The purpose of these interpretive discussions is to provide a perspective on regional- and national-scale variations in element and mineral distributions in soils and their likely causes. The significant spatial variations shown by most elements and minerals can commonly be attributed to geologic sources in underlying parent materials, but other spatial variations seem clearly related to additional factors such as climate, the age of soils, transported source material, and anthropogenic influences. We attempt to distinguish the influence of these various factors on a regional and national scale. Numerous more local features might similarly be related to these same factors, but these features also have some probability of being an artifact of a random sampling of variable compositions, so that there is some probability of samples with similar compositions occurring in clusters of two or more adjacent sites by chance. Distinguishing such random occurrences from true variability is beyond the scope of the data from which these maps are constructed. Some caution, therefore, is advisable in interpreting the significance of these more local features unless some unique sources or processes can clearly be related to them.
The members of the feldspar group of minerals are the most common rock–forming mineral series. Feldspars have limited solid solution among three end members —potassium (K) feldspar, sodium (Na) feldspar, and calcium (Ca) feldspar. The plagioclase feldspar series includes six minerals with varying proportions of Ca and Na, and chemical formulas ranging between NaAlSi3O8 and CaAl2Si2O8. Plagioclase is the most common mineral in many mafic and intermediate igneous rocks. Igneous rocks with abundant plagioclase can include basalt, andesite, gabbro, diabase, and anorthosite. Plagioclase is stable at nearly all metamorphic grades, and thus is common in many metamorphic rocks. Plagioclase is also common in sedimentary rocks, including sandstone and shale, both as a detrital and an authigenic mineral. Unconsolidated materials such as alluvial, eolian, or glacial deposits also may contain plagioclase. Plagioclase is susceptible to weathering in soils, with the weathering rate of calcium (Ca)–plagioclase greater than that of Na–plagioclase. With time in a sufficiently humid climate, plagioclase breaks down, resulting in leaching of dissolved Ca and Na and formation of aluminum (Al)–rich clay minerals, principally kaolinite (Al2(Si2O5)(OH)4). Thus, the distribution of plagioclase in soils is dependent on the occurrence of the mineral in the initial parent material, climate factors, and soil age.
In our data, plagioclase has a median concentration of 7.9 weight percent (wt. %) in the soil C horizon, and 8.6 wt. % in the soil A horizon (see the summary statistics [open in new window]). The map patterns for both soil horizons are similar to one another.
Plagioclase occurs in soil where parent materials include mafic and intermediate igneous rocks, sedimentary rocks, and more recent alluvial and eolian deposits sourced from similar rock types. Examples of such areas (and dominant soil parent materials) include:
- Pacific Northwest (mafic rocks, including the Columbia River basalts (Swanson and Wright, 1981) and Cascade Range basalt and andesite);
- Parts of the Central Rocky Mountains and Southern Rocky Mountains and the High Intermountain Valleys (USDA, 2006), Colorado, Wyoming, Montana, and Idaho (a variety of igneous rock types, including basalt and andesite);
- Southern Sierra Nevada Mountains and Carson Basin and Mountains (USDA, 2006), California and Nevada (basalt and andesite);
- Trans–Pecos volcanics (Schruben and others, 1997), Texas (rhyolite, alkalic volcanic rocks, and related alluvium);
- Southeastern Arizona Basin and Range (USDA, 2006), Arizona (basalt and andesite); and
- Adirondack highlands (Fenneman and Johnson, 1946), New York (anorthosite, gabbro, and metamorphic rocks).
Soil in southeastern Wyoming and northeastern Colorado that developed on feldspar–rich sandstone and claystone, and soil across much of northern and western Nebraska, including the Nebraska Sand Hills (USDA, 2006) (an area of unconsolidated sand dunes and sand sheets) have higher plagioclase concentrations than soil developed in eolian deposits in southeastern Colorado and soil formed in loess sheets in Kansas and eastern Nebraska.
The distribution of plagioclase in the Upper Midwest is related to glacial provenance. In this region, melting of glacial ice following late Wisconsinan period advances (16,000 to 12,000 years ago) left the region north of the southern glacial limit (Soller and others, 2012) mantled with a blanket of mixed, immature sediments, from which present–day soils developed. Individual ice lobes (Grimley, 2000) created distinct patterns in soil mineralogy and geochemistry because of varying provenance and ice transport paths. 'Gray' (carbonate– and shale–rich) till in the James and Des Moines lobes (Grimley, 2000) was derived from Cretaceous sedimentary rocks (dolostone, limestone, and shale). Gray tills were transported significant distances to the south and southeast from their source and deposited in areas underlain by Precambrian bedrock. 'Red' quartz– and feldspar–rich tills deposited in northeastern Minnesota, northern Wisconsin, and northwestern Michigan have a Precambrian crystalline rock provenance (Rainy lobe (Grimley, 2000)) and Precambrian red sandstone (with variable quantities of quartz and feldspar) provenance (Superior lobe (Grimley, 2000)). In northeastern Minnesota, soil on thin glacial deposits strongly reflects local Precambrian basalt and gabbro bedrock of the Duluth Complex. Thus, the concentration of plagioclase is markedly different among the different glacial lobes depending on local and distant provenance.
Soils throughout the Piedmont (Fenneman and Johnson, 1946) are old and have undergone long–term weathering in a humid to semi–humid environment. Many soils, including some that developed on parent materials that originally contained plagioclase, are highly leached, clay–rich soil with no remaining feldspar. However, in a few areas, such as central North Carolina, local occurrences of plagioclase persist in soil underlain by metamorphic rocks.
Large parts of the conterminous United States never contained plagioclase in soil parent material. Examples and their dominant soil parent materials are:
- Colorado Plateau (USDA, 2006) (quartz–rich sandstone and eolian deposits);
- Texas carbonate terrane (limestone, dolostone, chalk and marl bedrock); and
- Gulf and Atlantic Coastal Plain (Fenneman and Johnson, 1946) (quartz–rich sedimentary rocks and unconsolidated sediments).
The Gulf and Atlantic Coastal Plain (Fenneman and Johnson, 1946) is bisected by the Southern Mississippi River Alluvium and Southern Mississippi Valley Loess (USDA, 2006). Alluvial sediments have deposited in the Mississippi River valley as the river flooded in recent geologic time. When these sediments dried, winds picked up the fine material and deposited it in thick loess sheets, mainly along the east side of the river valley. The youngest loess sheets are about 10,000 years old. A pattern of higher plagioclase concentrations in soils developed on these young sediments reflects long–range transport of material from areas containing plagioclase from the upper part of the Mississippi River drainage basin.
Statistics - 0 TO 5 CM
Statistics - A Horizon
|Number of samples||4,802|
|LLD||0.2 wt. %|
|Number below LLD||682|
|Minimum||<0.2 wt. %|
|5 percentile||<0.2 wt. %|
|25 percentile||3.5 wt. %|
|50 percentile||8.6 wt. %|
|75 percentile||16.2 wt. %|
|95 percentile||35.4 wt. %|
|Maximum||70.5 wt. %|
Empirical cumulative distribution function
Statistics - C Horizon
|Number of samples||4,773|
|LLD||0.2 wt. %|
|Number below LLD||830|
|Minimum||<0.2 wt. %|
|5 percentile||<0.2 wt. %|
|25 percentile||2.4 wt. %|
|50 percentile||7.9 wt. %|
|75 percentile||15.5 wt. %|
|95 percentile||35.9 wt. %|
|Maximum||67.0 wt. %|