4  Parent material

Parent material can be explored in terms of geological units, lithology, and mineralogy.

4.1 Geology

Geological maps are the primary data source representing the spatial distribution of soil parent material. In particular, surface and near-surface stratigraphic maps are an essential resource for understanding the landscape. However, the scale and focus of the available products don’t always align with soil observation needs. Some caution is needed when interpreting geological maps in a soils context. Keep the following in mind:

• Read the accompanying reports for a given area as well as looking at geological maps. They contain a wealth of contextual information.
• When working in an area covered by transported soil materials, get familiar with the ‘source’ geologies - upstream, uphill, upwind. Those will be the main contributors to the soil parent material.
• In landscapes where transported materials make up the uppermost materials of the land as unconsolidated cover beds (such as in central North Island), it is common for such cover beds to be missing from geological maps (although they will likely be noted in the accompanying text, e.g., see Leonard et al. 2010). This because the map units selected in geological mapping tend to reflect the underlying, large-scale deposits that have been land-forming in emplacement.
• The unit boundaries of broad-scale geology maps don’t always align cleanly with landscape features on the ground, so they may not make reliable inputs for digital models of soil distribution without substantial refinement.

Due to these scale and purpose differences, recording the geological map code at a profile is optional, and if recorded, the surveyor should be confident that code is correct at the scale of the site. The source of the code should also be noted.

Geology mapping in New Zealand can be accessed through resources provided by Earth Sciences New Zealand. These include QMAP, a unified 1:250,000 scale geological map of the country, and a large set of legacy geology maps produced at scales as detailed as 1:25,000. In areas where volcanic ash deposition is extensive (e.g., the central North Island), tephra isopach maps showing the thickness and location of ash deposits from particular eruptive events will also be essential for understanding the landscape (Hopkins et al. 2021).

4.2 Lithology

The specific rock(s) contributing to the soil parent material, or its lithology, offers a more specific and soil-relevant categorisation than geology mapping supplies. Lithology should be recorded when describing the soil’s fine earth fraction (mineral material <2 mm), surface and in-profile rock fragments (mineral material >2 mm), subsolum rock, and profile-adjacent rock outcrops. Specific recording requirements for all of these characteristics are defined in Chapter 10, Chapter 12 and Chapter 14. Table 4.1 below provides a list of parent rocks common in New Zealand that can be applied in each case. Where lithology is difficult to determine, users may record using the second-level groupings (e.g. in mixed-origin alluvia), but it is always preferable to use the third level for rock fragments, subsolum material and outcrops. Table 4.2 provides some additional mineral modifiers that may be applied to the clastic sedimentary rocks.

Lithology data is required for classifying some orders of the New Zealand Soil Classification (e.g., Melanic Soils) and is also required when determining the Family and Sibling (Webb and Lilburne 2011). The code list used for this task is highly generalised but can be mapped to the rock types list in Table 4.1.

Table 4.1: Lithology classification for New Zealand soil survey (adapted from Laffan and Mew (1988), Rattenbury and Isaac (2012) and IUSS Working Group WRB (2022))

