Igneous Rock Formation

Question 1

What are the three major rock types.

Answer 1

Sedimentary
Metamorphic
Igneous

Question 2

How are Igneous rocks formed?

Answer 2

-Magma or lava cools and crystallises to form minerals
-Pyroclastic materials are consolidated to form a solid from the previously loose particles

Question 3

Name the two main categories of Igneous rocks

Answer 3

Extrusive and Intrusive

Question 4

Define extrusive (Volcanic) igneous rocks

Answer 4

Rocks that form from materials erupted at the Earth’s surface result from the cooling of lava flows or consolidation of pyroclastic materials

Question 5

Define intrusive (Plutonic) igneous rocks

Answer 5

Rocks that form from magma that cools below the Earth’s surface

Question 6

What is the main constituent of magma?

Answer 6

Silica

Question 7

Name the types of magma.

Answer 7

Ultramafic
Mafic
Intermediate
Silicic

Question 8

What lava and rock types come from ultramafic /mafic magma

Answer 8

Ultramafic (Peridotite)
Mafic (Basaltic lava – Basalt, Dolerite, Gabbro)

Question 9

Describe Bowen’s Reaction Series

Answer 9

Bowen’s Reaction Series shows how differentiation and partial melting work to create and modify magma composition. If a rock is re-heated, the minerals in Bowen’s Reaction Series will melt in reverse order

Question 10

Where does magma originate & accumulate?

Answer 10

Most magma originates in the upper mantle or lower crust. Magma forms from the partial melting of ultramafic material in the asthenosphere. Magma accumulates in reservoirs known as magma chambers

Question 11

Where are magma chambers found?

Answer 11

Beneath spreading centres or divergent boundaries (i.e. mid-oceanic ridges) magma chambers exist at a depth of only a few kilometres

At convergent boundaries, magma chambers are commonly a few tens of kilometres deep

Question 12

Name the factors affecting the generation of magma

Answer 12

Temperature

Pressure

Water content

Question 13

Describe the influence on Temperature on magma melting

Answer 13

The heat responsible for melting rock and generating magma comes from the Earth’s core. This heat is conducted towards the Earth’s surface through the solid parts of the mantle and crust.

Convection cells in the mantle distribute heat from the lower mantle towards the upper mantle and crust (through partially melted or semi-solid layers).

Question 14

Describe the influence on Pressure on magma melting

Answer 14

The melting point of a mineral increases with
increasing pressure. Both temperature and pressure increase with depth in the Earth’s crust. For this reason, a rock that melts at a given temperature at the Earth’s surface, will require a higher temperature to melt deep in the crust.

Question 15

Describe the influence on Water Content on magma melting

Answer 15

The presence of gas, especially water vapour,
significantly lowers a mineral’s melting
temperature. This occurs because water under high pressure helps break the silica bonds in the
mineral.

Question 16

State 4 ways magma can evolve.

Answer 16

Partial Melting (Bowen’s Reaction Series)

Differentiation

Magma contamination (Assimilation)

Magma mixing

Question 17

Describe the process of Partial Melting

Answer 17

If pre-existing rock is melted, the minerals will melt in the reverse order in which their created, following Bowen’s Reaction Series. Partial melting of ultramafic rock in the mantle, generally produces mafic (basaltic
rich) magma.

Question 18

Describe the process of differentation

Answer 18

This is the process by which ferro-magnesian minerals separate from a magma leaving behind a more silica-rich
magma.

This process takes place in magma through crystal settling, which is the downward movement of minerals that are denser.
These settle (more dense) at the bottom of the magma chamber to form cumulates.

Question 19

Describe the process of assimilation

Answer 19

Very hot magma could melt some of the surrounding country rock and assimilate the newly molten material into the magma.

If the magma is mafic, and melts continental country rock, the magma becomes
diluted with silica and becomes intermediate and cooler.

Question 20

Describe the process of magma mixing

Answer 20

Two magma types merge in the crust to produce an intermediate magma. Mixing equal amount of mafic magma and silicic magma would result in an intermediate magma producing diorite if it solidifies beneath the surface, or erupt on the surface to solidify as
andesite.

Question 21

What are enclaves?

Answer 21

Incomplete magma mixing can lead
to the formation of areas within a
pluton with a different composition
to the rest of the body.

Question 22

Outline the origin of magma at ocean ridges (divergent boundaries)

Answer 22

Mafic magma is created by decompression and partial melting of the asthenosphere at divergent plate boundaries. Localised mantle plumes rise beneath ridge, pressure decreases
and melting begins, thus yielding magma.
The presence of water also lowers the
melting temperature

Question 23

What type of rock is the asthenosphere made of and what magma does it create?

Answer 23

ultramafic rock (peridotite)
mafic-rich magma

Question 24

Outline the origin of magma at subduction Zones

Answer 24

When the descending oceanic plate reaches
the depth at which the temperature is high
enough to initiate partial melting, the water in the oceanic crust also enhances melting and magma formation by lowering
the melting point of the rock

Question 25

What magma is associated with converging/subducting plates?

Answer 25

Intermediate or silicic magma

Question 26

What other magma types can be found at subduction boundaries? Why?

Answer 26

Mafic magma rising through the lower continental crust will assimilate silica-rich materials

Question 27

Describe the compositions of oceanic and continental crust

Answer 27

Oceanic - Mafic (Basalt)
Continental - Intermediate

Question 28

Outline the origin of magma at collision zones

Answer 28

The base of the continental plates can be partially melted as it is forced into hotter material. This can produce magma at depth. This Intermediate magma then rises through the crust and assimilates with the lower temperature minerals, adding more silica and
making the magma Silicic.

Question 29

What are Diapirs?

Answer 29

Rising melts of magma

Question 29

Define the term Intrusive Igneous Structures

Answer 29

Bodies of intrusive rock named according to their size and shape, as well as their relationship to the surrounding country rock.

Question 29

Define “Mantle Plumes”

Answer 29

This is where localised heating of the mantle by a plume or hotspot causes partial melting of the mantle material.

Question 29

Describe and explain how magma rises

Answer 29

Magma moves upwards when it is less dense than the surrounding material. It will continue to do this until its density matches or is greater than the surrounding material. It will move through cracks and fractures due to its buoyancy. It will also rise by stoping.

Question 30

Chicken says

Answer 30

KEEP GOING YOURE DOING GREET

Question 31

What are the three S’s Regarding texture

Answer 31

Size
Shape
Sorting

Question 32

Define Euhedral, Subhedral and Anhedral

Answer 32

Euhedral - Well formed, regular crystal shapes
Subhedral - Partially formed crystals with some straight edges
Anhedral - Poorly formed crystals that had little time to grow.

Question 33

What is a Pegmatite?

Answer 33

Extremely coarse-grained igneous rock, grains are larger than 5cm. Pegmatites can be made-up of Diorite, gabbro, or granite.

Question 34

What are porphyritic rocks?

Answer 34

Igneous rock with a distinct difference in crystal size (can be quite minor difference) larger crystals, called phenocrysts, embedded in a finer-grained groundmass.