Rock Strata

Reading the stratigraphical notebook is not just a question of opening the pages.  A geological map of the world today is in effect a patch work of different environmental conditions, and the same was true at each stage in the past. The limestone was forming in one place a hundred million years ago does not imply that limestone was forming everywhere at that time. While rocks are deposited in one place, another place may be uplifted into mountains and eroded.

 There are however, gaps in the record, and things are not always what they seem. Although younger rocks are deposited on top of older ones, folding and faulting can be so intense that the younger rocks end up underneath the old. Layers can be folded up to steep angles, or the original layers may be sloping such as current bedding at a river delta or a continental slope. Thick shale beds may have take a few hundred thousand years to form and be tens of meters thick; in the same strata there may be another layer only a few centimeters thick that was five million years in the making. So thickness is no certain clue to age; neither is apparent depth of water. Sea level can change by tens of meters, land level by hundreds. Nor is present latitude much help, when continents have skated across the globe. Britain was once on the equator and there were glaciers in what is now the Sahara desert. But there are plenty of clues for geologists and with their knowledge of processes at work can compute the stratigraphical evidence from around the world to confirm their theories on geological maps.

Earth Crust

The Earth’s crust only accounts for 0.6 percent of the planet’s volume, yet we have not penetrated right through even that. Oceanic crust is 5 to 10 Km thick and made of less dense rocks, including granite and thick accumulations of sediment. The continental crust resembles the scum on the surface of a big cauldron.

The oldest regions of continents, made of material that has been piling up for billions of years, have been “cooked” by heat and pressure and are made of crystalline metamorphic rocks. At the base of the crust is a boundary that reflects seismic waves, called the Mohorovicic discontinuity, or Moho. Beneath it are the rocky slabs of lithospheres mantle, composed mostly of iron and magnesium rich peridotite, on which the cruse floats.  The more the weight laden on the crust, the lower it sinks. Mountainous areas have “roots” within the mantle which are significantly greater than the height of the mountains above. The balance maintained is called Isostasy.

The crust is divided into relatively rigid plates, some made of ocean crust, some of continents. Over geological time the plates jostled around as the convection mantle moves beneath them triggering earthquakes. Ocean crust is created where molten rock wells up along mid ocean ridges, and is destroyed where it is subsumed under continental plates. Continental crust can be roasted, stretched and split by mantle plumes, or uplifted, warped and eroded at the surface, but ancient continental cores remain intact.

It is possible to trace the history of the movements of continents’ waltzes around the globe over hundreds of millions of years. Their edges fit together like the pieces of a jigsaw; the similarities between their fossils and rock strata, the past climates they experienced and the orientation of magnetic minerals, frozen in volcanic rocks like tiny compass needles, all record their travels. Precise laser measurements taken from satellites reveal the present rate of continental drift. It is roughly comparable to the rate at which finger nails grow- a mere 4 to 5 Cm in a year across the Atlantic, 12 to 14 Cm in a year across parts of the Pacific