Igneous intrusions and lava flows

Question 1

Dyke

Answer 1

A discordant sheet like intrusion

Question 2

Transgressive sill

Answer 2

A sheet like intrusion that cuts across the bed

Question 3

Sill

Answer 3

A concordant sheet like intrusion

Question 4

Country rock

Answer 4

The rock into which an igneous rock has been intruded

Question 5

Chilled margin

Answer 5

Where the igneous rock has cooled rapidly so has fine crystals

Question 6

Intrusion

Answer 6

Composed of igneous rock formed below the earth’s surface, where magma is forced into pre-existing rocks

Question 7

Batholith

Answer 7

A large igneous intrusion which may be an aggregate of plutons

Question 8

Hypabyssal

Answer 8

When igneous rocks form at relatively shallow depths below the earth’s surface

Question 9

Plutonic

Answer 9

When igneous rocks form deep below the earth’s surface

Question 10

Concordant

Answer 10

Intrusions are parallel to the existing beds

Question 11

Discordant

Answer 11

Intrusions cut across the existing beds

Question 12

Baked margin

Answer 12

In the country rock where it was heated by the intrusion and altered

Question 13

Assimilation

Answer 13

The melting process that incorporates blocks of country rock, freed by stoping, into the magma

Question 14

Xenolith

Answer 14

Pieces of rock within an igneous rock which have not been derived from the original but have been introduced from elsewhere. These are usually from the surrounding country rock.

Question 15

Stoping

Answer 15

The process that accommodates the magma, as it moves upwards into the country rock, by the mechanical fracturing of the surrounding country rock.

Question 16

Partial melting

Answer 16

It occurs when only a portion of rock is melted. When a rock is heated, those minerals with lower melting temperatures will melt. Those with higher melting temperatures will remain solid

Question 17

Diapir

Answer 17

A body of relatively low density material that pierces and rises up through overlying material of higher density.

Question 18

Volcanic plug

Answer 18

An intrusion formed when magma crystallises in the conduits or vent of the volcano.

Question 19

Ring dykes

Answer 19

Roof collapses when the magma pressure decrease in the underlying pluton or magma chamber. Magma oozes out into the fractures surrounding the collapse and produces dykes that dip away from the centre of the underlying pluton or magma chamber.

Question 20

Cone sheets

Answer 20

Magma pressure pushes up and fractures the overlying roof rocks. Magma is forcibly injected into the fractures producing a conical emplacement with the dykes dipping toward the buried centre of the magma source.

Question 21

Cumulate

Answer 21

An igneous rock produced by gravity settling of crystals in a magma

Question 22

Palaeosol

Answer 22

A soil horizon that was formed in a past geological age

Question 23

What’s the difference between sills and lava flows?

Answer 23

Lava flows cool at the earth’s surface and are extrusive igneous rocks whilst sills cool below the earth’s surface and are intrusive igneous rocks.

Question 24

How can baked margins be used to distinguish sills form lava flows?

Answer 24

Sills have two baked margins where as lava flows only have one

Question 25

How can crystal size be used to distinguish sills from lava flows?

Answer 25

Lava flows cool rapidly so have fine crystals where as sills have medium to coarse crystals. The crystals at the centre of a sill cool more slowly and have a coarser crystal size.

Question 26

What are the features of the upper surface of a lava flow?

Answer 26

They have an irregular surface consisting of scoria and or rubble. If exposed to weathering it can develop a reddened surface. Prolonged exposure can allow a palaeosol to develop.

Question 27

How can igneous textures be used to distinguish sills from lava flows?

Answer 27

A vesicular texture is common on the upper part of a lava flow. Flow banding within silicic lava flows form due to friction. Tabular phenocrysts can become aligned parallel to the flow banding.

Question 28

What is the main rock type in sills and lava flows?

Answer 28

sill- dolerite

Lava flow- basalt

Question 29

Where do you find xenoliths in sills and lava flows?

Answer 29

Sills above and below as they rip up material during emplacement
Lava flows only above

Question 30

Ophiolite

Answer 30

Sections of the earth’s oceanic crust that have been tectonically moved onto continental crust