Volcanic Dike Intrusions Madeira, Portugal

Dikes are typically formed when magma intrudes into fractures in the Earth's crust, cutting across existing rock layers at high angles.

The intrusion of basaltic dikes on Porto Santo Island (Madeira Archipelago, Portugal) during the Middle Miocene marked a phase of heightened volcanic activity. These magmatic sheet intrusions critically shaped the island's geological structure and altered its volcanic rocks by baking the surrounding material, increasing hardness and erosion resistance, and modifying their chemical composition.

Dikes are sheets of rock formed within fractures of pre-existing rock bodies, with magmatic dikes arising from solidified magma injected into cracks, either cross-cutting layers or penetrating a contiguous rock mass. Clastic dikes, in contrast, form from sediment infilling pre-existing fractures.

Intrusion of Basaltic Dikes in Porto Santo Island (Madeira Archipelago, Portugal) Photo: Prof.  Mário Cachão.

Key Characteristics of the Dikes

• Composition: Primarily basaltic and trachytic.
• Morphology: Narrow (typically <1 m wide), vertical, and traceable for kilometers.
• Emplacement: Intruded into pre-existing volcanic rocks, including hyaloclastites, tephra, and submarine lavas.