22.3: Seafloor spreading

Wegener’s catalog of evidence for the past motion of the continents was revitalized when a new context was offered in the aftermath of World War II. The new context shifted the focus from the continents (~30% of the planet’s surface) to the rocks that floor the ocean basins (~70% of Earth’s surface). The new idea was that the seafloor was an ephemeral entity, forming at oceanic ridge systems, and destroyed elsewhere through the process of subduction.

The shape of the ocean basins

World War II was fought on many fronts. The relevant aspect of the conflict for the current discussion is submarine warfare. The United States Navy fought the German U-boats in the Atlantic Ocean, attempting to prevent tragedies like the sinking of the Lusitania. To aid in the war effort, sonar was utilized to map the seafloor to a greater degree than had previously been accomplished. When the data was declassified after the war ended, it revealed a fascinating aspect of the planet that had previously been hidden: the seafloor wasn’t flat!

Figure \(\PageIndex{1}\): The oceanic ridge system comes in many segments, offset along perpendicular fracture sets. (Visualization via GeoMapApp.)

Instead, the longest range of mountains on the planet, the oceanic ridge system, wrapped around the planet much like the seams wrap around a baseball. Even the stitching on the baseball is relevant here, as it evokes the fracture zones that emanate from the ridge, perpendicular to the ridge’s trend. And right there are the crest of this odd ridge system was a valley, a rift valley. It looked like a scar.

The oceanic ridge system is 70,000 km (43,000 miles) long. In both the Atlantic and Indian Ocean basins, the ridges were in the middle of the ocean, equidistant from the continents on either side. This led many geoscientists to adopt the term “mid-ocean ridge” to describe them, but it’s worth noting that the East Pacific Rise is not in the middle of the Pacific Ocean. So oceanic ridge is the more inclusive term.

Figure \(\PageIndex{2}\): “Discovering Plate Boundaries” topography/bathymetry map (Dale Sawyer)

There were also very deep spots in the ocean – long furrows that were quickly dubbed “trenches.” Curiously, these trenches paralleled chains of active volcanoes, and were in areas known to have lots of earthquakes.

Figure \(\PageIndex{3}\): Marie Tharp and Bruce Heezen, with their map.

At Columbia Lamont-Doherty Earth Observatory, Bruce Heezen and Marie Tharp collaborated on detailed mapping of the seafloor, systematically constraining the variation in bathymetry with a series of profiles. They are celebrated today not only for their meticulous measurements, but because of a landmark visualization of these findings; they commissioned artist Heinrich Berann to paint a rendition of their seafloor map that popped in a way few scientific visualizations have ever matched. The finalized Heezen & Tharp map (with labels, etc.) was published in 1978.

Painting by Heinrich Berann, via the Library of Congress

How to explain this exciting new information about the shape of the largest region on our world?

An essay in geopoetry

Figure \(\PageIndex{4}\): Harry Hess, the Naval captain and ‘geopoet’ who suggested seafloor spreading.

Harry Hess was a a geophysicist on the faculty at Princeton, and served as a captain in the United States Navy during World War II. He had experience with seafloor mapping using sonar on his own ship, and was deeply impressed with the Heezen & Tharpe ridge-top rift data when Heezen presented it at a departmental seminar in 1957. Hess’s paper outlined the case to be made that new seafloor was generated at the ocean ridges: that active spreading at those sites triggered magmatism that sealed the crack shut with new seafloor. But Hess went further: he didn’t think the planet was getting larger over time through ever-expanding ocean basins, but thought instead that a steady state ocean coverage could be maintained if some seafloor was recycled into the “jaw crusher” (his phrase) of the newly discovered oceanic trenches. Alpine geologist André Amstutz called these subduction zones, and though he wrote in French, the name was soon adopted in geoEnglish, too. Powering it all was convection in the underlying mantle: warm (but solid) rock rising beneath the oceanic ridges, and cold rock sinking beneath the subduction zones. Hess was conscious of how revolutionary these ideas were, and he modestly covered up the audaciousness of seafloor spreading by hedging his bet with self-deprecating humor. He waved his arms, and called his paper “an essay in geopoetry.”

Figure \(\PageIndex{5}\): Key features in a subduction zone. (Modified from an original by KDS4444.)

