igneous rocks – lava textures lava is an extrusive form of igneous activity it represents molten...
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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 volcanoes
Lava generally cools quickly (over months/years/decades)
depending on the thickness of the flow
Igneous Rocks – Lava TexturesThe rate of cooling determines crystal size
Lavas erupted onto the Earth’s surface cool relatively rapidly resulting in crystals less than 0.5mm in diameter
Igneous rocks with crystals <0.5mm in diameter are termed VOLCANIC
Types of Lava
Basalt - Hawaii Andesite – Mount St. Helens
Rhyolite - Yellowstone Pumice – Jemez Mts. Mexico
Lava Textures - Pillow LavasFormed by submarine eruptions of basaltic magma whereby the exterior chills and crystallises rapidly against cold seawater at around 5°C
The outside layer will have a glassy texture with vesicles (gas bubbles) trapped below the
surface
Lava Textures – Glassy
1cmFormed by
instantaneous cooling of lava so
that there is no time for crystals to form
The structure is very similar to glass and
shows good conchoidal fracture
when broken with a hammer
Lava Textures - Pahoehoe
Similar to skin on custard! As the external layer cools and 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 blocks of basalt. The cooled surface layer breaks up into
these fragments as molten lava continues to flow beneath
Lava Flows-Typical Structure
Lava 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)
Baked Margin
Lava Flows- Lava Tubes
Lava tubes are sub-surface tunnels within lava flows that have been formed as the fluid lava 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
The holes (or vesicles) in this rock represent gas bubbles that were trapped in the lava as it cooled and solidified.
1cm
Lava Textures – Vesicular/Pumice
This 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!
1cm
Lava Textures - Amygdaloidal
The holes (or vesicles) in this rock represent gas bubbles that were trapped in the lava as it cooled and solidified. Later mineral rich waters percolated through the rock
and deposited quartz (agate in this case) from solution
1cm
Flood Basalts-Mega Eruptions of Lava
These are basaltic eruptions that last between 0.5 and 2 million years and have 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 Jointing
Columnar 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 Jointing
Columnar 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 JointingThe 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!
The End