Download Name Student # Lab Time: 8 am 2 pm (Please

Name
Student #
Lab Time: 8 a.m.
2 p.m. (Please circle)
GLY 2010 Assignment 7 KEY, Summer 2013
Write a brief response to the following questions. Due at the beginning of the next class
(7/17/13). Answers must be submitted on this form. Copies are not acceptable. If you need
additional room, attach pages and staple them together. Be sure your name is on each page
1. What type of magnetic filed does the earth have (how many poles)? What is the polarity
of the magnetic pole currently located near the north geographic pole? What happens
during a polarity reversal? Explain the Vine-Mathews-Morley idea of how magnetic stripes
develop on the ocean floor. (3 points)
The earth has a dipolar field (two poles). The polarity of the magnetic pole near the
north geographic pole is south, currently. During a polarity reversal, the polarity of
the north and south magnetic poles interchange. According to
Vine-Mathews-Morley, polarity reversals and sea-floor spreading, both radical
ideas when they proposed the hypothesis, account for the magnetic stripes. New
oceanic crust (basalt) is magnetized according to the field of the earth when the
basalt forms. The field of the rock plus added to the earth’s magnetic field creates a
slight positive anomaly, recorded in the rock as a stripe. During the last magnetic
reversals, the field of the rock was against the present field of the earth, creating a
negative anomaly, or reverse stripe.
2. What does the abbreviation SID (as in SID earthquake) stand for? What are transform
plate interactions associated only with S earthquakes? Why are ocean-ocean and oceancontinent interactions associated with SID earthquakes, whereas continent-continent
interactions are associated with only SI earthquakes? ( 2 points)
SID stands for shallow, intermediate, and deep focus earthquakes. Shallow focus
earthquakes have foci between 0-70 kilometers below the surface, intermediate
between 70-300 kms, and deep focus > 300 km. Transform interactions are shearing
type motion, with no subduction. The interacting plates are moving sideways past
each other, and the earthquakes are caused by friction near the surface, so they are
always shallow focus. Both the O-O and O-C collisions involve subduction of an
oceanic plate. Since the subducting plate travels downward to 700 kilometers and
generates earthquakes as it goes, the earthquakes are shallow, intermediate, and
deep focus earthquakes. Continent-continent collisions are similar to head-on
collisions between cars. No plate is subducted, but the impact forces some of the
crust deep into the earth in order to maintain isostatic balance with the mountains
that are created. The cross-section of the Afghanistan situation (Lecture 14, slide 34)
illustrated this.
3. How is an oceanic trench created? How is it related to a volcanic island arc, or to a chain
of stratovolcanoes on a continent? (2 points)
Oceanic trenches are created when a subducting plate drags material (mostly
sediment) from the surface downwards, leaving a void at the boundary between th
colliding plates. If the collision is an ocean-ocean collision, the trench will be toward
the oceanic plate that is being subducted. The subducted plate will generate magma
in the region between 100-150 kilometers beneath the surface. The magma rises,
melting through the second ocean plate, and creating a series of volcanoes known as
a volcanic island arc. If the collision is on ocean-continent collision, the oceanic plate
is again subducted, creating magma which moves upward through the continental
plate, with substantial partial melting of the continent. This makes the magma more
felsic, and causes stratovolcanoes, such as the Cascades, to form.
4. What are magnetic stripes? Who first tried to publish an explanation of the formation of
magnetic stripes? What two highly controversial ideas did that person use in explaining
magnetic stipes? Summarize his explanation of magnetic stripes. What two people
subsequently formulated almost the same explanation, but got it published? How was it
possible that they got published first. (3 points)
The first person to offer an explanation of magnetic stripes was L. Wilson Morley, a
Canadian geoscientist. He sent an article to the British journal Nature in January,
1963. In his article he assumed two then controversial ideas were true:
1. Sea-floor spreading, first proposed by Hess and Dietz
2. Reversals of the polarity of the earth’s magnetic field
Morley said that as magma hardened into basalt at a spreading center, the
magnetite in the basalt would have a magnetic field parallel to the present field of
the earth. Measurements of the magnetic field over the spreading center would
combine the earth’s field plus the much weaker field of the rock, to produce a value
higher than the regional average magnetic field (a positive stripe). As we move away
from the spreading center in either direction, the rocks get older. When the age of
the rocks corresponded to a period of reversed magnetic polarity, the magnetic field
of the rock would be against the present field of the earth, and would produce a
negative magnetic stripe. The two people who published nearly the same
explanation in Nature in September, 1963 were graduate student Frederick J. Vine
and his supervisor, Drummond Mathews. Mathews was a professor at Cambridge
University in Great Britain. His well-established reputation as a scientist made it
much more difficult for the editors of Nature to reject the article. Neither Vine nor
Mathews had any clue about Morley’s work. There was no Internet at that time,
and ideas were spread at international meetings or through publications. Because
Morley and Vine-Mathews were unknown to each other, it happened that they all
came up with the same explanation without knowledge of the others work.