Igneous Rocks and Processes
Igneous Rocks and Textures
Igneous Rocks
Mafic Felsic
Dark Light
Cooling Rate Grain Size
Basic Acidic
Composition
Very Quickly Glassy Obsidian
Extrusive Quick Fine Grained
Basalt Andesite Rhyolite
(<1mm)
Slow Medium
Hypabyssal Grained (1- Dolerite Microdiorite Microgranite
3mm)
Coarse Grained
Plutonic Gabbro Diorite Granite
(>3mm)
Quartz
Olivine Orthoclase
Mineralogy Augite Mica
Plagioclase (+ Hornblende
& Plagioclase)
Only specimen we can be given in exam
Rock Descriptions
Colour – low, medium or high colour index – quote the % of each colour in the specimen
Texture:
All igneous rocks have a crystalline texture
• Size – fine (<1mm), medium (1-3mm), coarse (>3mm)
• Sorting – porphyritic, equicrystalline or glassy
• Shape – euhedral, subhedral, anhedral
• Arrangement of Crystals – preferred orientation; amygdales or vesicles
Mineralogy – identify the mineral within the rock – estimate the % of each mineral present
Key Words:
• Euhedral – describes a complete crystal, has the perfect crystal shape
• Subhedral – describes a partially complete crystal
• Anhedral – describes a crystal with no crystal shape
• Porphyritic – crystals have different sizes
• Equicrystalline – crystals are the same size
• Groundmass – small crystals between phenocrysts in porphyritic rocks
• Phenocryst – the big crystal in a porphyritic rock
• Vesicles – trapped gases in the rocks creates holes/cavities
• Amygdales – vesicles that have been filled in by crystals
• Crystal Alignment – crystals are aligned
• Random Orientation – crystals are not aligned
• Flow Banded – form lines
, Drawing Rock Textures to Scale
1. Describe the rock, record the size, shape and sorting including the proportion of different crystal sizes
2. If given the scale, draw example crystals outside the circle which match the sizes and shapes
3. Within the circle, draw the texture reflecting the relative crystal proportions – REMEMBER: igneous
rocks are crystalline so there is no space between the crystals
4. Label an example of each mineral or texture
To calculate the degree of magnification – size of image
size of real object
Structure of Oceanic Crust
Oceanic Crust
Trend 1 – The crust gets older the further you move from the ridge:
Oceanic sediment becomes thicker the further away you move from the
ridge as it has had longer to accumulate sediment which settles to the
bottom of the ocean.
Trend 2 – Oceanic lithosphere gets thicker the further you move from the
ridge:
The lithosphere has lost heat through conduction therefore the 1300°C
isotherm is lower into the mantle.
Pillow Lava
Formation – When lava is erupted into water it causes the lava too cool and crystallise very rapidly. This causes
it to form a pillow shape and the edges of the pillows will be glassy.
Features:
Rounded Top
• Rounded top
• Pointed keel
• Fine grained basalt
• Vesicles
• Glassy rind (skin) Pointed Keel
• 1m tall
Way Up Indicator:
The pillow has a rounded top and a pointed bottom (keel). If the keel can be seen, the pillow lava is the wrong
way around
Igneous Structures
Flow Top
Pahoehoe and a’a are used to describe the tops of basalt lava flows
A’a has a rubbly brittle top – has a high viscosity and cools quickly so the
crust forms quickly
Pahoehoe is smooth and ropy – has a lower viscosity and cools slower
Igneous Rocks and Textures
Igneous Rocks
Mafic Felsic
Dark Light
Cooling Rate Grain Size
Basic Acidic
Composition
Very Quickly Glassy Obsidian
Extrusive Quick Fine Grained
Basalt Andesite Rhyolite
(<1mm)
Slow Medium
Hypabyssal Grained (1- Dolerite Microdiorite Microgranite
3mm)
Coarse Grained
Plutonic Gabbro Diorite Granite
(>3mm)
Quartz
Olivine Orthoclase
Mineralogy Augite Mica
Plagioclase (+ Hornblende
& Plagioclase)
Only specimen we can be given in exam
Rock Descriptions
Colour – low, medium or high colour index – quote the % of each colour in the specimen
Texture:
All igneous rocks have a crystalline texture
• Size – fine (<1mm), medium (1-3mm), coarse (>3mm)
• Sorting – porphyritic, equicrystalline or glassy
• Shape – euhedral, subhedral, anhedral
• Arrangement of Crystals – preferred orientation; amygdales or vesicles
Mineralogy – identify the mineral within the rock – estimate the % of each mineral present
Key Words:
• Euhedral – describes a complete crystal, has the perfect crystal shape
• Subhedral – describes a partially complete crystal
• Anhedral – describes a crystal with no crystal shape
• Porphyritic – crystals have different sizes
• Equicrystalline – crystals are the same size
• Groundmass – small crystals between phenocrysts in porphyritic rocks
• Phenocryst – the big crystal in a porphyritic rock
• Vesicles – trapped gases in the rocks creates holes/cavities
• Amygdales – vesicles that have been filled in by crystals
• Crystal Alignment – crystals are aligned
• Random Orientation – crystals are not aligned
• Flow Banded – form lines
, Drawing Rock Textures to Scale
1. Describe the rock, record the size, shape and sorting including the proportion of different crystal sizes
2. If given the scale, draw example crystals outside the circle which match the sizes and shapes
3. Within the circle, draw the texture reflecting the relative crystal proportions – REMEMBER: igneous
rocks are crystalline so there is no space between the crystals
4. Label an example of each mineral or texture
To calculate the degree of magnification – size of image
size of real object
Structure of Oceanic Crust
Oceanic Crust
Trend 1 – The crust gets older the further you move from the ridge:
Oceanic sediment becomes thicker the further away you move from the
ridge as it has had longer to accumulate sediment which settles to the
bottom of the ocean.
Trend 2 – Oceanic lithosphere gets thicker the further you move from the
ridge:
The lithosphere has lost heat through conduction therefore the 1300°C
isotherm is lower into the mantle.
Pillow Lava
Formation – When lava is erupted into water it causes the lava too cool and crystallise very rapidly. This causes
it to form a pillow shape and the edges of the pillows will be glassy.
Features:
Rounded Top
• Rounded top
• Pointed keel
• Fine grained basalt
• Vesicles
• Glassy rind (skin) Pointed Keel
• 1m tall
Way Up Indicator:
The pillow has a rounded top and a pointed bottom (keel). If the keel can be seen, the pillow lava is the wrong
way around
Igneous Structures
Flow Top
Pahoehoe and a’a are used to describe the tops of basalt lava flows
A’a has a rubbly brittle top – has a high viscosity and cools quickly so the
crust forms quickly
Pahoehoe is smooth and ropy – has a lower viscosity and cools slower
