EARTH 2 Lab 

Lab 2: Plate Tectonics

Purpose of the lab:

• Walk in the intellectual footsteps of the scientists who first discovered plate tectonics

• Learn about relative plate motions on the Earth

• Appreciate various aspects of oceanic crustal formation at mid-ocean ridges

• Consider the pattern of volcanoes and earthquakes at subduction zones

• Use hot spot tracks to clock the speed of the plates

PART 1: Apparent polar wander – using magnetic Inclination

One of the first indications that the Earth’s surface was not static was the observation that the magnetic poles, as measured by the magnetic fields frozen into rocks, appeared to move. Of course, we now understand that the apparent motion is in fact due to the motions of the plates themselves. This question will illustrate how we can use these data to map the motion of the plates through time.

A. What is the difference between magnetic inclination and magnetic declination?

_________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________ [3]

A simple equation relates the magnetic field inclination (I) we measure at the surface to the latitude (λ):

tan I = 2 tan λ

Which can be shown in graphical form. to the left.

A. What latitude are we at here in Santa Barbara?

_________________________________ [1]

B. Using the equation or graph above, compute the inclination in SB today: _________________________˚ [2]

C. If you were to make a new rock in Santa Barbara today, freeze in the inclination, and then transport that rock (keeping it perfectly horizontal so that the inclination doesn’t change!) down to Oaxaca, Mexico (Latitude = 17˚), by how much would the frozen-in magnetic inclination deviate from the local measured inclination?

___________________________________˚ [3]

D. Consider the following section of layered rocks recorded on a small tectonic plate. Each layer has a measured magnetic inclination and age indicated. Assuming you know the path taken by the plate, use the inclination and age data to compute the paleolatitudes at each time – record your answers in the table provided and draw on the diagram when the plate was at various positions along its path. [6]

Note “Ma” – literally “Mega annum” – just means “millions of years ago”.

Also: while we have provided the plate path in the question above, in real paleomag. studies, researchers have to use a combination of magnetic declination and inclination to recreate the path.

Age (Ma)             Latitude ( ˚ )

0

2

5

10

20

PART 2: Magnetic stripes and seafloor spreading

The observation and understanding of magnetic stripes on the seafloor were key to our understanding of seafloor spreading and the motion of the oceanic plates. These measurements provided irrefutable proof of plate tectonic theory. These magnetic isochrons (“equal-time” stripes) allow us to “unwind the tape” and reconstruct the history of plate motions. This phenomenon arises because of periodic reversals in polarity of the Earth’s magnetic field.

A. The figure below shows several snapshots of the slow opening of an ocean as two continents separated over the last 4 million years. On the left you are given a key showing the (simplified) magnetic polarity history for the last 4 Ma. Using this key, shade in the seafloor at each age on the figure to indicate the polarity of seafloor in this ocean. You may assume constant spreading rate through time. [8]

NOTE: new crust generated in one time step should show up in all subsequent time intervals…

NOTE ALSO: that during several of these time periods there are polarity reversals!!

B. If you are told that at the present day, the two continents are 240 km apart, what is the full spreading rate of this mid-ocean ridge, in km/million-years?

________________________________ km/Myr [2]

C. What is the half-spreading rate (the rate at which either side moves away from the ridge)?

________________________________ km/Myr [1]

D. What is the full spreading rate in units of mm/yr ?

________________________________ mm/yr [2]

Different mid-ocean ridges spread at different rates. With increasing age, the oceanic plates progressively cool, get denser, and sit lower in the mantle (think about a fully loaded cargo ship versus an empty cargo ship). This results in a systematic, predictable relationship between plate age and ocean depth.

The diagram below shows the relationship between ocean depth and distance from the mid-ocean spreading ridge for two different oceans; the Atlantic (blue) and the Pacific (red).

E. What is the spreading rate for each ridge:

Pacific seafloor spreading rate is ________________________________ km/Ma [3]

Atlantic seafloor spreading rate is ________________________________ Km/Ma [3]

F. Which ridge has a faster spreading rate? [1] ________________________________

G. By drawing smooth lines through the uneven profiles, estimate the depth of 20 million-year-old seafloor in each of these oceans:

Pacific seafloor aged 20 Ma is ________________________________ m depth [3]

Atlantic seafloor aged 20 Ma is ________________________________ m depth [3]

H. How do these two estimated depths compare to each other? How do you explain this answer?

_________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________ [3]

I. The inset shows a close up of the axial region for each ridge. The difference in spreading rate also affects the shape of the axial region. What is the difference between the axial region for slow versus fast spread crust?

