How did scientists puzzle this story together?

Nobody has ever drilled a hole into the Earth's mantle (the deepest drill hole is about 10 km). Much of the structure under ground was gained from the quest for oil. Geologists detonate explosives to 'ping' the earth, like a depth sounder would. Arrays of microphones receive the reflected sound, while computers construct a picture from these signals. Underground nuclear tests have provided geologists with a wealth of opportunities. Waves also radiate out from earth quakes and these are very strong at times, while originating at depths, not reachable by humans. All around the world a network of seismic stations has been built to record earthquakes and detonations from nuclear weapons. 

It is known that sound travels faster in a denser medium. From measurements of the speed of sound, the densities of the various layers could be calculated. Underneath the continental crust, the Mohorovicic Discontinuity or Moho (Yugoslav geologist Andrija Mohorovicic, 1906) was discovered, marking the boundary between the solid crust and the upper mantle at about 40km depth.
In 1926, the German scientist Beno Gutenberg, discovered another discontinuity, about 150 km deeper down under continents and 70 km under the sea, marking the boundary with the deeper plastic mantle. It is now accepted that the tectonic plates extend 70 km deep, the lithosphere (Gk lithos = stone),  which float on a softer mantle, the asthenosphere (Gk asthenes = weak).

How did the continents drift?

Why the continents suddenly started to drift about 300 million years ago (The age of Earth is about 4500 million years), is a puzzle (also see box). Until that time, the continents formed one land mass, named Pangea (Greek pan, pas = the whole; geo = earth). The first split between the northern half, now named Laurasia (Europe and Asia), and the southern half, named Gondwanaland (South America, Africa, India, Antarctica, Australia and New Zealand), began 300 million years ago (300Mya). It is possible that the gap between the two continents was sufficiently large to form the first circumglobal sea, allowing ocean currents to travel around the world. In any case, the seas that formed between the continents, were warm and rich in nutrients and marine life, laying down all the mineral oil we now mine.

On the map one can see how the continents travelled, propelled by ocean plates that have changed shape enormously. The red curves are areas of collision, whereas the blue lines are those where the seafloor was spreading. Where the continents collided, high mountain ridges were formed, such as in Europe (colliding with Africa): The Pyrenees, the Alps and more. An interesting case is India which travelled all the way north to collide with Asia, forming the Himalayan mountains. Australia broke away from Antarctica at a later stage, pulling New Zealand with it. But how did scientists puzzle this story together?

The secret lies in a weak property of rock: its magnetic field. Inside many rocks is found the element iron, a very common element on this planet. Iron and some of its oxides can be magnetised by the magnetic field of Earth. This won't happen as long as the rock is liquid, but by the time it cools sufficiently to become rock, the Earth's magnetic field is 'frozen' in place. Geologists drill deep sampling cores and analyse the magnetic field orientation. To make matters worse and easier at the same time, the magnetic poles have reversed several times and they have been wandering around somewhat. So the data needs to be corrected by what is known about the oscillations in the Earth's magnetic field. But at the same time, the field reversals are also convenient time markers to age the layers in the core samples.

