Saudi Arabia's geomorphological history and harsh climate have led to geological wonders including extraordinary rock formations, volcanic craters, mineral rich mountains, caves and wadis. The geology can be divided into two main regions: the Arabian Shield and the Arabian Shelf or Platform.
The Arabian Shield is older and was part of the African continent before rifting caused the Red Sea to form between them 35 million years ago. It's composed of igneous and metamorphic complexes like basalt, gabbro, granite, shale, schist and marble. The Arabian Shelf is younger and covered in sedimentary deposits of sandstone, limestone and huge quantities of sand.
Half of the sedimentary area of Saudi Arabia is blanketed by aeolian (windblown) sand with immense salt and gravel plains stretching to the edges of the country's three main deserts: the Rub’ al Khali, the largest contiguous sand desert in the world; Ad Dahna, silica-rich red sand desert; and Al Nufud al Kabir, the great stony desert. There, you can find all dune types: transverse (simple or compound barchan, or crescent-shaped dunes), longitudinal, Uruq (long, parallel, sharp-crested dunes separated by broad valleys) and sand mountains (complex barchan dunes, pyramid and star dunes).
Alongside the Red Sea coast, lies the highest and largely limestone Asir Mountains. Further north, sits the colourful, mineral-rich Hijaz Mountains and running through the country’s centre is the Tuwayq escarpment, containing marine fossils and shells dating back to the Upper Jurassic period.
The volcanic west features columnar basalts, pillow lavas and granitic outcrops eroded into fantastic shapes. Intrepid explorers can visit the White Volcanoes of Harrat Khaybar, lava tubes or jet-black lava flows around Maqla'a Tamyah Crater (Alwa'aba). Deep, lush wadis are common around Jizan and AlUla, and Tabuk offers spectacular red sandstone pinnacles.
CHERT AND FLINT
Chert is a microcrystalline sedimentary rock often found in layered deposits of chalcedony, silica, and quartz. When broken, it forms sharp edges and in the Stone Age, people would fashion them into cutting tools and primitive weapons. Flint is a hard sedimentary form of quartz that occurs in chalks and limestones. It produces a spark if struck or scraped. Along with being of great value to our ancestors, it is still used as a survival tool today.
FOSSIL CORALS
The Tuwayq escarpment is one of the places where you will find fossil corals, foraminifera, and stromatoporoids (sponges) that look surprisingly similar to species that live in the oceans today. Millions of years ago, they formed underwater reef gardens that thrived in the warm Tethys Ocean. When the Arabian Shelf lifted, the ocean retreated, and now the fossils are scattered 500m above sea level, close to the country’s capital. Hard coral species, ammonites, oysters and shells can be spotted among the sloping hills – they're often darker than the surrounding rock.
GYPSUM CRYSTALS
Gypsum is an evaporite mineral found in sedimentary deposits in areas that used to be under the sea. Its crystals can be translucent and needle-like (satin spar) or large and clear (selenite). They occur in sections of the Tuwayq escarpment in white, powdery substrate.
LIMESTONE
Limestone is a carbonate sedimentary rock made up of countless bodies of tiny plankton that fell to the bottom of the ocean and became lithified (turned to stone). The rock erodes by gradual dissolution – usually as a result of acidic rain – in a process called 'Karstification'.
MARINE SHELLS
In areas of the Tuwayq escarpment, where the sediment is soft and crumbly, marine fossils of molluscs, rudists, bivalves, crinoids, oysters, gastropods, and foraminifera (one-cell organisms) can be found on the ground or embedded in wadi walls. They represent marine species that inhabited shallow waters when the sea level was much higher.
MICA SCHIST
When buried deep inside the Earth under extreme heat and pressure, rocks transform into different rock types called metamorphic rocks. Mica schist is a medium-grade metamorphic rock formed from mudstone or shale. It is silver-grey, has a slight sheen and is common in the Arabian Shield.
PERIDOTITE
This light-green, coarse-grained igneous rock consists mainly of silicate minerals such as olivine, pyroxene, magnesium and iron. It can be found around the volcanic craters of Harrat Kishb and Khaybar. In Saudi Arabia, it's a popular gem because of its green colour – the symbol of Islam and the Saudi flag.
QUARTZ CRYSTALS
Quartz crystals are abundant because of their hardness and durability. They occur in different colours depending on the mineral impurities they contain (milky quartz, smoky quartz, pink quartz and amethyst). They form within cavities in other rocks or even corals. Locally known as 'desert diamonds', the clear, high-quality quartz crystals are often used in jewellery.
SANDSTONE
Sandstone is a clastic sedimentary rock composed of sand-sized silicate grains. Its colour depends on its composition and mineral content, such as hematite-red or limonite-yellow.
SANDSTONE CONCRETIONS
These odd-looking balls of all shapes and sizes are masses of mineral matter embedded within rock layers. They form when minerals precipitating out of water collect around a nucleus like a pebble, leaf, shell or bone.
SHARK TEETH
Similar to humans, shark's teeth are made of dentin –hard calcified tissue that is denser than bone. They have several rows of teeth, which they shed and replace throughout their lifetime. Shark tooth fossils are regularly washed out by heavy rains around Khurais, Eastern Region.
VESICULAR BASALT
This lightweight, dark volcanic rock has many small holes, formed when dissolved gasses bubbled out of the magma as it approached the surface. It is abundant in volcanic fields and around Maqla'a Tamya Crater (Alwa'aba).
