In this series of articles, Utah artist J. Brad Holt talks about what artists are seeing as they look at the landscape. Holt studied geology in college and is attentive to what the rocks suggest in the scenes he paints.
In 1915 a German geophysicist named Alfred Wegener published a controversial theory on the origin of continents and oceans. The idea of continental drift was prompted by the remarkable fit of the coastlines between South America and Africa. The fit got even closer when improved mapping revealed not the shorelines, but the edges of the continental shelves. Wegener proposed the past existence of a massive super continent he called Pangaea, which began to break up around 200 million years ago. Wegener’s ideas were met with outright hostility at the time, despite the considerable amount of evidence that he brought forward. Distribution of fossil evidence on both sides of the Atlantic Ocean matched closely, as well as similarities in rock types. Especially interesting was a close match in rock types between the Appalachian Mountains in Eastern North America, with mountains in Scotland, and Norway. When the continental margins were brought together and rotated slightly, these ranges, which were of similar age, appeared to form a continuous north-south chain. Additional evidence was provided by marks of glacial scouring appearing in vast regions of subtropical Australia and India, where ice sheets had no business existing.
Wegener’s primary failure is that he was unable to propose a plausible mechanism governing the drift of the continents. He vaguely hypothesized that tidal forces caused the continents to drift about on thin oceanic crust, like colossal icebreakers, but the argument rang hollow with no evidence of plowed up seafloor ringing the continents. It was not until extensive sonar mapping of the sea floors began after the World War II, that a new picture began to emerge. It was found that lines of mid-ocean ridges ringed the planet, like the seams on a baseball. In other areas, deep ocean trenches were found, and seemed to be always adjacent to regions of seismic and volcanic activity. Deep dredging of the ocean floors turned up a couple of surprising facts: Nowhere was the ocean crust older than 200 million years–which is young to middle age in geological terms–and nowhere was the sediment on the seafloor as deep as expected.
“Coconino Sandstone, North Rim,” by J. Brad Holt, 2014, oil, 12 x 16 in.
Earth scientists began to understand that the mid-ocean ridges were spreading centers, where new seafloor was being created, and rifted apart. At the deep ocean trenches, the seafloor was being subducted back into the mantle of the earth. The seafloor and continent together comprised a single crustal plate, moving together, away from the spreading center, toward the subduction zone. The earth is divided into seven major lithospheric plates, along with half a dozen other minor plates. The plates appear to move with an average speed of two inches per year.
Boundaries between plates are of three types. Divergent boundaries are where new crust is being created by rifting at the mid-ocean ridges, and at a few rare continental rifts, such as East Africa. Convergent boundaries are where crust is being subducted back down into the earth. As this occurs heat, stress, and the introduction of a great deal of water create partial melting of the rock. These magma bodies rise to create igneous intrusions, or surface volcanism. The so-called Ring of Fire around the Pacific Ocean basin follows an extensive line of subduction zones. Occasionally, areas of continental crust collide. One such collision between the African plate and the Eurasian plate resulted in the creation of the European Alps. The Himalayan Mountains were created by the collision of the Australian-Indian Plate, and the Eurasian Plate. In some convergent boundaries, slivers of oceanic crust, island arcs, or even pieces of continental crust are scraped off and deposited at the plate boundary, while subduction continues to occur beneath. These additions to the edge of a continent are called terranes. The western sixth of North America is composed of such terranes. There are also many boundaries where the plates slide past one another. These are called transform fault boundaries.
“Kinesava From the Anasazi Plateau,” by J. Brad Holt, 2014, oil, 16 x 20 in.
Strong evidence in support of the theory of plate tectonics is provided by paleomagnetism, and hot spot volcanism. When molten rock cools it preserves a sort of fossilized imprinting of the direction of the earth’s magnetic field. Complete polar reversals of the magnetic field have occurred about every million years of earth’s history. By drilling core samples throughout the ocean floor, geophysicists have compiled maps showing magnetic reversals in the rock of the seafloor. These maps reveal bands of similar polarity spaced on either side of the mid-ocean ridges, like mirror images. The sea floors are like broad conveyer belts of rock, moving away from the spreading center in opposite directions, with each side carrying fossilized evidence of what the magnetic field was like at the time the rock solidified. Hot spot volcanism occurs within plates, in both continental crust and ocean crust. The Hawaiian Island chain is an example of this phenomenon. The oldest island is Kauai, at about 4 million years, the youngest is the main island, which is less than a million years old, and of course lava is still erupting there. The chain continues beneath the surface of the Pacific, all the way to Midway Island, which is dated at around 27 million years of age. A plume of heat from the mantle of the earth continues to generate these volcanic islands. The chain is generated as the Pacific Plate moves over this plume, with new islands being formed about every million or so years. The Yellowstone region in North America is another example of a mantle hot spot.
So why do the plates move? The precise dynamic is not known, but it is theorized that the subduction of cold dense oceanic crust helps initiate a convection current within the mantle of the earth. This convective motion is strengthened by rising heat along the spreading centers. More complex models with zones of opposing convections at several depths within the mantle have been proposed. What is generally agreed upon is that the subduction of ocean floor creates pull, which helps sustain rifting on the opposite side of a plate. Once initiated, the upwelling and solidifying of basalt at the ocean ridges generates a sustaining force of its own. Together this pull and push combine into the dance of the plates, with the force to tear continents apart, and rear mighty mountain ranges five miles into the sky.