Download Plate Tectonics: Ridges, Transform Faults and

Plate Tectonics: Ridges, Transform Faults and Subduction Zones
Goals of this exercise:
1. review the major physiographic features of the ocean basins
2. investigate the creation of oceanic crust at mid-ocean ridges
3. explore different types of plate boundaries
4. examine the origin of the arc-trench gap in subduction zones
Introductory material
After World War II, detailed surveys of the ocean floor revealed complex topography that
required geologists to rethink completely their concepts about the structure of the ocean basins.
Prior to this pioneering oceanographic work, the ocean floors were assumed to be simple flat
plains that collected sediment over the vastness of geologic time. The new oceanographic
surveys revealed the topography to be incredibly diverse, consisting of mountain ranges and deep
trenches that dwarfed anything seen on land. By the mid-1960s the extent of these subma rine
features had been mapped, and six major topographic features were defined: mid-ocean ridges,
fracture zones/transform faults, seamounts, trenches, abyssal plains and continental margins.
The single largest topographic feature on the planet is the mid-ocean ridge (MOR) system.
This huge mountain range extends for 65,000 km and cuts across all major ocean basins. At its
widest, the MOR is nearly 1500 km across and reaches heights of 3 km above the ocean floor.
One perplexing discovery was that of linear features that are generally oriented at right angles to
the ridge and that offset the MOR system. Some of these structures extend nearly half the way
across the ocean basins. Among the unusual features of these structures were:
Significant differences in sea- floor elevation across them
Termination at continental margins
Confinement of seismic activity to the region between offset ridge crests
Offset of magnetic lineations across the structures
Significantly different ages of ocean floor on adjacent sides of a structure
In 1965 J. Tuzo Wilson interpreted these features as transform faults. Wilson’s concept of the
transform fault was a keystone in the development of plate tectonic theory.
The abyssal plains occur in the deep parts of ocean basins and represent regions where oceanic
sediment covers the topographic roughness of the ocean floor. Abyssal plains are much more
common and better developed in the Atlantic than in the Pacific Ocean.
Deep-sea trenches occur along the coasts of many continental landmasses. They are filled with
large quantities of sediment eroded from the continent, as well as material scraped off of a
descending plate. Trenches that are located far from continents (where ocean crust descends
beneath an oceanic plate) are very deep and contain little sediment. The deepest spot measured
on Earth is in the Marianas Trench near the Philippines – it is 11,021 meters (36,160 feet) below
sea level. Trenches may extend for thousands of kilometers along the sea floor; they are 40-120
km (25-75 miles) wide and are associated with both earthquakes and volcanoes.
Note – figures in this lab are reproduced with permission from Houghton Mifflin Company.
Plate Tectonics – Ridges, Transform Faults, and Subduction zones
Name:
Date:
Activity 1: Ridges, Isochrons and the Creation of Oceanic Crust
1. Figure PT-1 shows a ridge at three different times after spreading began – i.e., 30 my, 60 my
and 100 my.
a. Along the bottom of each map, mark the mid-ocean ridge with an R.
b. Draw arrows (above or below the isochrones) the show the direction of plate motion.
c. Place an “O” above the isochron the represents the oldest crust.
d. Write a “Y” above the isochron that represents the youngest crust.
2. What does the spacing of the isochrons tell us about the spreading rate?
3. If the full spreading rate of the ridge is 2.5 cm/yr, and the ridge has been spreading for 100
my, what is the total length of new crust added to the ocean floor? “Full spreading rate”
refers to crustal addition on both sides of the active MOR, whereas “half-spreading rate” (a
term more commonly used) refers only to crus tal addition on one of the ocean plates.
