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Paleomagnetic anomalies on the seafloor
After lava comes out of midocean ridges, it cools and solidifies into rocks called basalt. Some of the minerals that crystallize line up according to the Earth's magnetic field that surrounds them, in the same way that iron filings line up in a particular pattern around a bar magnet.
The Earth's magnetic field doesn't point in the same compass direction all the time. Sometimes it behaves as if there were a bar magnet within the Earth with the positive side (north magnetic pole)pointing towards the north geographic pole. At other times, the magnetic field is reversed, as if the north pole of the bar magnet were located towards the geographic south pole. The magnetic field flips or reverses at seemingly random times, usually spaced millions of years apart. The flipping itself seems to take less than x years.
Today, the Earth's magnetic field is aligned to the north geoographic pole. That's considered normal. When the magnetic field is opposite, it's considered reversed.
Since the rocks that make up the seafloor solidify at times when the Earth's magnetic field is both normal and reversed, the rocks become magnetized in stripes of various widths, depending on how long the field remainined in one position and how fast the seafloor was spreading.
The pattern of magnetic stripes is actually called the "paleomagnetic anomaly pattern." It's easy to understand that it's a pattern and it's magnetic. The "paleo" part is a prefix indicating "ancient." Most of the seafloor pattern excapt at midocean ridges was formed in the past, i.e. ancient.
What's an anomaly? An anomaly is something that is out of the range of what's expected. When scientists measured the intensity of the magnetic field around the oceans, they found alternating strips of areas that .. stronger and weaker magnetic fields. They weren't measuring the polarity or the angle of magnetization. They were just measuring the strength or intensity of the field. The field was more intense over areas of the seafloor that were magnetized in the same direction as the existing field. Then both the field and the one from the underlying rock, amplified each other. The field was less intense when the underlying rock waws magnetized in the reverse, ... the existing field.
These phrases are used in this context:
Marine magnetic anomaly, seafloor paleomagnetic anomalies, paleomagnetic stripes, magnetic stripes.
Questions for thoughtThe pattern of marine paleomagnetic stripes is one of the key pieces of evidence for plate tectonics and seafloor spreading. Why?
Sedimentary column
Age scale
Sediments.
The Earth's magnetic field doesn't point in the same compass direction all the time. Sometimes it behaves as if there were a bar magnet within the Earth with the positive side (north magnetic pole)pointing towards the north geographic pole. At other times, the magnetic field is reversed, as if the north pole of the bar magnet were located towards the geographic south pole. The magnetic field flips or reverses at seemingly random times, usually spaced millions of years apart. The flipping itself seems to take less than x years.
Today, the Earth's magnetic field is aligned to the north geoographic pole. That's considered normal. When the magnetic field is opposite, it's considered reversed.
Since the rocks that make up the seafloor solidify at times when the Earth's magnetic field is both normal and reversed, the rocks become magnetized in stripes of various widths, depending on how long the field remainined in one position and how fast the seafloor was spreading.
The pattern of magnetic stripes is actually called the "paleomagnetic anomaly pattern." It's easy to understand that it's a pattern and it's magnetic. The "paleo" part is a prefix indicating "ancient." Most of the seafloor pattern excapt at midocean ridges was formed in the past, i.e. ancient.
What's an anomaly? An anomaly is something that is out of the range of what's expected. When scientists measured the intensity of the magnetic field around the oceans, they found alternating strips of areas that .. stronger and weaker magnetic fields. They weren't measuring the polarity or the angle of magnetization. They were just measuring the strength or intensity of the field. The field was more intense over areas of the seafloor that were magnetized in the same direction as the existing field. Then both the field and the one from the underlying rock, amplified each other. The field was less intense when the underlying rock waws magnetized in the reverse, ... the existing field.
These phrases are used in this context:
Marine magnetic anomaly, seafloor paleomagnetic anomalies, paleomagnetic stripes, magnetic stripes.
Questions for thought
- Playing with iron filings. 2-d pattern on a piece of paper. 3-D pattern in space.
- Would filings be aligned parallel, perpendicular or at an angle to the ground around the equator? At the pole? Depends on latitude.
- Crystals make up rocks. Are there crystals in lava?
- More or less flips, which is better to guess time?
- Make your own stripes.
- Make a stripe. Set it to a different spreading speed. Can you still make the same width?
- Use this diagram. How fast is the sea floor spreading during the time when ...
- The seafloor spreads more slowly in the Atlantic than the Pacific. Which one pair of diagrams is likely to show the pattern of paleomagnetic anomalies that represent the same time span, one from the Atlantic and the other from the Pacific?