"The magnetic field of the Earth reverses at irregular intervals! The reversal takes 5,000 years or less, with the intensity of the field declining to almost zero and then building back again -- but then the field intensity builds up in the opposite direction! That means that the magnetic north pole hasn't always been in the northern hemisphere. It is thought that the Earth's magnetic field has reversed 17 times in the last 4 million years.
Many rocks record the orientation of the Earth's magnetic field at the time they are formed. It's like a little record of the magnetic field got frozen into that rock. So, by looking at ancient rocks, we can tell the polarity of the magnetic field of the Earth even back to ancient times!
In the 1950's, magnetometer instruments started to be towed by ships traversing the ocean. The magnetometer would make a magnetic survey of the sea floor. Magnetic signatures were more intense than those found on land and they were arranged in bands parallel to mid-ocean ridges. In fact, areas around mid-ocean ridges showed that the magnetic signatures in rocks on one side of the ridge were mirrored by magnetic signatures on the other side of the ridge. So survey results might look something like this (with N denoting northward orientation and S denoting southward orientation):
It was deduced that mid-ocean ridges are places where new ocean crust is being created by upwelling material from the deep interior of the Earth. Another way to say this is that mid-ocean ridges are places of seafloor spreading. Here molten rock rises up from the depth of the Earth and makes new crust. The new crust forms in bands on either side of the mid-ocean ridge, with the newly created band pushing the already-existing crust a little farther away from the mid-ocean ridge. And of course, as each new band forms, the current orientation of the Earth's magnetic field is frozen into it."
This passage is a Windows to the Universe Original.
The magnetic north pole of the Earth
a. is always in the northern hemisphere
b. is always in the southern hemisphere
c. alternates between the northern and southern hemisphere
d. comes out from the equator of the Earth
According to the passage, which of these is a valid magnetic signature for a mid-ocean ridge region?
a. N-S-ocean ridge-N-S
b. S-S-ocean ridge-S-S
c. N-N-ocean ridge-S-S
d. S-N-ocean ridge-S-N
Mid-ocean ridges are NOT:
a. places where old crust sinks into the Earth
b. places where new crust forms
c. places where magnetic surveys can be taken
d. areas of sea floor spreading
2. Now, go to the sea floor model that your teacher has set up in the classroom. Use your magnetometer to perform a magnetic survey of the sea floor model. Draw the line representing the mid-ocean ridge. Draw an upward facing arrow when your magnetometer reads north and a downward facing arrow when your magnetometer reads south. Draw these arrows in order on either side of the mid-ocean ridge. Label your arrows "north" or "south". Sketch:
"Some ore deposits contain minerals that possess physical characteristics that can be measured by suitably sensitive instruments. Exploration based on the principles of physics is called geophysics. Exploration techniques utilize such physical properties as density, magnetic behavior, electrical conductivity, and radioactivity. Six basic geophysical exploration methods--gravity, seismic, magnetic, electromagnetic, electric, and radiometric-are commonly employed in the search for minerals.
Certain minerals distort the earth's magnetic field, and where sufficiently large concentrations of such minerals occur, variations can be measured by magnetometers mounted in aircraft, in ground vehicles, or positioned at stations on the ground. Magnetite iron ores have been found in many areas of the world using the airborne magnetometer.
In one case in the western United States, a very large iron deposit has recently been discovered beneath several hundred feet of barren volcanic flow rock erupted over the ore deposit. Magnetic copper skarn, magnetic nickel ore, and asbestos-bearing serpentine associated with certain magnetic intrusive rocks have been found, using the magnetometer. Some geophysicists propose the use of the magnetometer to detect gold placer deposits, because of their common association with black sands largely consisting of the mineral magnetite."
This passage comes from the Anatomy of a Mine web site produced by the United States Geological Survey (USGS).
A magnetometer tests which of these physical properties of a mineral?
d. magnetic behavior
How many basic geophysical exploration methods are commonly used to search for minerals?
According to this article, in order to search for ores, the magnetometer instrument can be used on which of these vehicles?
Which of these has NOT been found using a magnetometer?
b. iron ore
c. nickel ore
d. copper skarn
2. Now, go to the ore deposit model that your teacher has set up in the classroom. Sketch the grid that you see on top of the file folder. Make sure you have the same number of squares as you see on the folder! Now use your magnetometer to find any ores that might be in this area. Put an "X" on squares where there seems to be a magnetic disturbance (if a disturbance seems to be in more than one square, do mark all of the squares in which you find the disturbance). Be sure to also mark in that square whether or not the disturbance was "north" or "south".
Last modified December 10, 2001 by the Windows Team
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