Major rock type	Composition	Composition code	Rock type	Rock type code
Intrusive igneous rocks	Felsic	IF	Granite	GRAN
			Quartz-diorite	QZDI
			Granodiorite, tonalite	GRDI
	Intermediate	II	Diorite	DIOR
			Syenite	DYEN
	Mafic	IM	Gabbro	GABB
			Dolerite	DOLE
			Lamprophyre	LAMP
	Ultramafic	IU	Peridotite	PERD
			Pyroxenite	PYRX
			Ilmenite, (titano)magnetite	ILMG
Extrusive igneous rocks	Felsic	EF	Rhyolite	RHYL
			Felsic tuff and tuffite	FTUF
			Felsic breccia or agglomerate	FBRC
			Pumice	PUMC
			Ignimbrite	FIGN
			Obsidian	OBSD
	Intermediate	EI	Andesite	ANDS
			Trachyte	TRAC
			Intermediate tuff and tuffite	ITUF
			Intermediate breccia or agglomerate	IBRC
			Intermediate scoria	ISCR
			Dacite	DACT
			Ignimbrite	IIGN
	Mafic	EM	Basalt	BSLT
			Basanite, tephrite	BSNT
			Mafic tuff and tuffite	MTUF
			Mafic breccia or agglomerate	MBRC
			Mafic scoria	MSCR
	Ultramafic	EU	Picrite	PICR
Metamorphic Rocks	Felsic	MF	Quartzite	QTZT
			Felsic gneiss, migmatite, mylonite	FGNS
			Schist	FSCH
			Semi-schist slate, phyllite	FSLT
	Mafic	MM	Mafic gneiss, migmatite, mylonite	MGNS
			Amphibolite, Eclogite	AMPH
	Ultramafic	MU	Serpentinite	SERP
			Greenstone	GNST
	Carbonatic	MC	Marble	MBLE
Sedimentary rocks	Clastic	SC	Conglomerate, breccia (hard)	CONH
			Conglomerate, breccia (soft)	CONS
			Sandstone (hard), greywacke, arkose	SAHD
			Sandstone (soft)	SAFT
			Silt-, mud-, claystone (hard)	SIHD
			Silt-, mud-, claystone (soft)	SIFT
	Carbonatic	SO	Limestone	LMST
			Dolostone	DOLO
			Marl, chalk, and similar soft mixtures	MARL
	Organic	SB	Coal, lignite, bitumen	COAL
	Evaporites	SE	Anhydrite, gypsum	GYPS
			Halite	HALT

Table 4.2: Mineral modifiers for clastic sedimentary rocks

Code	Name	Description
Cb	Carboncaeous	containing a large proportion of organic material
Ct	Carbonatic	containing a large propotion of carbonate minerals (but below the threshold for a limestone)
Si	Siliceous	containing a large proportion of secondary silica minerals
Qz	Quartzitic	containing a large proportion of quartz grains
Qf	Quartzo-feldspathic	containing a large proportion of quartz and feldspar minerals
Gl	Glauconitic	containing a large proportion of the greenish mineral glauconite
Ch	Chloritic	containing a large proportion of the greenish mineral chlorite
Be	Bentonitic	containing a large proportion of the swelling clay mineral bentonite
Mi	Micaceous	containing a large proportion of the platy, shiny mineral mica
Mf	Mafic	containing a large proportion of mafic or ultramafic minerals
Tf	Tuffaceous	containing a large proportion of pyroclastic materials

Note 4.1: What are soft rocks?

‘Soft’ rocks in this context can at most be broken readily with a light hammer blow, and can be scraped and peeled by a pocket knife. ‘Hard’ rocks can at best be scratched with a knife but not scraped or peeled, and require a firm hammer blow to crack (Lynn and Crippen 1991, p. 11).

Note 4.2: Geological research vocabularies and other approaches to recording lithology

To encourage consistent geological terminology in scientific communication, several research-grade vocabularies for describing lithology have been developed in recent years, including Geoscience Australia (2016) and McCormick and Heaven (2023). These offer a detailed set of rock and mineral definitions in a hierarchical relationship structure, but are currently missing one or more terms essential to the New Zealand context. Additionally, the hierarchies don’t always align cleanly with those used in New Zealand geology. Research vocabularies are expected to develop over time and may be recommended in future editions of this handbook.

Other, simpler lithological vocabularies available in New Zealand and elsewhere often conflate lithology with mode of deposition and particle size when describing unconsolidated, transported materials (e.g. NZLRI rock types, Lynn and Crippen (1991) and WRB parent material, IUSS Working Group WRB (2022)). Parent material origin and particle size distribution are described separately for soil materials in this handbook (Section 11.8.3, Chapter 14). However, workers describing soils as part of an LUC assessment may still wish to use the NZLRI rock types list for compatibility reasons (Lynn and Crippen 1991).

Soil parent material classifications have largely focused on major geological origin (igneous, metamorphic, sedimentary), grain size, and some obvious compositional classes. (2016) contend that the major differentiator for soil formation should first be silica (SiO₂) content and propose an alternate lithology classification on that basis.

4.3 Mineralogy

Soil mineral composition influences nutrient availability, pH, buffering capacity and many other chemical properties. Some plants have a preference for, or are specifically adapted to, soils with particular mineral profiles. Mineralogy information can also be used to help trace the origin of transported soil materials back to their parent rocks. An comprehensive review of minerals in soils can be found in Churchman and Lowe (2012).

