Although many geologists have produced papers about Lundy, the first complete study was undertaken by Dr A T J Dollar who described Lundy's granite mass as: “the denuded core of far more lofty mountains piled up during the Armorican folding ... half liquid magma pushed up into the cavities at the base of the mountainous folds of rock and solidifies. Then, as millions of years go by with their millions of seasons of rain and frost ... denude the masses ... until a time may be reached when the granite core, the solidified magma so much harder than the overlying rocks, is all that remains.”

Research since then has shown that Dollar’s assumption that the Lundy granite was of the same age as that seen on Dartmoor and Bodmin Moor was incorrect, the Lundy granite being much more recent at between 59 and 52 million years old. This means that rather than being formed during the Amorican folding (now the Varascan orongeny) they were formed as part of a period of active vulcanicity and formed part of the British Tertiary Volcanic Province (BTVP), best known in western Scotland and Northern Ireland.

The southeast corner of Lundy is all that now remains of the overlying slates and these now join the granite in a distinct line from the Sugar Loaf to the Rattles. This slate is similar to the Morte slates of North Devon and may be called Upper Devonian. The island plateau forms a marine plane, similar to many in Southern England at the 120m (400ft) contour.

There are also numerous dykes, mostly of dolerite, that have invaded cracks in the older rocks and since formed foci for erosion. The dykes are a feature of the BTVP and are only slightly younger than the granite at 56-45 Ma. As well as the overall NW-SE trend seen throughout the BTVP, some dykes show evidence of a local centre from which they radiate. This, and the presence of a positive anomaly in the earth’s magnetic field to the west of the island suggest that there was a volcano there which probably reached the surface (although any evidence for this will have eroded away). The Lundy granite formed from the original magma chamber (possibly 10km below the surface at that time) and the dykes from later episodes of vulcanism, their different composition the result of fractional crystalisation.

Dollar classified the granite into three types: G1 which is an even-grained white orthoclase variety; G2, a variety with phenocrysts of orthoclase and quartz set in a microgranite groundmass; and G3 and G3a which are microgranites. These distinctions probably relate to the way that the granite cooled rather than being distinct types.

The plateau surface of the granite massif is almost level and bounded on all sides by steeply inclined sidelands at the base of which are vertical cliffs. The erosive activity of the many small streams that drain the surface has been minimal and virtually all the streams have developed along weaknesses in the granite or along dykes, such as those in Gannets' Combe.

There has long been discussion of the effects of glaciation on the island but the latest research has suggested that the geomorphology of Lundy can be largely interpreted as the product of glacial processes. There is widespread smooting and lineation (running WNW-ESE) of surfaces, which together with grooved whaleback forms, can be interpreted as subglacial ice moulding. There are also channels, now dry, that can also be intepreted as having formed as subglacial meltwater channels. There are large areas of eratic gravels and cobbles in the north part of the island whose composition reflects the geology of Pembrokeshire to the NW of Lundy. Their location again suggests movement by subglacial meltwater. The two main stream systems (Millcombe and Gannets' Combe) appear to have been overdeepened by meltwater following lines of geological weakness.

Scientific dating has shown that the the rocks were exposed as the ice melted between 35,000 to 40,000 years ago, long before the maximum extent of glaciation was reached (the global Last Glacial Maximum, LGM, at around 26,000 to 21,000 years ago). This dating suggests that the extent and timing of glaciation is complex at the southern limits. It does show that the ice did reach this far south during the last glaciation which had been questioned before.

Following the end of the glacial, Lundy would have formed a significant hill in a wide plain where the Bristol channel now lies. As more ice melted, the sea would have risen and flooded to south and north leaving a peninsula that would have become an island at around 7500 BC. The present landscape is dominated by granite tors formed by the weathering of the granite by chemical and physical processes.

Minerals found on Lundy include copper ore which is found at the junction of the granite and slate at the south end of the island just east of Benjamin's Chair. During the mid-19th century three shafts were opened in the hope of finding workable quantities but the find was not worth commercial exploitation. A vein was also found near Long Roost and three adits were made, but the quality of ore was too poor to be worked. During the Second World War an inspector visited Lundy to see if the molybdenum ore, of which there was then a shortage, could be worked, but again the amount was not commercially viable.

Other minerals recorded are:

• Beryl - in small white-yellow columnar crystals.
• Feldspar - in white tubular crystals.
• Fluorite - crystalline and massive.
• Garnet
• Mica - in plates and hexagonal crystals.
• Rock crystal - transparent, frequently dark brown or black.
• Schorl
• China clay - formed from disintegrating feldspar, present in small; quantities but too impregnated with iron to be useful.

In the slate are are veins and strings of Gossan containing:

• Blende - sulphuret of zinc in traces.
• Towanite - copper pyrites.
• Magnetite - magnetic iron ore, found in a vein below Benjamin's Chair.
• Quartz - amorphous and crystalline is found in veins crossing the slate in every direction. This is the most abundant non-metallic mineral.
• Limestone - a seam appears on the beach and passes southeastwards through Hell's Gates. This weakness of a soluble mineral probably accounts for the separation of Rat Island from the main island.

 Original text by Tony Langham, updated by Chris Webster