The continents may have been drifting much longer New evidence with respect to tectonic plate movements, suggests that the continents may always have been drifting apart, then together again. When continents collide, they form folded mountain ranges. A number of such ranges could be explained only by assuming that the continents have collided once or twice before. A difficulty in studying old mountain ranges is that they have all but completely eroded away. But as scientists are drilling more holes, their knowledge pieces together the continental crust movements dating back to 500 million years. In the drawing of the world map with all continents joined together, the red mountain ranges could be explained by subduction of ocean plates, uplifting the continents at their margins. Yet some of these ranges are folded extensively. The purple ranges are all folded and are thought to have occured by the continents colliding towards the Pangea configuration. On left the mountain range from Ouachita belt, through Appalachian belt and the Caledonian belt over Iceland, Scandinavia and Greenland. The Hercynian belt runs through  the Pyrenees and the Alps. The Uralian belts criss-cross through Asia and Siberia. This map shows continental movement back to 500 Mya, where the history of Pangea began, when continents were dispersed around the Iapetus Ocean. About 400 Mya, Laurentia (America) collided with Baltica (Europe) to form the super continent Laurasia. In the process the Apalachians, Iceland, Scandinavia and Greenland mountain belts were formed. The Iapetus Ocean vanished when Laurasia fused with Gondwana (all the other continents), creating Pangea (300 Mya). The mountain belts of the Atlas were formed and the Urals between Siberia and Europe and the many mountain belts in Asia. The plate motions changed as Pangea dispersed. North America moved north, then west away from Eurasia (180 Mya). In a cycle of 500-600 My, the continents may have been dancing to and fro for over 1500 million years. As scientists gather more data, this puzzle may be pieced together more accurately. (See also the Geologic Time Table) (Source: J Brendan Murphy and R Damian Nance: Mountain belts and the supercontinent cycle, Sci Am Apr 1992 p34-41

A possible mechanism But what could the mechanism for such movement be? A simple explanation is shown in the four drawings on right. It is generally accepted that the crustal movement is caused by convection currents, originating from differences in temperature between spots in the liquid mantle. The Earth cools its interior by leaking heat to the outside. Where the crust is thin, such as underneath oceans, heat is lost more easily, resulting in a cooler area. But since continents are four times thicker, they also insulate better. A hotspot could appear underneath a large continental slab. As the solid mantle heats up, the coninent is bulged up and cracks. It allows a convection zone to form, pushing the broken halves apart and ending up as a spreading mid-ocean ridge. At some other place on the globe, continents ae pushed together again, and a new hot spot is formed, while the old hot spot shrinks. Continents break up again and reverse their travel.

Growth and formation of continental crust One of the remaining problems is that land erosion happens so fast, that all the continents should have disappeared below sea level, a very long time ago. Particularly in the azoic era (till 2.5 eons ago) without any cover on the land, erosion would have been very high. By measuring the very low concentrations of the rare-earth elements (EER or Lantanides, see the Periodic Table of Elements), geologists discovered that the sedimentary rocks, originating from mud washing into the oceans, represent a very good average of the composition of the continental crust. The oldest mineral (zircon) is found in sedimentary rock in Australia and is 4.2 eons old. In north-west Canada, the Acnasta Gneiss formation yielded the oldest rock (granite) that was not formed from sediment.  Now that a very large number of measurements have been made on sedimentary rocks of various ages, the actual volume of the continental crust can be plotted. In the figure on right, the bottom graph shows how the volume of crust has been growing, sometimes slowly, sometimes more rapidly. The time scale is in eons (thousand million years). Almost immediately after Earth accreted from particles and meteors, a small amount of crust was formed and during the first eon, this amount did not increase very much (erosion was very high). Only when the ocean plates started moving, was continental crust formed, first fast because the interior of Earth was hotter, then more slowly. The crust that formed in the first episode of fast growth (granite?), is different from later rocks, perhaps due to  the possibility that oceanic crust was recycled much faster than today. (due to the presence of a deep ocean??). The top diagram shows a slab of oceanic crust subducting under a continent. By pushing sediments up, the continent grows from accretion. It also acquires more volume from volcanoes. It is now thought that sedimentary rock melts, not only from friction but mainly from the amount of water it contains, which acts as a flux additive in a foundry, inducing first the sedimentary rock to melt and then the continental rock as it moves upward to the surface. Magma chambers do not always reach the surface but can convert to granite, which is included as part of the continental crust. Current oceanic crust forms mainly by the eruption of basaltic lava along a globe-encircling network of mid-ocean ridges. More than 18 million cubic km of rock are produced this way, each year. The simple concept of continents made from granite, formed when the mantle was liquid, while staying the same size, needs to be changed in the light of these new findings. Source: S R Taylor & S M McLennan: The evolution of continental crust. Sci Am Jan 1996. p76-81