Geology - F2 Igneous

How is pillow lava formed?

Eruption of basaltic lava underwater cools rapidly when it comes in contact with sea water

What is the texture of pillow lava?

V-shaped bottom, domed top, glassy surface, vesicles at the top

What is Pahoehoe?

Lava formation caused by the cooling of the surface and the lava underneath carries on moving so pulls the surface layers with it, forming a rope-like texture

What is Aa?

Lava formation caused by surface layer cooling and breaking apart as the lava beneath carries on moving, leaving a sharp and angular texture

What are lava tubes?

Tunnels formed when the surface layer of low viscosity lava cools and forms a roof whilst the lava underneath carries on flowing

What is columnar joining?

Contraction due to cooling in shallow igneous intrusions forms a polygonal pattern of vertical joints

What is a flood basalt?

Type of eruption that lasts 0.5-2 million years and have a global impact on climate and life

What are the main differences between a lava flow and a sill?

Sills	Lava flows
Two chilled margins as it is intruded between two layers of rock	On lower chilled margin
Two baked margins	One lower baked margin
Xenoliths from above fall into sill as it intrudes. Xenoliths from below are broken off by intrusions as it flows	Xenoliths only from below as lava picks up loosened material as it travels
Medium grained crystal 1-3mm	Fine grained <1mm
Regular upper surface	Weathered/rubbly upper surface as exposed to the atmosphere
Vesicles are rare	Vesicles are common
Most made of Dolerite	Most made of basalt
Regular upper surface	Aa or pahoehoe texture on the surface
No sill fragments in the rock above. Rock is older than the sill	Lava fragments in the rock above as lava gets weathered and eroded over time

Do lava flows or sills get weathered?

Lava flows - can involve oxidation so iron can be found on the top surface

What does orientated mean?

There is no clear direction, it just follows a trend

What does directed mean?

There is a clear direction, but doesn’t follow a trend

What does equicrystaline mean?

All crystals in an igneous rock are the same size

What does porphyritic mean?

Consists of 2 crystal sizes - larger crystals called phenocrysts and smaller crystals called groundmass

What rate of cooling is represented by a glassy texture?

Very fast

How is a vesicular rock formed?

Fast cooling of lava traps gas bubbles, which leave holes in the rock

What does amygdaloidal mean?

Vesicles in a rock have been filled in by the precipitation of groundwater, depositing sediment in the holes

What is flow banding?

Bands of light and dark minerals are produced that lie parallel to the direction of lava flow

What is tuff?

Rock formed from volcanic ash

What minerals are found in silicic igneous rocks?

Quartz, K feldspar, Plagioclase feldspar, biotite, and muscovite

What minerals are found in intermediate igneous rocks?

K feldspar, plagioclase feldspar, biotite, and horneblend

What minerals are found in mafic igneous rocks?

Olivine, augite, and plagioclase feldspar

What minerals are found in ultra-mafic igneous rocks?

Augite and olivine

What is country rock?

Any rock which an igneous rocks intrude into

What is a chilled margin?

The inside edge of an igneous intrusion with fine grained crystals due to fast cooling

What is a baked margin?

Area of country rock which has experienced contact metamorphism from an igneous intrusion

What is a dyke?

Sheet like igneous intrusion that is discordant

What is a sill?

Sheet like igneous intrusion which is concordant

What is a bedding plane?

The boundary between two layers of sedimentary rock

What is a laccolith?

An igneous intrusion that has forced between two layers of sedimentary rock due to the pressure of injecting mama from a dyke

What is a volcanic neck?

Magma within a volcano solidifies and the rock around it erodes and leaves behind the solidified magma

What is a ring complex?

When the roof above a magma chamber collapses and allows magma to intrude upwards into the fractures

What is a batholith?

Large scale igneous intrusion formed below the earths surface which hardens before it can reach the surface

What is a pluton?

Columns of magma attached to a batholith which extend much higher than the batholith

What is a metamorphic aureole?

Large scale baked margin around a pluton

What is a xenolith?

Block of pre-existing country rock broken off and surrounded by an igneous rock from magma cooling around it

What does assimilation mean?

A xenolith has been fully melted into magma

What is stoping?

When magma makes its way to the earths surface

What is an intrusive igneous rock?

A rock formed from the cooling of magma below the earths surface

What is an extrusive igneous rock?

A rock formed from the cooling of lava above the earths surface

An igneous rock with a light colour indicates…

a high silica content

What size are fine grained crystals?

<1mm

What size are medium grained crystals?

1-3mm

What size are coarse grained crystals?

>3mm