3 SEA-FLOOR MAGNETISM

䊳 11.3 SEA-FLOOR MAGNETISM

Figure 11–2 Alvin is a research submarine capable of diving Recall from Chapter 2 that Alfred Wegener proposed that to the sea floor. Scientists on board control robot arms to

continents had migrated across the globe, but because he collect sea-floor rocks and sediment. (Rod Catanach, Woods

was unable to explain how continents moved, his theory

Hole Oceanographic Institution)

was largely ignored. Then, in the 1960s, 30 years after Wegener’s death, geologists studying the magnetism of

approach is both effective and economical. The echo sea-floor rocks detected odd magnetic patterns on the sea sounder is commonly used to map sea-floor topography.

floor. Their interpretations of those patterns quickly led It emits a sound signal from a research ship and then

to the development of the plate tectonics theory and

Sound sent downward from ship

Sound reflected from sea floor

(a) Figure 11–3 (a) Mapping the topography of the sea floor

with an echo sounder. A sound signal generated by the echo sounder bounces off the sea floor and back up to the ship, where its travel time is recorded. (b) A seismic profiler record of sediment layers and basaltic ocean crust in the Sea of Japan. (Ocean Drilling Program, Texas A&M University)

(b)

182 CHAPTER 11 OCEAN BASINS

proved that Wegener’s hypothesis of continental drift had history. When a magnetic reversal occurs, the north been correct.

magnetic pole becomes the south magnetic pole, and To understand how magnetic patterns on the sea

vice versa. The orientation of the Earth’s field at present floor led to the plate tectonics theory, we must consider

is referred to as normal polarity, and that during a time the relationships between the Earth’s magnetic field and

of opposite polarity is called reversed polarity. The magnetism in rocks. Many iron-bearing minerals are per-

Earth’s polarity has reversed about 130 times during the manent magnets. Their magnetism is much weaker than

past 65 million years. But polarity reversals do not occur that of a magnet used to stick cartoons on your refriger-

on a regular schedule. A period of normal polarity dur- ator door, but it is strong enough to measure with a mag-

ing the Mesozoic Era lasted for 40 million years. netometer.

As the Earth’s magnetic field is about to reverse, it When magma solidifies, certain iron-bearing miner-

becomes progressively weaker, but its orientation re- als crystallize and become permanent magnets. When

mains constant. Then the magnetic field collapses to zero such a mineral cools within the Earth’s magnetic field,

or close to zero. Soon a new magnetic field with a po- the mineral’s magnetic field aligns parallel to the Earth’s

larity opposite from the previous field begins to grow. field just as a compass needle does (Fig. 11–4). Thus,

The reversal takes 3000 to 5000 years, a very short time minerals in an igneous rock record the orientation of the

compared with many other geologic changes. Earth’s magnetic field at the time the rock cooled. Many sedimentary rocks also preserve a record of

SEA-FLOOR SPREADING: THE BEGINNING OF the orientation of the Earth’s magnetic field at the time

THE PLATE TECTONICS THEORY the sediment was deposited. As sediment settles through

water, magnetic mineral grains tend to settle with their As you learned in Chapter 4, most oceanic crust is basalt magnetic axes parallel to the Earth’s field. Even silt

that formed as magma erupted onto the sea floor from particles settling through air orient parallel to the mag-

the mid-oceanic ridge system. Basalt contains iron- netic field.

bearing minerals that record the orientation of the Earth’s magnetic field at the time the basalt cooled.

Figure 11–5 shows magnetic orientations of the MAGNETIC REVERSALS oceanic crust along a portion of the mid-oceanic ridge

The polarity of a magnetic field is the orientation of its known as the Reykjanes ridge near Iceland. The black positive, or north, end and its negative, or south, end.

stripes represent rocks with normal polarity, and the in- Because many rocks record the orientation of the Earth’s

tervening stripes represent rocks with reversed polarity. magnetic field at the time the rocks formed, we can con-

Notice that the stripes form a pattern of alternating struct a record of the Earth’s polarity by studying mag-

normal and reversed polarity, and that the stripes are netic orientations in rocks from many different ages and

arranged symmetrically about the axis of the ridge. The places. When geologists constructed such a record, they

central stripe is black, indicating that the rocks of the discovered, to their amazement, that the Earth’s mag-

ridge axis have the same magnetic orientation that the netic field has reversed polarity throughout geologic

Earth has today.

Shortly after this discovery, geologists suggested that a particular sequence of events created the alternat- ing stripes of normal and reversed polarity in sea-floor

North

rocks:

1. New oceanic crust forms continuously as basaltic magma rises beneath the ridge axis. The new crust then spreads outward from the ridge. This move- ment is analogous to two broad conveyor belts mov- ing away from one another.

2. As the new crust cools, it acquires the orientation of

South

the Earth’s magnetic field.

Iron minerals acquire orientation

3. The Earth’s magnetic field reverses orientation on

an average of every half-million years. Figure 11–4 An iron-bearing mineral in an igneous rock ac-

of Earth’s magnetic field

4. Thus, the magnetic stripes on the sea floor record a quires a permanent magnetic orientation parallel to that of the

succession of reversals in the Earth’s magnetic field Earth’s field, as the rock cools.

that occurred as the sea floor spread away from the

The Mid-Oceanic Ridge 183

Sea floor spreads N

away from ridge W

E Oceanic

Oceanic ridge

Magma rises

Mid-Atlantic

at ridge

Ridge

62 ° Figure 11–6 As new oceanic crust cools at the mid-oceanic ridge, it acquires the magnetic orientation of the Earth’s field. Alternating stripes of normal (blue) and reversed (green) po- larity record reversals in the Earth’s magnetic field that oc-

60 ° curred as the crust spread outward from the ridge.

Figure 11–5 The mid-Atlantic ridge, shown in red, runs gin of all oceanic crust. 1 In a short time, geologists com- through Iceland. Magnetic orientations of sea-floor rocks near

bined Wegener’s continental drift hypothesis and the the ridge are shown in the lower left portion of the map. The

newly developed sea-floor spreading hypothesis to de- black stripes represent sea-floor rocks with normal magnetic

velop the plate tectonics theory. You read in Chapter 2 polarity, and the intervening stripes represent rocks with

that this theory explains how and why continents move, reversed polarity. The stripes form a symmetrical pattern of

mountains rise, earthquakes shake our planet, and volca- alternating normal and reversed polarity on each side of the

noes erupt. It also explains the origin and features of the ridge. (After Heirtzler et al., 1966, Deep-Sea Research, Vol. 13.)

Earth’s largest mountain chain: the mid-oceanic ridge.