Activity 2: Magnetic Reversals and Ocean Floor Magnetic Anomalies
Using your knowledge of how magnetic anomalies form (we will go over this concept in class)
and this figure putlinkhere , answer the following questions.
a. Near what major geologic feature of the Earth’s surface is older oceanic crust always
located?
b. What is the age of the oldest oceanic crust found on Earth?
c. How does this age compare with the oldest continental rocks?
d. Assuming that the Earth is 4.6 billion years old, what percentage of Earth history is
recorded by the rocks of the ocean basins?
e. Why are there no “old” rocks found on the ocean floor?
f. When did the North Atlantic Ocean basin begin to open up?
Figure PT-1
g. Estimate the maximum age of volcanic rocks on Iceland using the magnetic striping
of the ocean floor.
h. With these answers in mind, what can you say about Iceland when the North Atlantic
Ocean began to form?
i. When did the South Atlantic Ocean basin begin to form?
Activity 3. Transform Faults and Fracture Zones
Figure PT-2 shows the ocean floor around a ridge at three different periods of sea- floor
spreading. Stage 1 corresponds to the appearance of the ocean floor at the onset of spreading.
Stage 2 represents the ocean floor 15 my after spreading began. Stage 3 shows the ocean floor
after 30 my of spreading. Remember that the transform fault is located between the active ridge
axes, whereas the fracture zone extends away from the ridge crest across the ocean floor.
1. Record the following on the Stage 1, 2 and 3 maps:
a. Mark the ridge with an R.
b. Mark the fracture zone(s) with x and the transform fault(s) with a plus sign (+).
c. For the ridge segments, mark the direction of spreading with arrows.
d. Use arrows to show the motion of the plate segments on both sides of the fracture
zones and the transform fault.
2. How does the distance between the ridges change as spreading progresses?
3. How does the length of the transform fault vary with time?
4. How does the length of the fracture zones vary with time?
5. Where in this system are the plates moving past one another?
6. What type of geologic event would you expect to occur in this region?
7. Why are fracture zones aseismic (lacking earthquakes)?
8. Where in the region would you sample the youngest submarine lava flows?
Figure PT-2
(Ignore the dashed and dotted lines – they are there to help you but apparently are
somewhat confusing!)
Activity 4. Subduction Zones and the Arc -Trench Gap
1. Figure PT-3 is a cross section of a subduction zone. We will use it to investigate the factors
that determine the width of the arc-trench gap and formulate a scientific model relating these
dips and the arc-trench gap. This model can be used to predict plate dip and seismicity based on
the width of the arc-trench gap.
a. Draw a straight line (representing the top of the subducting plate) from the trench to
each subduction angle shown on the bottom and the right-hand side of the diagram.
b. Noting that the volcanic arc is located 100 km above the subducting plate, project the
position of each volcanic arc to the Earth’s surface. Mark the position with a V and a
subscript indicating the dip angle – e.g., V10 for the slab dipping 10o .
c. Mark the earthquake pattern along each plate by showing shallow earthquakes (0-100
km) with an S, intermediate ones (100-200 km) with an I, and deep earthquakes
(>200 km) with a D.
d. Using the following directions, transfer the location of the volcanic arc and the
earthquake distribution pattern to maps of the ocean floor shown in figure PT-4.
i.
Measure the horizontal distance from the trench to the volcanic line in
figure PT-3. Draw a line of circles on figure PT-4 showing the location of
the arc volcanoes.
ii.
Measure the distance to the intermediate-deep earthquake boundary.
Indicate this location on figure PT-4.
iii.
Indicate the distribution of each type of earthquake on figure PT-4.
Figure PT-3
Figure PT-4
2. Using the maps you have drawn in figure PT-4, it is possible to construct a scientific model
that can predict features of subduction zones when you do not know the subsurface structure.
a. Measure the width of the arc-trench gap for each subduction angle and enter these
values in figure PT-5. Connect the data points with a smooth curve.
b. From this plot, estimate the dip of the subducting plate if the arc-trench gap is 175 km.
3. How does the dip of the subduction zone influence the arc-trench gap?
4. Where are most subduction zones located globally?
5. Why is this distribution so important in terms of planning for geologic hazards?
Figure PT-5