The New Zealand Soil Classification (Hewitt and MWLR Pedology Staff, 2025) references a previous version of the Soil Taxonomy mineralogy classification, adapted to New Zealand conditions and extensively tested (Whitton and Childs 1989; Childs and Whitton 1990). The classes are mentioned in relation to soil Orders but are not used in the key or the diagnostic materials, so there is no requirement to record these in the field. Where mineralogy is of interest during fieldwork, the current mineralogy classes of Soil Taxonomy (Soil Survey Staff 2022) may be estimated, but these will require laboratory confirmation and the ability to classify the profile against Soil Taxonomy at least to Great Group level.

Childs CW, Whitton JS 1990. A proposed revision of the key to mineralogy classes of Soil Taxonomy (1975). In: Transactions 14th International Congress of Soil Science, Kyoto, Japan, August 1990, Volume VII [Internet]. Vol. VII. Kyoto, Japan: IUSS; p. 10–16. https://old.iuss.org/meetings-events/world-soil-congress/index.html

Churchman JG, Lowe DJ 2012. Alteration, formation, and occurrence of minerals in soils. [Internet]. In: Huang PM, Li Y, Sumner ME, editors. Vol. 1. 2nd ed. [place unknown]: CRC Press; p. 72. https://doi.org/10.1201/b16386

Geoscience Australia 2016. Simple lithology. [Internet]. https://vocabs.ardc.edu.au/viewById/56

Gray JM, Bishop TFA, Wilford JR 2016. Lithology and soil relationships for soil modelling and mapping. CATENA [Internet]. 147:429–440. https://doi.org/10.1016/j.catena.2016.07.045

Hewitt AE, MWLR Pedology Staff, 2025. New Zealand soil classification. 4th ed. Wellington, New Zealand: Manaaki Whenua Landcare Research. https://doi.org/10.7931/DL1-LRSS-1-2010

Hopkins JL, Lowe DJ, Horrocks JL 2021. Tephrochronology in aotearoa new zealand. New Zealand Journal of Geology and Geophysics [Internet]. 64(2-3):153–200. https://doi.org/10.1080/00288306.2021.1908368

IUSS Working Group WRB 2022. World Reference Base for Soil Resources: International soil classification system for naming soils and creating legends for soil maps. [Internet]. 4th ed. Vienna, Austria: International Union of Soil Scientists. https://wrb.isric.org/

Laffan MD, Mew G 1988. Classification of rocks for soil differentiation and field description. [Internet]. Nelson, New Zealand. https://doi.org/10.7931/DL1-DOR-NS27

Leonard DS, Begg JG, Wilson CJN 2010. Geology of the rotorua area. Lower Hutt, New Zealand.

Lynn IH, Crippen TF 1991. Rock type classification for the New Zealand Land Resource Inventory. [Internet]. Lower Hutt, New Zealand: DSIR Land Resources. https://doi.org/10.7931/DL1-DLRSR-10

McCormick T, Heaven RE 2023. The British Geological Survey Rock Classification Scheme, its representation as linked data, and a comparison with some other lithology vocabularies. Applied Computing and Geosciences [Internet]. 20:100140. https://doi.org/10.1016/j.acags.2023.100140

Rattenbury M, Isaac M 2012. The QMAP 1:250 000 geological map of new zealand project. New Zealand Journal of Geology and Geophysics [Internet]. 55(4):393–405. https://doi.org/10.1080/00288306.2012.725417

Soil Survey Staff 2022. Keys to Soil Taxonomy. [Internet]. 13th ed. USA: National Resources Conservation Service, United States Department of Agriculture. https://www.nrcs.usda.gov/sites/default/files/2022-09/Keys-to-Soil-Taxonomy.pdf

Webb TH, Lilburne LR 2011. Criteria for defining the soil family and soil sibling : The fourth and fifth categories of the New Zealand Soil Classification. [Internet]. 2nd ed. Christchurch, New Zealand: Manaaki Whenua Press. https://doi.org/10.7931/DL1-LRSS-3-2011

Whitton JS, Childs CW 1989. The new zealand experience in applying the soil taxonomy (1975) key to mineralogy classes, and, a proposed revision of the key to mineralogy classes. [Internet]. Lower Hutt, New Zealand. https://doi.org/10.7931/dl-tech-lh1