GEOL 101
Outline 15: Plate Motions & The “Big Picture”
1. Paleomagnetism - continued
 What is a magnetic reversal?
 Normal versus reverse polarity
 Magnetic polarity time scale
 How do magnetic reversals lead to the pattern of magnetic anomalies we see on the sea
floor? The seafloor as a “magnetic tape recorder”.
 How do these anomalies allow us to determine the rate of seafloor spreading?
 Recognition of the implications of seafloor magnetic anomalies by Vine & Mathews
(1963) – the clinching piece of evidence for plate tectonics.
2. How can we determine absolute rates and directions of plate movement?
 GPS
 Hot spots – represent fixed points over which the plates move
 Age pattern of Hawaiian Islands
3. What makes the plates go?
 Mantle convection – hot mantle is rising beneath divergent boundaries; “cold” mantle
sinks beneath subduction zones
 Slab pull. Subducted oceanic plate (slab) is denser than the mantle; actually observed to
break off and sink in some cases. Idea is that subducting slab pulls the plate behind it.
Deeper it goes the denser it gets, the more it pulls.
4. Tying It All Together. Movement of the continents, growth and destruction of ocean
basins, volcanic activity, earthquakes, deformation, metamorphism, and mountain building are
all part of one big picture.
 We can begin with the rifting apart of a continent (or supercontinent)
 Rifting is driven by heat from below (and hot spots?) Example: E. Africa.
 Opening rifts are divergent boundaries beneath which hot mantle is rising.
 Results in decompression melting, production of basalt magmas; creation of new oceanic
crust at a mid-ocean ridge (MOR).
 Why do we have MOR’s? Not because plate is thicker – it’s just hotter. As plate moves
away from ridge it cools, contracts, and MOR disappears.
 Formation of new subduction zones – why does this happen?
 Subduction zone is marked by a trench. What happens in a subduction zone? Granitic
magmas formed at subduction zones – this is one way continents grow.
 Once subduction begins the ocean basin starts closing; this will eventually lead to a
continent-continent collision – fold and thrust mountains, regional metmorphism.
 Terrane accretion. “Smaller” collisions involving chains of islands, continental
fragments (Cuba), etc. These cannot be subducted and instead wind up accreted to the
edge of the continent. This is other main way that continents grow larger.
 Eventually subduction leads to continent-continent collision; formation of a
supercontinent, and the cycle begins again.
5. Wilson Cycle.
Question from Chapter 20 that we won’t cover in lecture:
1. What do we call the ancient stable nucleus of a continent? Where would you look in North
America to find these oldest rocks?
Tying it together questions. The following questions are intended to help you tie together
different topics that we have covered. They are based mostly on your notes although the
material is also in the book.
1. Some chains of volcanic islands are related to subduction (volcanic island arcs) and some are
related to hot spots. List as many ways as you can by which you could tell the two apart. You
might think about patterns or types of geologic activity, ages of the islands, etc.
2. What evidence could you use to demonstrate that an exotic terrane such as “Wrangellia” (see
p. 466-467) originally formed thousands of kilometers farther south than it is today?