Eduqas GCSE Geology · 1.2

Igneous Rocks and Processes

50 revision questions with answers for 1.2 Igneous Rocks and Processes in Eduqas GCSE Geology (spec C480QS). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, Five Degrees, Associations, or read every question and answer below.

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Question 1
What is an igneous rock?
Answer:
A rock formed by the cooling and solidification of magma or lava.
Question 2
What is magma?
Answer:
Molten rock beneath Earth's surface, which may contain crystals and dissolved gases.
Question 3
What is lava?
Answer:
Magma that has reached Earth's surface.
Question 4
Name the four igneous rocks specified by Eduqas.
Answer:
Peridotite, basalt, andesite and granite.
Question 5
What does equicrystalline mean?
Answer:
Most crystals in the rock are of approximately similar size.
Question 6
What is a porphyritic texture?
Answer:
Larger crystals, called phenocrysts, set in a finer-grained groundmass.
Question 7
How does a porphyritic texture develop?
Answer:
Early crystals grow during slower cooling; the remaining melt then cools faster, producing a fine groundmass.
Question 8
Why are crystals generally larger in slowly cooled magma?
Answer:
They have more time to grow before the remaining melt solidifies.
Question 9
Why are crystals generally small in rapidly cooled lava?
Answer:
Rapid cooling gives crystals little time to grow.
Question 10
What does random crystal orientation look like?
Answer:
Crystals lie in many directions rather than consistently aligning in one direction.
Question 11
According to the specification, what size is coarse in igneous rocks?
Answer:
Crystals larger than 3 mm are described as coarse.
Question 12
According to the specification, what size is fine in igneous rocks?
Answer:
Crystals smaller than 1 mm are described as fine.
Question 13
A crystal measures 12 mm in a photo with 4x magnification. What is its actual size?
Answer:
12 / 4 = 3 mm.
Question 14
A granite sample has crystals of 4, 6 and 5 mm. Calculate the mean.
Answer:
(4 + 6 + 5) / 3 = 5 mm.
Question 15
A rock contains large crystals in a fine matrix. Name both parts.
Answer:
The large crystals are phenocrysts; the fine material is the groundmass.
Question 16
How does a geological sketch of a crystalline rock differ from one of sand?
Answer:
Draw interlocking crystal boundaries for the crystalline rock; show separate grains in sand.
Question 17
What features commonly help identify granite?
Answer:
It is light-coloured and coarse-crystalline, with minerals such as quartz, feldspar and mica.
Question 18
What features commonly help identify basalt?
Answer:
It is typically dark and fine-crystalline, reflecting rapid cooling of relatively fluid lava.
Question 19
What features commonly help identify andesite?
Answer:
It is commonly intermediate in colour and fine-grained or porphyritic.
Question 20
What mineral is especially characteristic of peridotite?
Answer:
Olivine; peridotite is typically dark and rich in olivine.
Question 21
Why should a rock name be supported by more than its colour?
Answer:
Different rocks can share colours; crystal size, texture and mineralogy provide additional evidence.
Question 22
A dark fine-crystalline rock forms a lava flow. Which named rock is likely?
Answer:
Basalt is likely; check its mineralogy and other observations.
Question 23
A light-coloured rock has visible quartz, feldspar and mica crystals. Identify it.
Answer:
Granite.
Question 24
What is a pluton?
Answer:
A large body of igneous rock that crystallised from magma beneath the surface.
Question 25
What is a sill?
Answer:
A sheet intrusion emplaced approximately parallel to existing bedding or layering.
Question 26
What is a dyke?
Answer:
A sheet intrusion that cuts across the existing bedding or rock structure.
Question 27
What does concordant mean for an igneous intrusion?
Answer:
Its boundary lies approximately parallel to the host rock's layering.
Question 28
What does discordant mean for an igneous intrusion?
Answer:
It cuts across the host rock's layering or other pre-existing structure.
Question 29
A sheet of igneous rock follows bedding and has baked margins above and below. What is it?
Answer:
A sill; the two baked contacts support intrusion into pre-existing rock.
Question 30
A sheet cuts steeply across horizontal beds. What type of body is it?
Answer:
A dyke, because it is discordant to the bedding.
Question 31
How do baked margins provide evidence of an intrusion?
Answer:
Heat from the magma altered the adjacent existing country rock at the contact.
Question 32
Why does a large pluton tend to have coarse crystals?
Answer:
Magma insulated deep underground cools slowly, allowing crystals to grow.
Question 33
What is a lava flow?
Answer:
Lava that spreads over the land surface and solidifies.
Question 34
How can field evidence distinguish a lava flow from a sill?
Answer:
A flow may have a baked lower contact and weathered or vesicular upper surface; a sill can bake both contacts.
Question 35
How does pillow lava form?
Answer:
Lava erupts into water and cools rapidly at its surface, forming rounded pillow-shaped masses.
Question 36
What does pillow lava suggest about the eruption environment?
Answer:
The lava erupted beneath water, for example on the sea floor.
Question 37
How do columnar joints form?
Answer:
Cooling and contraction of lava cause fractures that produce roughly polygonal columns.
Question 38
Are columnar joints depositional bedding planes?
Answer:
No. They are cooling fractures developed as igneous rock contracts.
Question 39
What is viscosity?
Answer:
A measure of a fluid's resistance to flow.
Question 40
How does viscosity affect lava flow length, other factors being equal?
Answer:
Lower-viscosity lava generally flows more readily and farther than more viscous lava.
Question 41
How does cooling normally affect the viscosity of a given lava?
Answer:
As it cools, its viscosity generally increases and it flows less readily.
Question 42
What eruption style is associated with low-viscosity basaltic lava?
Answer:
Relatively passive, effusive eruptions, often from fissures.
Question 43
Why can high-viscosity magma produce more violent eruptions?
Answer:
It can resist gas escape, allowing pressure to build before fragmentation and eruption.
Question 44
How do shield volcanoes commonly differ from steep composite volcanoes?
Answer:
Shield volcanoes have broad gentle slopes from fluid lava; composite volcanoes are generally steeper and can erupt explosively.
Question 45
What is a fissure eruption?
Answer:
An eruption in which lava emerges along an elongated crack rather than only through a single central vent.
Question 46
What is a central vent?
Answer:
A localised opening through which volcanic material reaches the surface.
Question 47
In a jelly-lava simulation, what variable could model slope angle?
Answer:
The angle of the surface over which the jelly mixture flows.
Question 48
How could you test the effect of viscosity on simulated lava-flow length?
Answer:
Change mixture viscosity while keeping volume, slope and release method constant; measure and repeat flow length.
Question 49
Why repeat each simulated lava flow?
Answer:
Repeats help identify anomalous results and make the mean more reliable.
Question 50
Three flows measured 16, 18 and 20 cm. What is their mean length?
Answer:
18 cm; (16 + 18 + 20) / 3.

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Igneous Rocks and Processes

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