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 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 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 Lavas
Formed 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
Formed 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
1cm
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
Baked
Margin
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)
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
1cm
The holes (or vesicles) in this rock represent gas bubbles
that were trapped in the lava as it cooled and solidified.
Lava Textures – Vesicular/Pumice
1cm
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!
Lava Textures - Amygdaloidal
1cm
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
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 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!
The End