"I shall consider this paper an essay in geopoetry..."
"...In order not to travel any further into the realm of fantasy than is absolutely necessary I shall hold as closely as possibly to a uniformitarian approach…” - Harry Hammond Hess, "History of Ocean Basins," 1962
It all came together in the early 1960s. A number of scientists put the evidence together and concluded that mantle convection drove a process they called seafloor spreading. New seafloor was continually being created at mid-ocean ridges. Old seafloor was being destroyed at deep-sea trenches. This was the mechanism that drove continental drift.
An Essay in Geopoetry
Harry Hess was a geology professor and a naval officer who commanded an attack transport ship during WWII. Like other ships, Hess’s ship had echo sounders that mapped the seafloor. Hess discovered hundreds of flat-topped mountains in the Pacific that he gave the name guyot. He puzzled at what could have formed mountains that appeared to be eroded at the top but were more than a mile beneath the sea surface. Hess also noticed trenches that were as much as 7 miles deep.
Meanwhile, other scientists like Bruce Heezen discovered the underwater mountain range they called the Great Global Rift. Although the rift was mostly in the deep sea, it occasionally came close to land. These scientists thought the rift was a set of breaks in Earth’s crust. The final piece that was needed was the work of Vine and Matthews, who had discovered the bands of alternating magnetic polarity in the seafloor symmetrically about the rift.
The features of the seafloor and the patterns of magnetic polarity symmetrically about the mid-ocean ridges were the pieces that Hess needed. He resurrected Wegener’s continental drift hypothesis and also the mantle convection idea of Holmes.
Hess wrote that hot magma rose up into the rift valley at the mid-ocean ridges. The lava oozed up and forced the existing seafloor away from the rift in opposite directions. Since magnetite crystals point in the direction of the magnetic north pole as the lava cools, the different stripes of magnetic polarity revealed the different ages of the seafloor. The seafloor at the ridge is from the Brunhes normal; beyond that is basalt from the Matuyama reverse; and beyond that from the Gauss normal. Hess called this idea seafloor spreading.
Magma at the mid-ocean ridge creates new seafloor.
These animations show the creation of magnetic stripes of normal and reversed polarity at a mid-ocean ridge: http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A49.gif and http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A55.gif.
Since new oceanic crust is created at the mid-ocean ridges, either Earth is getting bigger (which it is not) or oceanic crust must be destroyed somewhere. Since the oldest oceanic crust was found at the edges of the trenches, Hess hypothesized that the seafloor subducts into Earth’s interior at the trenches to be recycled in the mantle. This subduction explains why the the oldest crust abruptly stops at continental margins.
- As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis.
- If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle.
- The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.
Hess could also use seafloor spreading to explain the flat topped guyots. He suggested that they were once active volcanoes that were exposed to erosion above sea level. As the seafloor they sat on moved away from the ridge, the crust on which they sat become less buoyant and the guyots moved deeper beneath sea level.
The Mechanism for Continental Drift
Seafloor spreading is the mechanism for Wegener’s drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that, over millions of years, takes them around the planet’s surface. The spreading plate takes along any continent that rides on it.
Seafloor spreading is the topic of this Discovery Education video: http://video.yahoo.com/watch/1595570/5390151.
The history of the seafloor spreading hypothesis and the evidence that was collected to develop it are the subject of this video (3a): http://www.youtube.com/watch?v=6CsTTmvX6mc&feature=rec-LGOUT-exp_fresh+div-1r-2 (8:05).
- subduction: The downward movement of the edge of a plate of the Earth's crust into the mantle beneath another plate.
- Seafloor spreading wedded together the mantle convection idea of Holmes, the continental drift idea of Wegener, new bathymetric and magnetic data from the seafloor, and made a coherent single idea.
- Harry Hess called his idea “an essay in geopoetry,” possibly because so many ideas fit together so well, or more likely because at the time he didn’t have all the seafloor data he needed for evidence.
- Seafloor spreading is the mechanism for the drifting continents.
Use this resource to answer the questions that follow.
1. Where does seafloor spreading occur?
2. What is the average elevation of the ocean ridges?
3. What are the characteristics of the seafloor near these ridges?
4. Explain why a ridge exists.
5. How fast is the spreading occurring?
1. How does the pattern of magnetic stripes give evidence for seafloor spreading?
2. How does the topography of the seafloor give evidence for seafloor spreading?
3. How does seafloor spreading fit into the idea that continents move about on Earth’s surface?