Hess need not have been so coy; his ideas have been validated ever since. The seafloor spreads, adding new oceanic crustal material (basalt and gabbro) at the oceanic ridge system into the gap between diverging continents. Old oceanic crust is destroyed through subduction at deep sea trenches.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which geoscientist is recognized as first articulating the concept of seafloor spreading?

a. Matthew Vine

b. Alexander du Toit

c. Marie Tharp

d. Alfred Wegener

e. Harry Hess

Answer

e. Harry Hess

Exercise \(\PageIndex{2}\)

What is found halfway across the Atlantic Ocean, mid-way between eastern North America and northwestern Africa?

a. A deep sea trench

b. A continental transform fault

c. An oceanic ridge

d. Glacial striations

Answer

c. An oceanic ridge

Exercise \(\PageIndex{3}\)

Which team of geophysicists made a map that presented the newfound structure of the seafloor in visually astonishing detail?

a. Alfred Wegener and Antonio Snider-Pellegrini

b. Marie Tharp and the Deftones

c. Bruce Heezen and Harry Hess

d. Harry Hess and Alfred Wegener

e. Marie Tharp and Bruce Heezen

Answer

e. Marie Tharp and Bruce Heezen

Exercise \(\PageIndex{4}\)

What is a "subduction zone?"

a. A place where new oceanic lithosphere is generated through the process of seafloor spreading. It's marked by a ridge and a central rift valley at its crest.

b. A place where old oceanic lithosphere dives down into the mantle. It's marked by a deep sea trench and a parallel volcanic arc.

c. A place where short segments of oceanic lithosphere on adjacent plates slide past one another. The crust on one side is measurably older than on the other side.

Answer

b. A place where old oceanic lithosphere dives down into the mantle. It's marked by a deep sea trench and a parallel volcanic arc.

Exercise \(\PageIndex{5}\)

What conflict led to a new push for detailed mapping of the seafloor?

a. The Viet Nam War

b. The American Civil War

c. World War I

d. The conquests of Ghenghis Khan

e. World War II

f. The War of 1812

Answer

e. World War II

Age of the oceanic crust

Figure \(\PageIndex{6}\): Age of the oceanic crust on Earth. (NOAA map.)

Oceanographic expeditions such as the Deep Sea Drilling Program sampled the seafloor in many places, gathering cores of seafloor sediment down to the top of the oceanic crust. The age of the oldest sediment could be deduced through study of the microfossils it contained, and that would constrain the age of the crust beneath to be older. Scientists then used correlation with key features of the seafloor (ridges, fracture zones, etc.) to extend points where the age of the oceanic crust was known to cover wider regions. The overall pattern was that the youngest crust was located atop the oceanic ridges, and it got older perpendicular to the ridge, in both directions. In the Atlantic Ocean, for instance, the pattern offered a beautiful symmetry, with crust of 0 Ma age at the Mid-Atlantic Ridge, and Jurassic (~180 Ma) oceanic crust immediately adjacent to the North American and African continental shelves.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Based on the age of the the seafloor, which of the following continents did Africa separate from first?

a. South America

b. Greenland

c. Arabia

d. North America

Answer

d. North America

Exercise \(\PageIndex{2}\)

Almost all the oceanic crust on this planet is less than _________ old.

a. 5 million years

b. 50 million years

c. 200 million years

d. 6000 years

Answer

c. 200 million years

Exercise \(\PageIndex{3}\)

The oldest seafloor is _____________ to the oceanic ridge of that same plate.

a. adjacent

b. beneath

c. closest

d. furthest

Answer

d. furthest

Paleomagnetism

Like many planets, Earth generates its own magnetic field. This magnetic field manifests from flow of a magnetically-conductive fluid: the molten iron/nickel alloy of the outer core. What happens in the core doesn’t stay in the core: the magnetic field penetrates the mantle, crust, oceans, and atmosphere, and extends far out into space. This protects our planet from the atmosphere-eroding effects of the solar wind.

Figure \(\PageIndex{7}\): Bar magnets have two poles, like the Earth’s magnetic geodynamo.

Closer to the surface, where rocks form that can later be studied by historical geologists, the magnetic field also makes an impression. Certain minerals are magnetically sensitive. There are several, including ilmenite and hematite, but for our purposes here, let’s focus on magnetite. Crystals of magnetite have freedom of movement within a molten lava flow or within a body of sediment settling through a water column. Given the freedom to move, they will rotate into alignment with Earth’s prevailing magnetic field. When the rest of the surrounding lava is crystallized, or when the surrounding sediment is lithified, the grains of magnetite are no longer free to move. At this point, they are “fossil” evidence of Earth’s magnetic field, and if they are moved to new locations, or new orientations, they take their “stamp” of the old magnetic field with them. We call the study of old magnetic orientations paleomagnetism.

Geophysicists who study paleomagnetism (who cheekily call themselves “paleomagicians”) collect very precisely oriented samples of magnetite-bearing igneous or sedimentary rocks and bring them back to a lab where a well-shielded magnetometer measures the remnant magnetism.

Inclination

Figure \(\PageIndex{8}\): The shape of Earth’s magnetic field is a torus. Lines of magnetic force are vertical at the poles, and horizontal at the equator, with intermediate values in between. (Callan Bentley illustration.)

