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A beach walk into deep time

If you walk Kiritehere Beach, just south of Marokopa on the west coast of New Zealand, you come across rocky outcroppings of the Kawhia syncline.
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If you walk Kiritehere Beach, just south of Marokopa on the west coast of New Zealand, you come across rocky outcroppings of the Kawhia syncline. Within the rocks - and sometimes fully exposed - are the fossil remains of Triassic shellfish from around 200 million years ago.

Our lifespan is such a short period of time, we rarely notice the world changing significantly but in these rocks is evidence of life from a time we can only imagine. We might not notice the rise of mountains, the disappearance of seas, or the shifting of the continents but an ancient sea floor forming a rocky coastal outcrop puts things in perspective.

Geological time has a way of boggling the mind. All of recorded human history is encompassed in the last 5,000 years or so. This is a blink in the eye of the Earth as a whole - nothing compared to the 200 million year old fossils found strewn across a rocky beachhead.

One of the great accomplishment of the last century is the development of the scientific tools and expertise which allows us to look at the history of the world with fresh eyes. The age of rocks, for example, can be measured using isotopic analysis, mineralogy and stratification. Geologists can literally read the history of the world by staring at an exposed cliff face and analyzing its content.

Or by simply picking up a rock on Kiritehere Beach.

Since the mid-1800s, collecting geological samples and developing this understanding of Earth's ancient history has shifted from the amateur scientist to the professional geologist. And while we have learned a great deal, much remains to be discovered.

But some of the things we have learned tell us a great deal about the evolution and development of life on our planet. For example, in a search for iridium in rock samples traversing the Cretaceous-Tertiary boundary, Luis and Walter Alvarez discovered a narrow peak of extremely high concentration. Its position in the rock coincided with the mass extinction which took place at the time.

The layer of iridium is so thin, it represents a span of time less than 100,000 years and is a perfect marker for the disappearance of many forms of life. Many creatures, of which dinosaurs are the most prominent, disappear from the fossil record across this thin strip of rock. No one has found any remains in younger rock strata.

The iridium signal has been found in locations around the world at the same age. Clearly, the source of the iridium represented a global event. More to the point, the elemental composition pointed to an extraterrestrial source.

With the discovery of the Chixculub impact crater near the modern Yucatan peninsula, the rock record provided an explanation for the extinction of the dinosaurs and approximately 75 per cent of all species on the planet. A killer asteroid impacted Earth 65 million years ago leaving its footprint in the geological record.

But it wasn't the only significant geological event at the time. It has been hypothesized that the impact of the asteroid could have had a rippling effect through Earth's mantle resulting in a magna plume and the eruption of the Deccan Traps in what is now southeast India.

Such a dual event - impact followed by massive volcanism - seems a more likely scenario as the cause of the mass extinction because most other large impact craters do not correlate well with mass extinction events. It would seem a double whammy is necessary.

Geology might also provide answers for the evolution of species. Thirty million years before the shellfish of Kiritehere Beach lived and died, the Earth appears to have entered into the Carnian pluvial episode. After millions of years of dry climates - possibly a result of the super landmass Pangaea - the Earth's climate shifted to a wet period lasting between one and two million years.

The evidence is in the fossil record as nearly any place geologist look for rocks of that age, they find signs of weathering and a wet Earth. The Carnian pluvial was first noticed by Alistair Ruffell and Michael Simms. They first published their analysis in 1989 but after years of academic obscurity, the episode has become a major focus of research.

The reason is that the Carnian pluvial may have kickstarted the evolution of a group of early reptiles giving rise to two major groups of dinosaurs - the ornithischians, from which developed Stegosaurus and Triceratops, and the saurischians, which include huge long-necked species such as Brachiosaurus and theropods such as Tyrannosaurus rex.

Dinosaurs weren't the only creatures impacted by the wet conditions. Reef-building corals and marine plankton evolved into more modern forms. The period might have also seen the emergence of the first mammalian species.

Geological history is long and not easily read but when we can read it, it tells us a lot about life on Earth.