Buried sandstone cements itself by dissolving the points where its grains touch
Under kilometres of sediment, sand grains press so hard against each other that the strained contact points dissolve. That dissolved mineral then settles into the open pores and glues the grains together. This slow chemistry turns loose sand into sandstone, a rock that also stores groundwater and oil.
Sandstone is a clastic sedimentary rock: sand-sized silicate grains, between 0.0625 and 2 mm across, bound by a mineral cement. It makes up about a fifth to a quarter of all sedimentary rock. Most grains are quartz or feldspar, the common minerals that best survive weathering at the surface, and trace impurities can colour the rock tan, red, pink, grey, white or black. Striking cliffs have tied some colours to places, such as the red rock of Arches National Park in the American Southwest.
The grains start as weathered bedrock, eroded fastest in rugged terrain such as volcanic arcs, rift zones and mountain belts. Rivers and wind carry them to basins where tectonic movement has made room for sediment, from forearc basins to grabens along continental margins.
Burial begins a three-stage transformation called diagenesis. In eogenesis, within a few tens of metres of the surface, burrowing creatures stir the sand and minerals change; the red hematite of red-bed sandstones probably forms here. Deeper, during mesogenesis, most compaction and cementing happens. Grains shuffle closer, soft mica flakes bend and pore space shrinks. Mechanical squeezing works mainly above 1,000 metres, chemical compaction by pressure solution continues to about 2,000 metres, and most cement is laid down between 2,000 and 5,000 metres, where heat speeds the process.
When erosion later strips away the overlying rock, telogenesis follows: fresh rainwater can dissolve some cement and open new pores. That porosity is what makes sandstone valuable, since fluids percolate through it and it can hold large volumes of water or petroleum. Squeeze quartz sandstone hard enough in the collision zones of mountain building, and metamorphism converts it into quartzite.
Source: Sandstone