_______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ ___________________________________________________________________________________________________________________ [3]

PART 3: Sea Floor spreading – fracture zones and transform. faults

The observation of large scars in the seafloor – fracture zones – was an important early indicator that the oceans were mobile. Later, these features were recognized as crucial to explaining how relatively linear plate boundaries snake over the surface of our curved planet. Tuzo Wilson showed how the pattern of ridges, transforms, and fracture zones all made sense through the theory of plate tectonics.

Activity: Find the seafloor spreading model. Make the sea floor spread by pulling the wooden continents apart. Carefully observe the sea floor spreading centers, the active transform. faults and the fracture zones. (By definition, the fracture zones include both the active transform. faults and the inactive, fossil traces of transform. faults.)

Questions:

A. As sea floor spreading widens the ocean, do the spreading center segments change length?

(circle one) Yes or No [1]

B. As sea floor spreading widens the ocean, do the active transform. faults change length?

(circle one) Yes or No [1]

C. As sea floor spreading widens the ocean, do the fracture zones change length?

(circle one) Yes or No [1]

D. The figure to the right is a map of a spreading system between two hypothetical plates A and B, with three slightly offset spreading centers segments…

- Label each active spreading center: SC

- Label each active transform. fault: TF

- Label each inactive fracture zone: FZ

- Put arrows on the two sides of each active plate boundary segment showing the relative motion between the two sides (i.e., five pairs of arrows).  [8 total]

E. The green star shows an earthquake. Would we expect this earthquake to involve plates:

separating apart from each other or sliding horizontally past each other ? (select one) [1]

F. Consider points 1 and 2 in the figure above, on opposite sides of the dotted line. Is the oceanic crust at these points:

the same age or 1 older than 2 or 2 older than 1 ? (select one) [1]

PART 4: Plate boundaries

The most tectonically active parts of the planet are at the plate boundaries – the interfaces between one plate and another. The plates are – by definition – moving with respect to each other at these boundaries, which creates earthquakes, volcanoes, and dramatic topography.

The following questions ask you to analyze carefully the figure of the South America-Pacific plate boundary on the next page. This figure contains a lot of information, so take some time to look over it and ensure you understand it.

A. What is the name for this type of plate boundary?

_________________________________________________ [1]

B. What is the relative motion of the plates across this boundary?

_________________________________________________ [1]

Figure 1: Figure made using GeoMapApp software showing South America - Pacific plate boundary. Circles show locations of earthquakes since 1960, colored by depth and scaled by magnitude. The red stars are active volcanoes. A topographic profile across the plate boundary at ~30˚S is inset.

C. What is the overall relationship between the depth of earthquakes and distance from the plate boundary here (i.e., from the trench just offshore from the continental margin)? How do you explain this trend?

_________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________ [3]

D. What is the approximate distance between the volcanic arc (the chain of volcanoes, marked by red stars) and the plate boundary?

_________________________________________________ [1]

E. Draw a cross section that spans the region of the topographic inset at 30˚S in the space below. Show the structure of the plates from the surface down to 200 km depth in the Earth. Make sure to illustrate and label the slab, trench, Andes mountains, volcanoes, and earthquake locations. [5]

F. Explain what leads to volcanism in the Andes (and other plate boundaries of this kind).

_________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________ [3]

PART 5: Hotspot tracks

Hotspot tracks form. as plates move over hot plumes from the Earth’s deep interior. These plumes are thought to initiate at the core-mantle boundary and undergo melting as they rise to the surface. The melted rock then punches through the plate above the hot spot, creating volcanic seamounts, islands, atolls and guyots (depending on the amount of volcanism and age/maturity of the volcanic edifice). Hotspots remain relatively immobile while plates move over them and can therefore be used to work out the speed and direction of plate motion.

The following questions rely on interpretation of the Hawaiian ocean island chain shown in the following figure. The figure gives you compass directions and a horizontal scale, in km, measured along the chain from the youngest ocean island volcano – Kilauea, which is presently active. In red are localities of rocks dated using geochronological techniques.

NOTE, one of the ages is actually wrong… so be careful!

A. Using this hotspot track, calculate the speed of the plate’s motion over the Earth’s surface, in mm/yr.

_______________________________ mm/yr [5]

B. Roughly in which compass direction is the plate moving? (we’re looking for an answer like “north” or “southeast”)

_______________________________ [1]

C. Which of the rock ages is wrong and what should the age of rocks at that locality actually be (be sure that the plate speed that you report for part A is not based on the incorrect point)?

The age of ________________ Ma is wrong – rocks on that island should be _______________________ Ma [4]