mantle, core, lithosphere, asthenosphere, plate tectonics, magma, lava,
subduction, fault, Mohorivičić discontinuity (Moho), convergent boundary, divergent boundary, convection, magnetic
stratigraphy, paleomagnetism.
Mohorivičić discontinuity (Moho) – The boundary between the crust and the mantle, located at an average depth of 8km
under oceans and 32 km under continents. It is marked where there is a big change in the velocity of seismic waves.
The waves travel much slower in the crust (~7 km/s) than in the mantle (~8 km/s).
Magnetic stratigraphy – the study of the magnetic orientations in the oceanic crust.
Paleomagnetism – the fixed orientation of a rock’s magnetic minerals as originally aligned at the time of the rock’s
formation.
2. Draw a cross-section diagram of the Earth. Include and label all of the following features: inner core, outer core, mantle,
asthenosphere, lithosphere, Mohorivičić discontinuity (Moho), oceanic crust, continental crust. For the different layers
indicate the thickness and temperature for each.
3. Intense pressure is one reason for the high temperature in the core. What is another major factor?
Nuclear decay inside the Earth produces a significant amount of heat energy.
4. When and by who was plate tectonic theory first proposed?
Alfred Wegener first proposed the theory of continental drift in 1912.
5. List and describe four pieces of evidence that support plate tectonic theory. (both resources)
1. Fossils – fossil distributions are continuous when crustal plates are put together.
2. Magnetic Stratigraphy – Parallel bands of oceanic crust have alternating magnetic orientations caused by the reversal of
the Earth’s magnetic poles. The magnetic orientation is solidified into the rock during the cooling of lava into basalt.
The parallel bands are the same on both sides of the mid-ocean ridges suggesting that each band was formed at the same
time and has moved away from the ridge.
3. Glacial evidence – evidence of glacial movement from the same period found in S. America, Australia, etc.
4. Jigsaw puzzle pieces of continents – the western coast of Africa is very similar in shape to the eastern coast of S. America.
5. Stratigraphic sequences – similar rock layers of the same age in S. America, S. Africa, and Antarctica.
6. Sea floor spreading – direct observation of mid-ocean ridge/divergent boundary
7. GPS – the actual distance of plate/crust motion can be measured accurately.
6. Use the maps provided to describe fossil and glacial evidence for plate tectonic theory.
Fossils –Cynognathus and Mesosaurus both have a continuous distribution from S. America into Africa when the two
continents are put together. Lystrosaurus and Glossopteris fossils span more than one of the continents when put
together. Glossopteris offers an added piece of evidence in that its seeds are much too large to have been blown across
the Atlantic Ocean.
Glacial – Glacial scarring and lithographic evidence indicate that a glacier was present at the same time on several continents.
Continental glaciers form on continents so there must have been a continent at the South Pole at the time of the glacial
accumulation and motion.
7. How is Earth’s magnetic field produced? (ESH p. 158-159; Earth’s Magnetic Field document)
The truth is that no one knows exactly how it works. It is presumed that the rotation of the Earth affects the liquid outer core
in such a way as to produce currents that generate a magnetic field.
8. How are iron bearing rocks magnetized by Earth’s magnetic field? (ESH p158-159)
Magma coming to the surface at mid-ocean ridges has a high iron content. While the magma is liquid, the iron has not
definite orientation. As the lava begins to cool on the surface (under the water), the Earth’s magnetic field aligns the iron
atoms in one direction. The solidified rock then has all (ok, almost all) of its iron atoms aligned with the direction of the
Earth’s magnetic field.
9. What magnetic striping (paleomagnetic banding)? How is it used to support plate tectonic theory? (both; ESH p. 219-220)
Based on the known fact of the flipping of the Earth’s magnetic field, it was found that the oceanic crustal rocks on both sides
of a mid-ocean ridge have similar magnetic orientations. These orientations are the same on both sides of the rift. The
resulting “stripes” can then be used to show that sea floor spreading is indeed happening and with an assumption made
about relatively constant growth rates, the oceanic crust can be dated. This evidence supports PTT by showing that crust
is being formed and is moving.