Latitude on Earth today (position on Earth’s surface between the Equator and the poles) corresponds strongly with the inclination of Earth’s magnetic field. The lines of magnetic force emanate from the South magnetic pole, wrap around the planet, and dive back into the planet at the North magnetic pole. The lines are vertical at these polar locations (i.e., perpendicular to Earth’s surface), but horizontal at the magnetic Equator (i.e., parallel to Earth’s surface).

To interpret paleolatitude, we apply this modern observation to the past through the lens of uniformitarianism. If we find a sedimentary rock layer with magnetite crystals that show a bedding-perpendicular orientation, we would infer a polar latitude of origin for the original sedimentary deposit. (We apply the principle of original horizontality to infer the bed was laid down more or less horizontally, so a bedding-perpendicular paleomagnetic inclination means “vertical” when it formed.) Conversely, if we find a sedimentary rock layer bearing magnetite crystals that have their magnetic orientation parallel to the bed, then that would imply the sediment was deposited near the paleomagnetic Equator. Intermediate latitudes have values that are intermediate between horizontal and vertical, and the variation occurs in a systematic way.

Declination

Now consider another behavior of a free-spinning magnet in a magnetic field: a compass needle. Like a crystal of magnetite, the compass needle is magnetically susceptible, and will align itself with the planet’s magnetic field, pointing toward magnetic north. Fossil magnets pull this same trick. We call the variation between a given direction and north by the name declination. If you want to track how a given landmass has moved through geologic time, paleomagnetic declination comes in handy. In this case, you want a sequence of sedimentary strata and/or lava flows, spanning the relevant portion of geologic time. If you are successfully in extracting paleomagnetic declination information from them, and you can successfully constrain the ages of the layers, you will get a series of time-stamped declination directions. For instance:

500 Ma	“The north magnetic pole was at 190\(^{\circ}\) to the modern north pole, at a latitude distance of 40\(^{\circ}\).”
350 Ma	“The north magnetic pole was at 230\(^{\circ}\) to the modern north pole, at a latitude distance of 60\(^{\circ}\).”
200 Ma	“The north magnetic pole was at 300\(^{\circ}\) to the modern north pole, at a latitude distance of 80\(^{\circ}\).”
50 Ma	“The north magnetic pole was at 350\(^{\circ}\) to the modern north pole, at a latitude distance of 100\(^{\circ}\).”

In this hypothetical example, four dates/orientations spanning half a billion years constrain the movement of that block of crust increasingly southward (crossing the equator sometime between 200 Ma and 50 Ma), and rotating clockwise all the while.

This “testimony” from the strata of a given location is called an apparent polar wander path. It is basically how one particular spot on a given continent might view the north magnetic pole through time.

Figure \(\PageIndex{9}\): Apparent polar wander (APW) paths for several continents. Orange ellipses show uncertainties. (Callan Bentley graphic, after an original from Geotimes.)

But different continents tell different stories – they have apparent polar wander paths that do not make any sense if the continents are eternally fixed in place. If continents were not permitted to drift, then the implication from a half dozen different continents’ apparent polar wander paths is that in the past there were numerous magnetic north poles, arrayed all over the place, and over geologic time they have gotten closer to the modern north pole, and merged into one. This makes no sense, in terms of what we know about how magnets work. But, if the continents are free to drift, and the seafloor to spread in between them, or subduct in front of them, suddenly you only need one planetary magnetic field to make sense of the continents’ plurality of parochial perspectives.

In other words, paleomagnetic inclination gives us a way to quantify the past motion of a given plate (continent) relative to a more or less stationary magnetic north pole.

Polarity reversals

Figure \(\PageIndex{10}\): The last 5 million years of magnetic reversals. (Public domain via Wikimedia.)

The final aspect of paleomagnetism that bears consideration has to do with reversals of the magnetic field. We’ve previously discussed (a) inclination and (b) declination as being related to angles between a line of magnetic force and (a) Earth’s surface and (b) the direction to the north pole. But that line is not just a line; it is an arrow. In other words, all lines of magnetic orientation have a direction to them – a direction in which they point. A rock forming at the modern day south pole would have a vertical magnetic inclination, but so would one forming at the north pole. How can we tell them apart? Earth’s magnetic flow exits the planet at the south magnetic pole and re-enters at the north magnetic pole. So the modern South Pole rock would have an arrow pointing “up” (away from Earth’s surface), while the modern North Pole rock would have an arrow pointing “down” (toward Earth’s interior).

