Igneous Rocks – Lava Textures Lava is an EXTRUSIVE form of Igneous Activity It represents molten rock that has been erupted or extruded onto the earth’s surface from volcanoesLava generally cools quickly (overmonths/years/decades) depending on the thickness of the flow
Igneous Rocks – Lava Textures The rate of cooling determines crystal size Lavas erupted onto the Earth’s surface cool relatively rapidly resulting in crystals less than 0.5mm in diameterIgneous rocks with crystals <0.5mm in diameter are termed VOLCANIC
Types of LavaBasalt - Hawaii Andesite – Mount St. HelensRhyolite - Yellowstone Pumice – Jemez Mts. Mexico
Lava Textures - Pillow Lavas Formed by submarine eruptions of basalticmagma whereby the exterior chills and crystallises rapidly against cold seawater at around 5°CThe outside layer will have a glassy texture with vesicles (gas bubbles) trapped below the surface
Lava Textures – Glassy Formed by 1cminstantaneous coolingof lava so that there isno time for crystals to formThe structure is very similar to glass and shows good conchoidal fracturewhen broken with a hammer
Lava Textures - PahoehoeSimilar to skin on custard! As the external layer coolsand solidifies it forms a rope-like pattern of rock as the liquid lava continues to move underneath.
Lava Textures - aa Aa lava has a surface made of sharp, angular and jagged blocksof basalt. The cooled surface layer breaks up into these fragments as molten lava continues to flow beneath
Lava Flows-Typical Structure Baked MarginLava flows typically have rubbly bases and tops (Sometimes showing pahoehoe or aa texture). The interior often shows columnar jointing and vesicles are often trapped in the upper part of the flow. The ground underneath the lava will be baked from the heat showing a zone of contact metamorphism (baked margin)
Lava Flows- Lava TubesLava tubes are sub-surface tunnels withinlava flows that have been formed as the fluidlava has continued to flow down slope. The solid top and base is formed as it cools and solidifies against the ground and the atmosphere
Lava Textures - Vesicular 1cmThe holes (or vesicles) in this rock represent gas bubbles that were trapped in the lava as it cooled and solidified.
Lava Textures – Vesicular/Pumice 1cmThis is a specimen of highly vesicular rhyolite (an acid lava flow)which is more commonly known as pumice. It is so light it actually floats on water. Also used to remove rough skin!
Lava Textures - Amygdaloidal 1cm The holes (or vesicles) in this rock represent gas bubbles thatwere trapped in the lava as it cooled and solidified. Later mineralrich waters percolated through the rock and deposited quartz (agate in this case) from solution
Flood Basalts-Mega Eruptions of LavaThese are basaltic eruptions that last between 0.5 and 2 million years andhave a global impact on climate and life. The mountains above are part of the Deccan Trapps in India which were formed about 65 Ma. At this time enough basalt was extruded to cover the whole of the USA to a depth of 1km It resulted in 75% of life on earth becoming extinct including the dinosaurs and ammonites.
Lava Textures – Columnar JointingColumnar jointing forms when thick lava flows (tens of metres)cool slowly. As the lava cools discrete cooling centres develop and the rock contracts towards these centres producing a polygonal pattern of vertical joints.
Lava Textures – Columnar JointingColumnar jointing produces columns of basalt with anything from 4 to seven sides. The base and top of flows are often rubbly as they have chilled against the ground and the air.
Lava Textures – Columnar Jointing The album sleeve for Led Zeppelin’s ‘Houses of the Holy’ was shot at The Giant’s Causeway in Northern Ireland. It depicts naked ladies clambering over the polygonal basalt columns Yet another example of why Geology Rocks!