The Earth Story

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Splash zone 💦

Cracking It

Essentially, sedimentary rocks can handle being squeezed together (compressed) — not surprisingly: piled up on the sea bed, sandstones and limestones have already been squashed for millions of years. Even the bottom block of a large pyramid or column at Karnak is not going to be under as much weight as it was hundres of metres below the bed of the Tethys ocean or Nile delta. However, while sedimentary rocks take readily to compression, they can’t stand tension (being pulled apart). If a layer of sandstone or limestone is forced to bend, the material on the inside of the bend undergoes compression while the outer edge is stretched. It takes only a small amount of tension on that outer edge to prise open the ‘glue’ between the rock grains. When that happens, cracks appear — just as, on a rather smaller scale, a chocolate bar will show a set of breaks forming on the stretched side surface when it is bent. Even though the compressional side of the layer is perfectly comfortable with the stresses imposed on it, the layer as a whole snaps, creating a tension crack, or joint.

Like a furrowed brow, joints are the rocks’ first outward sign of strain. They are the simplest and most common geological structure and occur in virtually all rocks, but particularly well-layered sedimentary rocks. Stresses from distant land movements and collisions can ripple for hundreds or even thousands of kilometres through such rigid strata, forcing the rock layers to flex, and consequently crack. In the layers the Egyptians used for building stones, the cracks are roughly every two or three metres. Just the right size, it would seem, for the stonemasons to extract ready-made building blocks.

The trouble was that what happens in nature also happens in buildings. The tendency for sedimentary layers to bend and crack under tension put severe limitations on what the Egyptians could build. For a start, roofs were a problem. To make the pillars of the Great Hypostyle Hall, the Egyptians cut the sandstone into blocks, which they rounded off into discs. Then, by stacking the discs on top of each other, they created huge tall columns. Across the top of the columns they placed great sandstone roof beams. At either end of the stone beams the weight of the rock was held up by the columns, but between then the slab sagged under the weight of gravity, forcing it to bend and the roof slab to crack. To get around this, the Egyptian builders were forced virtually to fill the hall with giant pillars to support the heavy roof slabs that would have once covered the whole ceiling. The dense hall of columns of the Great Hypostyle Hall in Karnak is supposed to represent a forest, or primeval swamp of papyrus, but it is difficult to see how else the builders could have constructed it with the limitations of the sandstone blocks to hand.

By pushing the properties of sedimentary rocks to the limits, the Egyptians had established a new fundamental way of holding up buildings. They were the first to get away from simply stacking stones on top of each other and, in doing so, they had invented one of the classic architectural forms, the post-and-lintel. This combination of a pair of vertical columns capped by a horizontal cross beam allowed them to start building roofs. Four and a half thousand years later we can still admire their achievements.

~ JM

Image Credit: http://bit.ly/1y7rmZq

More Info: The Great Hypostyle Hall: http://bit.ly/1F1ESQD

What is Sandstone: http://bit.ly/1DOkO2K

Post-and-lintel: http://bit.ly/1DCa9cV

Rock Joint: http://bit.ly/1F1INgm

Bruthans, J., Soukup, J., Vaculikova, J., Filippi, M., Schweigstillova, J., Mayo, A. L., ... & Rihosek, J. (2014). Sandstone landforms shaped by negative feedback between stress and erosion. Nature Geoscience, 7(8), 597-601.

In this phenomenal capture by Roberto Giudici, we see four waterspouts over the Adriatic Sea.

A sister of the tornado, waterspouts are generally less powerful. They occur when high layers of cool air blow across a body of water while warm moist air sweeps up from below. They appear as thin columns with the funnels sucking up water. Waterspouts can vary in size from a few feet to more than a mile in height, and from a few feet to hundreds of feet wide.

These water twisters can move anywhere from 3 to 130 kilometres per hour (km/hr). Winds within the waterspout can spiral around at 97-195 km/hr.

Waterspouts, like their land counterparts, can pick up and transport some interesting objects. They have sent showers of tadpoles in New York, and even toads in France. One in Providence, Rhode Island, rained fish down on the people, who promptly collected and sold them!

-Jean

For a video of a waterspout from this week, see here:  https://www.youtube.com/watch?v=v2cIWi0kjWc

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