By paying attention to which way these arrows point, we arrive at an interesting insight: they frequently switch back and forth! The reason why these reversals happen is not entirely clear, but it is plain that they do — and we can use that to our advantage. Not only does it open up a whole new avenue for dating rocks (reading their “magnetostratigraphy” like a bar code) but it also provides a tool for confirming that seafloor spreading is, in fact, real.

Figure \(\PageIndex{11}\): Magnetic intensity anomaly map of the North Atlantic. Note the “mirror imaging” of the anomaly pattern in the seafloor on either side of the Mid-Atlantic Ridge. Explore more here. (Modified from a presentation of the EMAG2 model by Maus, et al. (2009).)

When new oceanic crust is generated, magma wells up from the mantle below, where it is produced through decompression partial melting. Within that magma are magnetically-susceptible minerals (like magnetite). The magma injects into the crack between the two diverging plates, and solidifies. At that moment, the magnetic signature of that moment in Earth history is locked into the plate. Then tensional stresses build up anew, the crust cracks again, and more magma squirts in to seal it shut, again, taking on the magnetic signature of the Earth. If a reversal happened in between dike-injection events, that will be preserved as “stripes” of oppositely-oriented magnetism on the seafloor — something that we can readily measure using a submersible magnetometer towed behind a ship.

It turns out that the pattern of magnetic reversals in Earth’s oceanic crust is recorded twice – once in each direction leading away from the oceanic ridge system. This is a prediction of the Vine-Matthews-Morley hypothesis (named for three geophysicists who suggested it):

Figure \(\PageIndex{12}\): Spreading along several mid-ocean ridges, showing magnetic striping symmetry. (Animation by Tanya Atwater, UCSB.)

Paleomagnetic stripes in the seafloor are like a doubly-redundant tape recording of the reversals in Earth’s magnetic field. The “normal” polarity of the Bruhnes chron that we are currently in can be found astride the ridges all over the world, but head a short distance away (in either direction) and you’ll soon encounter the “reversed” signature of the older Matuyama chron. The two records are like mirror images of one another.

These patterns of magnetic “stripes” parallel to the oceanic ridge system and faithfully recording the changes in the planet’s magnetic polarity were perhaps the most compelling line of evidence yet in support of seafloor spreading, and thus plate tectonics.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What is the origin of Earth's magnetic field?

a. The Moon

b. A million mantle magnets

c. Convective flow in molten iron of the outer core

Answer

c. Convective flow in molten iron of the outer core

Exercise \(\PageIndex{2}\)

The study of ancient magnetic fields preserved in rocks is ___________.

a. Historical magnetology

b. Fossilferrum

c. Dipole deduction

d. Paleomagnetism

Answer

d. Paleomagnetism

Exercise \(\PageIndex{3}\)

Inclination is _______________.

a. The angle at which a magnetic field line intersects Earth's surface.

b. The orientation of the planet's magnetic field (i.e., north magnetic pole at south geographic pole, or vice versa).

c. The horizontal direction toward the magnetic North Pole.

Answer

a. The angle at which a magnetic field line intersects Earth's surface.

Exercise \(\PageIndex{4}\)

What is declination?

a. The horizontal direction toward the magnetic North Pole.

b. The orientation of the planet's magnetic field (north magnetic pole at south geographic pole, or vice versa

c. The angle at which a magnetic field line intersects Earth's surface.

Answer

a. The horizontal direction toward the magnetic North Pole.

Exercise \(\PageIndex{5}\)

What does "apparent polar wander" describe?

a. The north magnetic pole dances around over time, and every continent gets its own north magnetic pole.

b. From the perspective of fossil magnets preserved within stratified rocks on a given landmass, over time the relative position of the north magnetic pole changes, sometimes closer, sometimes further away, sometimes this way, sometimes that way. "Apparently," the pole is wandering, from the perspective of a stationary continent. Landmasses aren't actually stationary, though so really the continent is doing the wandering.

Answer

b. From the perspective of fossil magnets preserved within stratified rocks on a given landmass, over time the relative position of the north magnetic pole changes, sometimes closer, sometimes further away, sometimes this way, sometimes that way. "Apparently," the pole is wandering, from the perspective of a stationary continent. Landmasses aren't actually stationary, though so really the continent is doing the wandering.

Exercise \(\PageIndex{6}\)

In the geologically recent past, what have all continents' apparent polar wander paths come closer to?

a. The South Geographic Pole

b. The Mid-Atlantic Ridge

c. Middle Earth

d. The North Magnetic Pole

Answer

d. The North Magnetic Pole

Exercise \(\PageIndex{7}\)

What are the "stripes" of normal and reversed paleomagnetic orientation in the oceanic crust parallel to?

a. Deep sea trenches

b. The San Andreas Fault

c. The oceanic ridge system

d. Mountain belts in the continental crust

Answer

c. The oceanic ridge system