To learn about horizontal compression and its effects on rock, roll the cursor from left to right over the blue buttons.

A slice through the center of the Earth reveals a rigid, rocky shell, called the 'lithosphere', that overlies a hot, slowly churning interior layer called the 'middle and lower mantle'. The middle and lower mantle is solid rock, but because it is very hot, it flows like tar on a mid-summer day.

The movement taking place in the middle and lower mantle exerts a force on the overlying lithosphere, subjecting it to horizontal compression in some places, to tension in others.

These horizontal compressional and tensional forces cause the lithosphere to break into large pieces. Geologists call these pieces 'plates'. Propelled by the forces exerted on them by the churning deep mantle, the plates move with respect to one another. This geological theory of plate movement is referred to as 'plate tectonics'.

In regions of compression, the plates move towards each other; in regions of tension, they move away from each other. Where the plates move towards each other, one plate is forced under the other.

Let's look in more detail at what happens in a region before and after it is affected by horizontal compression. We shall expand our view of the area within the black rectangle.

Notice that in this location, the lithosphere has several parts: continental crust (green),oceanic crust (grey), and upper-most mantle (brown). The ocean water is shown in dark blue. A wedge of sediment (mostly sand and mud) that has been eroded from the continent and deposited on the adjacent floor of the ocean is yellow. The layers of the sediment wedge are gently inclined seaward. As yet, the lithosphere is not affected by compression.

Now, compression has becomeactive. The lithosphere has broken into two plates and one plate has been forced down ('subducted') under the other plate. Where the two plates are colliding, the compressional force has bowed down the ocean floor,creating an oceanic trench. The trench may be as much as 7 miles deep, two to three times the depth of 'normal' ocean floor. Melting takes place just abovethe upper part of the descendingplate. The molten rock (pink) moves upwards, some of it reaching the surface to formvolcanoes. As the plate on which the continent is located moves laterally and then down (subducts), the continent is carried towards the oceanic trench. When continental crust reaches the trench, subduction is halted: low density continental crust resists being thrust down into regions of higher pressure. Lateral platemovement, however, temporarily persists, and the continent-bearing plate rams into the plate bearing the volcanoes. Caught at the plate margin, the sedimentary wedge is compressed, shortened and thickened. The layers of which it is composed are massively deformed.

An enlarged view permits acloser look at what happens to the wedge of sediment. In thisdiagram, the sediment wedge has not yet reached the oceanic trench. The sediment continuesto accumulate as gently inclined layers. With time, the deeper layers of sediment turn into sedimentary rock. The sense of plate motionis indicated by the red arrow.

The sedimentary wedge reaches the oceanic trench. As the continent-bearing plate begins to try to descend beneath the other plate, the wedge, caught in the jaw-crusher between the plates, shortens and thickens, breaking into slices that ride upover one another. The elevatedsurface forms a mountain range. Because the earth's temperature increases with depth, the layers towards the bottom of the deformed wedge are heated up and thoroughly transformed ('metamorphosed'). The sedimentary rocks have become 'metamorphic' rocks.

Here we can see that the sediment layers have been bent into a series of complex folds. As part of this process, the sediment has been turned into rock. The rock, having been subjected to high pressure and temperature, no longer resembles the sediment out of which it was originally composed. It has been radically changed (metamorphosed).

A closer look will now be taken at the material within the red rectangle.

Here we can see the complexfolds developed as the deeperlayers were subject to heat and pressure. The material bearslittle resemblance to the original sedimentary layers.

Rocks exposed in parts of New York City display complex folding, like that developedwhere rock materials havebeen deformed and altered (metamorphosed) byconverging plates. These metamorphic rocks are found along the shore at Orchard Beach in Pelham Bay Park, in the Bronx.

So that they maye be seen more clearly, some of the folds have been outlined in yellow.

Brooklyn College geology students are seen here examining complexly folded metamorphic rocks on a field trip to Central Park.

David J. Leveson