1. 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

2. 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

3. Types of Lava
Basalt - Hawaii Andesite – Mount St. Helens
Rhyolite - Yellowstone Pumice – Jemez Mts. Mexico

4. 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

5. Lava Textures – Glassy
Formed by 1cm
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

6. 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.

7. 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

8. 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)

9. 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

10. 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.

11. 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!

12. 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

13. 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.

14. 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.

15. 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.

16. 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!

17. The End