magnetic evidence for seafloor spreading

seafloor age

By combining magnetic polarity data from rocks on land and on the seafloor with radiometric age dating and fossil ages, scientists came up with a time scale for the magnetic reversals. The first four magnetic periods are: Brunhes normal - present to 730,000 years ago. Matuyama reverse - 730,000 years ago to 2.48 million years ago. Gauss normal - 2.48 to 3.4 million years ago. Gilbert reverse - 3.4 to 5.3 million years ago. The scientists noticed that the rocks got older with distance from the mid-ocean ridges. The youngest rocks were located at the ridge crest and the oldest rocks were located the farthest away, abutting continents. Scientists also noticed that the characteristics of the rocks and sediments changed with distance from the ridge axis as seen in the Table 1.1. Rock ages At ridge axis With distance from axis youngest becomes older Sediment thickness none becomes thicker Crust thickness Heat flow thinnest becomes thicker hottest becomes cooler Away from the ridge crest, sediment becomes older and thicker, and the seafloor becomes thicker. Heat flow, which indicates the warmth of a region, is highest at the ridge crest. The oldest seafloor is near the edges of continents or deep sea trenches and is less than 180 million years old (Figure something was happening to the older seafloor. Seafloor is youngest at the mid-ocean ridges and becomes progressively older with distance from the ridge. How can you explain the observations that scientists have made in the oceans? Why is rock younger at the ridge and oldest at the farthest points from the ridge? The scientists suggested that seafloor was being created at the ridge. Since the planet is not getting larger, they suggested that it is destroyed in a relatively short amount of geologic time. Click image to the left or use the URL below. URL:

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seafloor magnetism

On our transit to the Mid-Atlantic ridge, we tow a magnetometer behind the ship. Shipboard magnetometers reveal the magnetic polarity of the rock beneath them. The practice of towing a magnetometer began during WWII when navy ships towed magnetometers to search for enemy submarines. When scientists plotted the points of normal and reversed polarity on a seafloor map they made an astonishing discovery: the normal and reversed magnetic polarity of seafloor basalts creates a pattern. Stripes of normal polarity and reversed polarity alternate across the ocean bottom. Stripes form mirror images on either side of the mid-ocean ridges (Figure 1.1). Stripes end abruptly at the edges of continents, sometimes at a deep sea trench (Figure 1.2). The magnetic stripes are what created the Figure 1.1. Research cruises today tow magnetometers to add detail to existing magnetic polarity data.

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instructional diagrams

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questions

how do scientists determine the age of a seafloor basalt?

a) they can get a radiometric age if they can collect a sample.

b) they can use the time scale for magnetic reversals.

c) they can look at fossils in the sediments on top of the basaltic rocks.

-->  d) all of these.

what is true about the seafloor near mid-ocean ridge?

-->  a) rocks are younger closer to the ridge.

b) the crust is thicker near the ridge.

c) the magnetic stripes are thinnest near the ridge and get thicker with distance away from the ridge.

d) all of the above.

submarines during wwii discovered the magnetic patterns on the seafloor.

a) true

-->  b) false

the magnetic stripes on either side of the mid-ocean ridge, the same distance out from the ridge, have opposite polarity.

a) true

-->  b) false

with increasing age, seafloor rocks

a) wear away to become thinner.

b) become hotter.

-->  c) collect an increasing amount of sediment on top.

d) all of these.

the oldest seafloor is less than 180 million years old.

-->  a) true

b) false

what is not true about the magnetic stripes on the ocean floor?

a) stripes alternate, normal and reverse polarity, across the ocean floor.

b) stripes indicate the age of the basaltic rock.

-->  c) stripes are evidence of apparent polar wander.

d) stripes end abruptly at the edges of continents.

navy ships use these to search for enemy submarines, but accidently discovered the magnetic polarity of the seafloor.

a) bar magnets

-->  b) magnetometers

c) electromagnets

d) echo sounders

observations of the seafloor support the idea that new seafloor is created at mid- ocean ridges.

-->  a) true

b) false

if it is true that new seafloor is created at mid-ocean ridges, then

a) the planet must be getting bigger.

-->  b) old seafloor must be destroyed somewhere.

c) new seafloor must sink directly into the mantle.

d) none of these.

diagram questions

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