Geological Time Periods

Plate tectonics affected by mantle hot spots

The work relied on tracking plate motion using magnetic readings. The Earth’s magnetic poles commonly (in terms of geologic time) reverse themselves, with the magnetic north pole spending time at the geographic south pole. This orientation is captured by crystals of magnetite that are locked in place as magma solidifies into rock. 

As a result of this process, stripes of normal and reversed polarity run parallel to the spreading centers on the ocean floor, where magma solidifies into new crust. Since the whole planet experienced the same magnetic events, variations in the width of each individual stripe tell you how much rock crystallized during that time period, which in turn tells you the velocity of plate motion at spreading centers.

The researchers used this information to build a more detailed timescale of the plate motions near Africa and India around 60 million years ago. At that time, Africa was rotating north toward central Europe, and India was an island continent on a collision course with Eurasia. When the significant events of this period were put on a single timeline, some remarkable coincidences emerged.

Around 67 million years ago, a mantle plume reached the surface, punching through southern India and causing an absolutely massive eruption that formed the Deccan Traps, a region of lava flows up to 2 kilometers thick that now cover an area of 500,000 square kilometers (stop for a moment and think about how much rock that is). At the same time, the northward motion of the Indian plate accelerated, more than doubling to a blistering (again, geologically speaking) 18 cm/yr. While this was going on, the velocity of the African plate slowed considerably.

Taken on their own, each of these observations could simply be coincidental with the time of the Deccan eruptions, but together they build an interesting case. Not only did these events all occur roughly 67 million years ago, they all ended around 15 million years after as the plates returned to their previous velocities and the Deccan eruptions ceased. 

The researchers also found that changes in spreading rates are centered on the position of the mantle plume. It appears that the motion of the plume alters plate velocities by exerting force from below. The physics of this interaction have proven difficult to model, so there seems to be some uncertainty about whether the plume could have provided enough force to fully explain the observed changes in plate velocity, but the basic concept seems solid.

Oblong Land Conservancy Blog: The Power of Ideas

The idea here is not so much to take up the point about conversation but rather to consider the power of ideas. In the dim and distant past it was believed that the world was flat. That was an idea that ruled the times for centuries and now we know differently. Tom Friedman may not agree but, of course, he is looking at the planet from a different perspective. Another idea that ruled the times was that the planets revolved around the Earth. Copernicus replaced that idea with what we now know to be the situation - we and the planets revolve around the Sun. History is littered with such examples and the remarkable thing about some of these ideas is their power to occupy the mind and prevent it from seeing something as it really is. One might think of it as a form of mass chronic denial that for a period of time huge numbers of people can completely deny the fact that the idea in question just does not stand up to critical analysis. One such idea that seems to hold sway today is that our activities are not having a material and/or deleterious impact upon the planet. One can think of this in various ways. The global population is expanding. It is estimated to be just shy of 7 billions now and could grow to 9 billions by 2050. Given that the planet is a finite resource this presents challenges as to how we will feed ourselves, where sufficient fresh water will come from, how we will manage the demands for energy including that from a depleting fossil fuel resource and a whole host of other issues. And we didn’t even mention climate change or global warming yet. In last week’s The Economist magazine there was a rather sobering briefing entitled The Anthropocene. The thrust of the article is simply that we now appear to have moved out of a period of relative stability in the way that the planet works. This is the province of something called Earth-systems science and it examines how the planet has “worked” over very long periods of time, geologic time measured in the hundreds of millions of years. The notion is that we are now in the Anthropocene – the age of man – and we are not just spreading over the face of the planet but changing the way that it works. Some of us will be familiar with Dr. James Lovelock and the Gaia Hypothesis. In simple terms this suggests that the Earth is a complex, living, self-regulating organism that operates to maintain the conditions for life on the planet, an homeostatis if you will.

Earth

The rotation period of the earth in early geological time

Abstract of ...... lecture notes on geological time periods

Philosophical magazine, a journal of theoretical, experimental and applied physics

Report of the Committee on the measurement of geologic time