Magma Formation and Composition

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Questions and Answers

What is the primary product of magma formed by partial melting?

Metamorphic rocks

Volcanic ash

Sedimentary rocks

Igneous rocks (correct)

Which factor is NOT relevant to the composition of magma during its formation?

Depth of source rock

Color of source rock (correct)

Texture of source rock

Temperature of source rock

What characterizes equilibrium melting?

Temperature remaining constant throughout

Addition of external chemicals

Removal of solid residue during melting

Constant overall composition with evolving melt (correct)

What happens to the solid residue during equilibrium melting?

It becomes enriched in high-T components Signup and view all the answers

Which of the following correctly describes fractional melting?

It produces a more evolved melt than the parent source Signup and view all the answers

What effect does decreasing pressure have on melting processes?

It decreases lithostatic stress Signup and view all the answers

The geothermal gradient varies due to which of the following factors?

Rock age and tectonic setting Signup and view all the answers

Which process is primarily responsible for basaltic magma generation at ocean spreading ridges?

Decompression melting due to rising mantle Signup and view all the answers

What must the order of multiple qualifiers reflect in rock nomenclature?

Order of increasing abundance Signup and view all the answers

Which prefix is designated specifically for metamorphosed igneous rock?

meta- Signup and view all the answers

Which type of rock must be classified using its phenocryst if no mode or chemical analyses are available?

Volcanic rock Signup and view all the answers

Which IUGS classification term ranks highest in the hierarchy?

Pyroclastic rocks & Tephra Signup and view all the answers

What is the role of the prefix 'micro-' in rock nomenclature?

Used for fine-grained plutonic rock Signup and view all the answers

Which of the following is a restriction when using the suffix '-bearing'?

Must have a consistent definition Signup and view all the answers

Which of the following rocks is not classified as igneous according to the IUGS hierarchy?

Sedimentary rock Signup and view all the answers

Rocks believed to be pyroclastic in origin must be formed by which process?

Fragmentation due to explosive eruption Signup and view all the answers

What distinguishes a rock as a carbonatite?

Contains over 50% modal carbonates Signup and view all the answers

Which of the following defines a melilite-bearing rock?

Contains 10% or more modal melilite Signup and view all the answers

What type of carbonatite is known for containing ankerite?

Ferrocarbonatite Signup and view all the answers

Which rock is characterized specifically as plutonic melilite-bearing?

Melilitolite Signup and view all the answers

When is a rock referred to as silicocarbonatite?

When it has 20% or more of other minerals Signup and view all the answers

How is a mixed pyroclastic-epiclastic deposit referred to?

Tuffite Signup and view all the answers

Which melilite-bearing classification is named if perovskite exceeds 10%?

Afrikandite Signup and view all the answers

What type of carbonatite is specifically found in the Oldoinyo Lengai volcano in Tanzania?

Natrocarbonatite Signup and view all the answers

Which of the following is NOT considered an accessory mineral?

Quartz Signup and view all the answers

What is the purpose of the Color Index / Mafic Index in igneous rock analysis?

To quantify the proportion of mafic minerals relative to felsic minerals Signup and view all the answers

Which of the following statements about Point Count Analysis is correct?

It uses a grid system to tabulate mineral points for accuracy. Signup and view all the answers

Which mineral group does not contain potassium-bearing minerals?

Quartz Signup and view all the answers

Which reagent is used in the CIPW Norm for normative mineralogy?

Iddings Signup and view all the answers

What does Modal Classification refer to in the context of mineralogy?

The identification of mineral types and their volume percentages by visual inspection Signup and view all the answers

What is the primary component range of Olivine?

Forsterite to Fayalite Signup and view all the answers

Which of the following minerals is a member of the Pyroxene group?

Augite Signup and view all the answers

Which mineral presence prohibits a rock from being classified as lamproite?

Sodalite Signup and view all the answers

What characterizes the molar K2O/Na2O ratio for Ultrapotassic lamproites?

