What is a lava plateau? What is a volcanic plug? What are basalt columns?

Lava plateaus are formed by the large outpourings of fluid lava from long narrow openings in the crust. During each eruption, the lava flows out from these openings, solidifies and builds up layer upon layer each time. An example would be the County Antrim Plateau in Northern Ireland.

About 430 million years ago the North American and Eurasian plates had collided to form a vast range of mountains called the Caledonian mountains- they would be fused for 370 million years. Around sixty million years ago- the plates began to diverge. As they diverged, huge fissures would have been created in the landscape, this allowed magma to rise from the mantle as lava and cover the surrounding chalk rock- these lava flows continued for over 2 million years. As the lava cooled and formed into basalt, a vast flat plateau was left behind.

/ Volcanic Plugs An example would Slemish in County Antrim, Northern Ireland. A volcanic plug is a volcanic landform created when magma hardens within a vent in an active volcano. If a plug is preserved, erosion/weathering may remove the surrounding rock while the resistant plug remains, producing a distinctive landform.

Fissures created by the divergence of the North American and Eurasian plates, allow magma to flow from the mantle as lava. Explosions deposited volcanic rocks around the fissure, creating a volcano; continued deposition would have lead to a cone developing. Lava would continue to flow from a vent in the cone, onto the surrounding plateau. In the vent of the cone, the magma cooled more slowly than usual, forming a harder rock than basalt called dolerite- this plugged the volcano vent. The less resistant rock surrounding the plug of the volcano has been weathered and eroded over time- leaving the distinct landscape feature.

Basalt Columns An example would the Giant’s Causeway in County Antrim, Northern Ireland. Large eruptions of basalt lava may create deep flows of molten rock. As the rock slowly cools it shrinks slightly. The stresses cause jointing in several different planes, and columns of rock form with a generally hexagonal shape, like pencils.

As the North American and Eurasian plates diverged magma, as lava, rose through fissures in the surface. The lava cooled to form basalt, as the rock formed, contraction occurred. Contraction can occurred with ease vertically, but horizontally this creates cracks within the rock. This contraction produces hexagonal columns. Quick cooling produces smaller columns, whilst longer cooling creates larger columns.