A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and deep trenches. Mid-ocean ridges are part of chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the longest mountain chain on Earth. These ridges are spreading centers or divergent plate boundaries where the upwelling of magma from the mantle creates new ocean floor.
Deep-sea trenches are long, narrow basins which extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle.
Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle.
A test of the hypothesis of sea-floor spreading was provided by studies of the Earth’s magnetism.
Age of the sea floor:
- The age of the sea-floor also supports sea-floor spreading.
- If sea-floor spreading operates, the youngest oceanic crust should be found at the ridges and progressively older crust should be found in moving away from the ridges towards the continents. This is the case.
- The oldest known ocean floor is dated at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea trenches.
- Magnetic surveys over the ocean floor in the 1960s revealed symmetrical patterns of magnetic “bands,” (zebra stripes)anomalies parallel to midoceanic rifts .
- The same patterns in relation to midoceanic rifts are present in different oceans.
- The magnetic anomalies coincide with the episodes of magnetic reversals that have been documented from studies on land, indicating that the andesitic rocks that form new oceanic crust in the tensional setting of the rift valley record the earth’s magnetic field as they cool.
- A rock has a normal (positive) polarity when its paleomagnetic field is the same as the earth’s field today.
- The positive magnetism adds to the earth’s magnetic field and creates a higher magnetic measurement at that location.
- Rocks are negatively polarizedwhen the earth’s field is reversed, which reduces the earth’s net field strength.
- Since the ages of these anomalies are known from dating the paleomagnetic reversals on land, the rate of movement of the ocean floor can be calculated.
- The fact that new ocean crust moves away from the midoceanic ridge at speeds that range from 2 to 10 centimeters per year has also been documented using satellite measurements and radar. For example, if it is known that a segment of sea floor that formed 10.0 million years ago is now 50 kilometers (5.0 million cm) away from the crest of the ridge, it can be calculated that it traveled that distance at about 2 centimeters per year.
- By using the calculated ages for episodes of paleomagnetic reversal, scientists can construct sea floor age maps, which confirm that the youngest oceanic crust is presently being formed at midoceanic ridges and that the oldest is about 150 to 200 million years old, or late Jurassic in age.
- This older material is the farthest from the spreading centers and is the next crust to be subducted.
- Sea floor age maps have been proven correct by the age dates calculated from hundreds of rock samples gathered from the ocean floor.
- More proof for sea floor spreading comes from seismic studies indicating that earthquakes occur along the rift valley of a midoceanic ridge and the cross‐cutting fractures that offset it.
- Rift valley earthquakes occur only along transform faults, those portions of the fracture zone located between the offset sections of a ridge and rift valley.
- Because of the way in which the sea floor spreads (that is, away from both sides of a midoceanic ridge), transform faults are the only areas along the fracture zone in which sections of the oceanic crust pass one another in opposite directions.
- The concentration of earthquakes in the transform‐fault sections of the fracture zones further supports the concept of ocean crust moving away from a midoceanic ridge.
Modern plate tectonic theory:
- By the 1960s, the theories of continental drift and sea floor spreading were supported by reliable scientific data and combined to develop modern‐day plate tectonic theory.
- The theory maintains that the crust and uppermost mantle, or lithosphere, is segmented into a number of solid, rigid slabs called lithospheric plates.
- These slabs move slowly over the asthenosphere, the 200‐kilometer‐thick zone of more plastic mantle material that underlies the plates.
- New oceanic crust is created at the crests of the midoceanic ridges and pushed laterally away by new accumulations of crust. It begins to cool as it moves away from the high heat flows at the ridge.
- By the time it is subducted at the convergent boundary with another plate, it is cold and dense enough that it begins to sink back into the mantle.
- Subduction is also probably a function of a down‐turning mantle convection current below the converging plates.