Greater than 3 Signup and view all the answers

Which type of Leucitite is characterized by foids being 60-90% felsic minerals and AF > P?

Phonolitic Leucitite Signup and view all the answers

Which of the following is NOT a characteristic of lamprophyres?

High SiO2 content Signup and view all the answers

Charnockitic rocks are characterized by the presence of which mineral?

Orthopyroxene Signup and view all the answers

Which of the following increases the complexity of lamproites?

Extent of partial melting Signup and view all the answers

What defines a Tephritic Leucitite?

Foids are 60-90% felsic minerals and P > AF Signup and view all the answers

What kind of alteration is often seen in lamprophyres?

Hydrothermal alteration Signup and view all the answers

Study Notes

Magma Formation

Magma consists of molten rock generated from the partial melting of the mantle and crust, primarily composed of silicates.
The composition of magma is influenced by temperature, pressure, and chemical makeup.

Types of Melting

Anatexis involves the partial melting of source rock, leading to liquid melt rich in low-temperature components while the remaining solid rock becomes enriched in high-temperature components.
Factors influencing anatexis include the composition and texture of the source rock and its melting history.
Equilibrium Melting maintains a constant overall system composition; as melting occurs, low-temperature components are enriched in the melt while high-temperature minerals dominate the solid residue.
Fractional Melting separates solids and melt into isolated fractions, resulting in a more evolved melt compared to the parent rock.

Melting Factors

Temperature increases with depth, represented by the geothermal gradient, which varies with tectonic setting and rock age.
Decreasing Pressure leads to adiabatic melting, where the reduction of lithostatic stress allows for melting at lower temperatures, especially at ocean spreading ridges.

Common Minerals

Quartz, feldspar (K-feldspar, plagioclase), and several mafic minerals like olivine and pyroxene are prevalent in igneous rocks.
Accessory minerals occur in less than 5% abundance and include magnetite, hematite, and zircon.

Chemical Composition Terminology

Mafic Index quantifies the proportion of mafic minerals:
- Formula: %mafic/(%mafic + %felsic) x 100.
Modal Classification identifies mineral presence and volume percentages through visual inspection or thin section analysis.

Point Count Analysis offers a detailed modal classification by tabulating mineral occurrences in thin sections through a grid method.
Normative Mineralogy (CIPW Norm) uses chemical analysis to mimic actual minerals, aiding in rock classification.

Igneous Rock Nomenclature Rules

Nomenclature involves adding qualifiers to describe mineral content and texture, ensuring clarity and compliance with established definitions.
A hierarchy is present in using IUGS classifications, prioritizing special rocks before standard classifications.

Pyroclastic Rocks and Tephra

Classification of pyroclastic rocks is reserved for deposits formed by explosive fragmentation, excluding those formed by autobrecciation.
Mixed pyroclastic and epiclastic deposits are referred to as tuffites, categorized by grain size.

Carbonatites Classification

Carbonatites must contain over 50% modal carbonates and can be classified as volcanic or plutonic based on mineral content.
Variants include Sövite (calcite-rich) and Alvikite (medium to fine-grained), with ferrocarbonatite indicating the presence of iron-rich carbonate.

Melilite-bearing Rocks

Defined by containing at least 10% modal melilite; classification varies for plutonic (melilitolite) and volcanic (melilitite) forms.
Specific names are given based on dominant mineral presence, with strict guidelines on priority and abundance.

Lamproites and Leucite-bearing Rocks

Lamproites difficulty separates chemically, distinguished by occurrence as dikes and specific mineral compositions.
Leucite-bearing rocks use prefixes indicating the presence of leucite and are subdivided based on mineral composition.

Lamprophyres and Charnockitic Rocks

Lamprophyres are characterized by specific porphyritic textures and essential biotite or amphibole.
Charnockitic rocks require the presence of orthopyroxene and perthite for classification within the charnockitic suite.

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Description

This quiz explores the processes involved in magma formation, including anatexis and the factors affecting its composition, texture, and depth. Understand how molten material is generated and its significance in forming igneous rocks.