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