10. What is convection? How can convection explain crustal plate motion?
Convection is the transfer of heat energy by the motion of fluids with different temperatures. The currents formed by the
motion of the fluid are called convection currents. It is believed that convection currents within the mantle are one of the
driving factors involved in moving crustal plates. As high temperature mantle material rises it displaces the mantle
material below the crust. It displaces the material in a lateral direction carrying crustal plates along with it. An analogy
can be drawn between this and wooden block being moved on the surface of water that is being heated from below.
11. What are the three forces thought to move the crustal plates? (both; ESH p. 226)
1. Convective motion in the asthenosphere applies drag to the base of a plate.
2. The ascent of magma at a spreading zone pushes the lithosphere upward, and the weight of the elevated ridge then causes
the lithosphere to spread laterally in both directions, pushing the plate on each side ahead of it.
3. At the other end of a plate, the cold, relatively dense slab sinks into the hot asthenosphere, dragging the rest of the plate
toward the subduction zone.
12. Distinguish between divergent and convergent plate boundaries.
Divergent boundaries occur where two pieces of crust are moving apart from each other. Most commonly, these occur
between pieces of oceanic crust. Convergent boundaries occur where two pieces of crust collide.
13. Describe why volcanoes can be present at both convergent and divergent plate boundaries. (both; ESH p. 18)
At divergent boundaries at mid-ocean ridges, magma comes to the surface and solidifies into crust. At convergent boundaries
where there is subduction, portions of the subducted plate and the material that it carries with it melt and rise back to the
surface. This magma can be released onto the surface from a volcano.
14. Describe what happens at a divergent boundary. Include the terms rift valley and mid-ocean ridge in the discussion.
A divergent boundary at a mid-ocean ridge is like a very long volcanic line. New crust is continuously being formed and
moved away (very slowly of course). The rising magma, coming from the mantle, pushes up the surrounding area,
making the ridge much higher (1-3 km) than the average depth of the surrounding ocean floor. The ridge is split along a
line that releases the magma; this split is called the rift valley.
15. Describe what happens at a convergent boundary between continental crust and oceanic crust. Use the following terms:
subduction, trench, volcano, earthquake, mountains, melting.
When a piece of continental crust collides with a piece of oceanic crust the higher density oceanic crust gets forced under the
continental crust is a process called subduction. In the region of subduction, a trench is formed; this is a deep region of
the ocean floor where the oceanic crust is pushed down. The subducted plate grinds along below the overriding
continental plate creating lots of earthquakes. The melting of the subducted plate can form volcanoes on the surface of
the overriding plate.
16. Describe what happens at a convergent boundary between continental crust and continental crust. Use the following
terms: subduction, volcano, earthquake, mountains, melting.
When two pieces of continental plate collide, there is no subduction and plate melting, and therefore probably no volcanoes.
There are lots of earthquakes as the pieces of crust are crushed, twisted and forced upward. The collision forms
mountain ranges like the Himalayas and the Alps.
17. Describe a transform fault. Give an example of a transform fault in continental crust and in oceanic crust.
When pieces of the crust move horizontally past each other, we call the region of the contact a transform fault. The San
Andreas is one of the most famous transform faults.
18. Section Review questions 1, 2, 3, 4, 6, 7 on page 566.
1. The inner core is under much higher pressure than the outer core, and even though it is at a much higher temperature, the
inner core remains solid.
2. a) transform plate boundary
b) convergent boundary
c) divergent boundary
3. Magma from the mantle moves up into the voided area.
4. a) divergent
b) convergent
c) divergent
d) convergent
6. Convergent boundary where subduction is occurring.
7. The sea floor is constantly being formed and subducted. Continents are not subducted.