The most common Welsh rocks are sedimentary rocks, i.e.
collections of
mineral and rock grains or organic fragments. Typical sediments are
sand and
mud, since hardened to sandstone and mudstone (or shale
if
finely layered), laid down on the floor of the ocean. The sandstones
are made
of rounded grains of quartz and other minerals, commonly a millimetre
or so in
diameter. The rocks are typically grey in the fresh cliff exposures,
though
they weather to a light brown colour. Some sand and mud formed on the
land, in
floodplains beside rivers or in deltas. Many of these rocks formed when
Gravel, now hardened to conglomerate, generally formed as ancient beach deposits. The pebbles may be up to five centimetres in diameter, typical of those found on beaches, but a few that form as scree near an ancient cliff may be much larger and are generally angular.
Limestone is another variety of sedimentary rock. Limestones are generally light grey or yellowish brown rocks, the collection of the remains of shelly animals. However, many limestones are collections of tiny animals that are too small to see; you may have to look closely to see larger fossils. The shells are made of the mineral calcite that is much softer than quartz, so if you scratch the rock with your car keys you will see a white streak where the key crushed the soft rock. Scratching sandstone leaves a metallic grey streak from your key on the rock--quartz is harder than steel.
Bedding is the common structure of sedimentary rocks. Layers of different rock types, such as sandstone alternating with mudstone, are called beds, and bedding planes separate the beds. Beds are usually horizontal when the rocks are laid down and so show how much the layers are tilted during folding. Exceptions are in deltas and dunes where cross-bedding is at the angle of rest of grains accumulating on the dune or delta face--typically at thirty degrees to the horizontal.
When the rocks are folded and heated many of them change to metamorphic
rocks. Most of the changes are subtle and you need a microscope to
see them.
But mudstone forms a new structure, called cleavage, when
compressed and
heated. Cleavage cuts across the original layering and forms at right
angles to
the direction of compression. The resulting metamorphic rock is a slate
that
was widely used as roofing material. The cleavage, formed by the
alignment of platy minerals such as mica, allows quarry workers to
split the slate into thin slabs.
The tiny mica grains in slate grow with further heating to a millimetre
or so
in diameter, big enough to see. The rock still splits along the
cleavage,
though it is much coarser and useless for roofs. These rocks are called
schist.
Cleavage gets weaker with further heating and the rocks look more like
igneous
rocks with weak layering--they are called gneiss--but these are
rare in
Igneous rocks form when other rocks melt. Temperatures high enough to melt rocks occur more than a hundred kilometres beneath the surface. The melts formed here rise along cracks in the crust to erupt as lava and cool to form basalt, a black rock with small--less than one millimetre--crystals of pyroxene and feldspar. Some magma solidifies in the cracks before reaching the surface to form dykes. Magma may also squeeze between beds of sedimentary rocks to form sills. (Dykes cut across layers, sills intrude along the layers.) The rocks in dykes and sills cool more slowly than those that erupt as lavas, so have larger grains--typically about a millimetre, so are visible. These coarser rocks, which are chemically identical to their finer grained equivalent (basalt), are dolerite--American diabase.
Some magma collects in large bulbous bodies-- small stocks and large batholiths-- beneath the surface and cool even more slowly. The mineral grains have time to grow and are commonly over two millimetres in size. The grains are often well formed crystals, rather than the worn and rounded grains of sedimentary rocks. The coarse-grained rock is a gabbro.
Magma travels a long way to reach the surface and may change along the way. Minerals with the highest melting point--the dark minerals pyroxene and olivine--may sink out of the magma, leaving the magma enriched in the later crystallizing minerals--including quartz and feldspar. Or the rock may tear off sedimentary rocks from the crack walls and react with them. Both processes produce rocks rich in quartz, and feldspar richer in sodium and potassium. Andesite has more feldspar and is generally dark grey in colour. Rhyolite, another fine grained extrusive rock, has even more feldspar and also quartz, so is lighter grey than andesite. Where rhyolite magmas solidify before reaching the surface the resulting coarse-grained rock is granite with crystals of milky feldspar, clear grey quartz, and white and black mica. Granite typically forms small stocks or the larger batholiths.
Some rhyolite magma erupts as a slow-moving sticky mass, but most
rhyolite
eruptions are explosive, with the magma breaking up in the air into
small solid
pieces that settle out of the air as ash--or tuff, or
roar down
slopes as a mixture of hot gases and ash--or a pyroclastic flow. A
typical tuff
is full of volcanic fragments, some up to a metre in size near the
volcano. In
some cases, the fragments in a tuff are still very hot when they land.
The
weight of the overlying tuff may be enough to flatten the fragments and
weld
them together like blobs of hot glass. These rocks are welded tuff,
and
many of them still look glassy. The eruptions that produced these
different
rhyolite rocks would have been similar to the recent explosive
eruptions at