From the time maps of the globe became available, people wondered about the arrangement of the continents and oceans. Hundreds of years later, valid explanations were constructed.
Leonardo da Vinci and Francis Bacon wondered about the possibility of the American and African continents having broken apart, based on their shapes. This thinking continued up into the early 20th century, to a meteorologist named Alfred Wegener.
Wegener revived the early idea of continental drift, contending that all of the present-day continents were connected, side-by-side, as long ago as the Carboniferous (~300 Myr). He called the supercontinental mass Pangaea, Greek for ‘all lands’.
Continental Drift: Fossil Evidence
Mesosaurus: purely freshwater reptile Glossopteris: seeds too large to be effectively wind-transported
Continental Drift: Glacial Evidence
Large ice masses carve grooves in the rocks over which flow. Such masses tend to flow outward (generally downhill) from a central locality.
Continental Drift: Rock Ages
Even before geochronology, the relative framework of rock ages showed strong correlation across the Atlantic, as did mountain ranges of similar age.
Mechanism of Continental Drift?
Wegener never lived to see the general acceptance of continental drift, largely because of the lack of a mechanism. Wegener considered the buoyant continents to be ‘plowing’ through the mantle, resulting in mountain belts on continental edges.
Beginning just after Wegener’s end, Arthur Holmes began to describe mantle heat flow in terms of convection. Deep materials, hotter than their surroundings (and hence buoyant), would tend to flow upward. In approaching the cool surface of the Earth, the material would lose its thermal energy, cool and sink, having lost buoyancy.
The motion of mantle material put into action by convection thus becomes a plausible mechanism for moving rigid pieces of the crust over some more actively flowing mantle material.
Materials that can flow tend to lose thermal energy by the convection process. This explains circulation in a pot of water that is being heated from below in the same way it describes the cooling of the Earth.
Harry Hess and Marine Geology
From the 1940’s to the 60’s, Harry Hess made many key intellectual contributions to the coming revolution
in geologic thought:
echo-sounding of sea floor revealed deep sea features like guyots and seamounts, and the topography of mid-ocean ridges
ridges are areas of high heat flow and volcanic activity
young age of ocean floor, based on thickness of sediment
He also speculated that the continents did not plow through ocean crust, but that the two are linked and move as a unit.
Topography and Age of the Sea Floor
As ocean crust ages, it cools and is less buoyant. The cool mantle root on this crust helps pull it down into the mantle, resulting in deeper sea floor progressively away from the ridges.
Harry Hess and Sea Floor Spreading
Hess rationalized all of his observations into a system linked by the old Holmes concept of mantle convection.
He conjectured that hot material rose at the oceanic ridges, thus explaining the high heat flow and basaltic volcanic activity, and why the ocean floor is bulged up at the ridges.
The logical next step is that where continent and ocean meet, at the trenches, ocean crust is being returned to the mantle at the same rate it is being generated at the ridges.
Sea Floor Spreading
Hess combined his observations with the earlier ideas of Wegener and the mechanism of Holmes into the concept of sea floor spreading, which lead to plate tectonics.
This hypothesis makes a number of testable predictions.
Earth’s Magnetic Field
The Earth has an invisible magnetic field, which has been critical to the earliest nautical navigation: all free-floating magnets at the Earth’s surface point to magnetic north.
Iron-rich minerals crystallizing from molten rock will orient towards magnetic north when they cool below the Curie point, the temperature above which permanent magnetism is impossible (580oC for magnetite). Thus lavas lock in the record of Earth’s magnetic field when they form.
How do we measure the ‘magnetism’ of a rock? Magnetic Reversals
Interestingly, the polarity of the magnetic field shifts every 0.5 – 1.0 Myr. That means rocks formed over time will record either ‘normal’ magnetic orientation (like today), or ‘reversed’. Since this is a global phenomenon, these changes can be used for global stratigraphic correlation.
Taking magnetic stratigraphy back in time is paleomagnetism.
Geomagnetic reversals • MECHANISM
How does the field reverse?
currents in outer core slowly change direction
new computer model demonstrates how
- currents flow and field reverses
- field weakens and loses dipolar form while changing direction
Geomagnetic reversals • CONSEQUENCES Effects of a future reversal
- solar wind will hit Earth more strongly
- increased radiation will cause greater skin cancer
- Disaster is unlikely—Earth has survived countless reversals in the past
Geomagnetic reversals • ANOMALIES
- worldwide data shows a reversal around 10,500 B.C.
- some data from same time shows no reversal
- coincides with mass extinction and end of ice age
Paleomagnetism on the Sea Floor
An amazing discovery was made when the magnetic profile of the sea floor around the Mid-Atlantic Ridge was mapped.
The maps showed parallel magnetic ‘stripes’ that were perfectly symmetrical across the ridge axis. Colored stripes represent rocks with present-day magnetic orientations (‘normal polarity’), grey represents rocks with reversed polarity.
Paleomagnetism and Sea Floor Spreading
Vine and Matthews interpreted the magnetic stripes as products of steady creation of new ocean crust over geologic time, supporting the hypothesis of Hess.
Magnetic Field: Direction and Inclination
Rock magnetism has two components: the direction of magnetic ‘pointing’ and the inclination of this with the Earth’s surface. Magnetic inclination goes from nearly horizontal at the equator to vertical at the magnetic pole.
Magnetic North vs True North Thus, magnetic records give an indication of where the rock was on the surface when it was magnetized. Magnetism and Wandering Continents
Another key contribution to the geology thought-revolution came from paleomagnetic studies on the continents. It was noticed that the magnetic pole positions indicated by rocks of known age were not constant.
If magnetic north remained in an essentially similar position over Earth history (despite the periodic polarity changes), then the different magnetic orientations meant that the continents had moved.
These results showed that some rocks on continents currently at equatorial positions had occupied high latitudes in the past.
Apparent Polar Wander Paths
The Key Features of Plate Tectonics
The Earth’s crust is constantly being created
and destroyed (recycled).
Ocean crust, formed at divergent margins, is mafic and dense.
As ocean crust ages and cools, its great density relative to the continents results in subduction as plates converge.[As a result, old ocean crust cannot persist, whereas old parts of the buoyant continents can survive for eons.]
The other kind of plate margins, transforms, are parallel to the current motion of the plates.
Testing Plate Tectonics
Like any theory, plate tectonics has been rigorously tested, and from a startling array of disciplines.
This model is consistent with the key tests thus far, including:
- sea floor spreading
age structure of the sea floor and continents
locations and focal depths of earthquakes
Mechanisms of Plate Tectonics: Mechanisms of Plate Tectonics: ———————————————————————————————————— Adam C. Simon Ph.D., University of Maryland, 2003 Research Associate Department of Geology University of Maryland College Park, MD 20742 p: 301 405 0235 f: 301 314 9661 e-mail: email@example.com Credits
Some of the images in this presentation come from: Plummer, McGeary and Carlson, Physical Geology, 8/e; Hamblin and Christiansen, Earth’s Dynamic Systems, 8/e; Press and Siever, Understanding Earth, 3/e; Paul Tomascak (University of Maryland)