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| Lava flows (dark material) cover surface to the lower near-right, middle left, and across the canyon on the opposing cliff surface. Some of the material on the lower surface is colluvium. Near Hurricane, UT, USA. |
lava flows - surface flows of molten magma
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| Lava flows (dark material) cover surface to the lower near-right, middle left, and across the canyon on the opposing cliff surface. Some of the material on the lower surface is colluvium. Near Hurricane, UT, USA. |
plutons - underground intrusions of molten magma
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| Independence Rock, the rounded peak in the center of this picture, is a classic granite pluton, formed by an underground volcanic intrusion and later exposed by weathering and erosion. Independence Rock was a major landmark and meeting place on the Oregon Trail. |
volcanic ash - often non-crystalline or poorly-crystalline
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| Volcanic ash deposit near Azrou, Morocco.
Note particle size stratification, presence of larger clasts within the beds. |
cinders - porous, gas-filled particles, usually composed of some crystalline minerals held together by a non-crystalline, glassy matrix. Highly porous parent material, often drouthy due to very high hydraulic conductivities.
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| Cross-section of a large (12 cm dia.) weathered cinder from Hurricane, UT. Note porous, vesicular structure and thick weathering rind that reaches nearly 2/3 of the way to the center. |
Igneous, sedimentary, or other metamorphic rocks can be altered by high temperatures, high pressures, or both. The rocks so produced are termed metamorphic rocks. Examples include the formation of slate from shale, marble from limestone, and serpentinites from mafic or ultra-mafic igneous rocks.
Sedimentary rocks have formed from formerly unconsolidated materials that have been deposited and consolidated or otherwise cemented together.
There are two major types:
crystallites - those which have formed by crystallization of aqueous solutes. Examples include limestone, chert, and numerous types of evaporites.
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| Weathered limestone with loess cap, southeastern Minnesota. |
accretionary materials - these materials were merely transported and deposited by wind or water. Virtually any type of mineral material may be transported and included in sedimentary rocks. Since deposition, they have become consolidated, usually through cementation by some chemical agent such as iron oxides, silica, or calcium carbonate. They are commonly classified on the basis of particle size: conglomerates, sandstones, siltstones, and mudstones or shales.
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| Conglomerate rock from Death Valley, CA, USA. Photo courtesy of Bill Zanner. |
Degree of crystallinity. Materials have a wide range of crystallinity (See figure below).
Crystalline minerals - have a repeating structure and a more or less constant composition. Common examples of crystalline minerals include quartz, feldspars, hornblendes, olivine, and micas. The type of minerals present influences the pH of the materials, the elemental composition, the ease of weathering of the parent rock, the nutrient status of the soil, and the types of secondary minerals formed during weathering.
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| Pyrophyllite crystal structure viewed from the c axis. |
Non-crystalline materials - do not have a repeating structure and typically have a wide range of compositions. Common examples include volcanic glass and ash, and some clay minerals, such as allophane, and some iron and aluminum oxides.
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| One should note that more than one type of structural disorder is represented along the x-axis of this figure. |
Para-crystalline materials - generally have a characteristic structure and a more or less constant composition, but cannot form a repeating structure. Examples include smectites (thin sheets) and halloysite, chrysotile, and imogolite (small tubes).
grain size - in young soils, the grain size is determined mainly by the size of mineral grains in the parent rock. In older, more highly weathered soils, grain size may depend more on the weatherability of the minerals present in the parent rock.
pH - soil pH is initially determined to a large extent by the parent material composition and ease of weathering of the parent rock.
nutrient status / trace element content - the ultimate source of nutrients and trace elements in soils is weathering of primary minerals. Rocks and minerals lacking in specific nutrient elements cannot provide them to soils.
potential secondary minerals formed -
The type of minerals present in parent materials, as well as their degree of crystallinity, is strongly dependent on their geologic and transport history. The mineralogy of igneous rocks depends not only on the composition of the melt, but also on the temperature, pressure, and the rate of cooling of the magma. The rate of cooling particularly affects the grain size of the minerals that form.
Transport processes that produce well-sorted materials can strongly influence the mineralogy of the sediments they leave behind because many minerals occur only in a selected size fraction range. For example, smectite clays are seldom larger than 0.2 µm, whereas the majority of quartz grains occur in the sand and coarse silt fractions. Outwash deposits (initially) contain only limited quantities of smectites, whereas quartz is present only in small quantities in lacustrine deposits.
The primary minerals present in parent materials strongly influence the nature of the secondary minerals formed during soil development. Clays formed from non-crystalline materials are often non-crystalline themselves. Examples include allophane and imogolite, which commonly form in weathered volcanic ash. Smectite, vermiculite, and kaolinite commonly form from crystalline parent materials. However, this is not a clear cut distinction: smectite and halloysite (a hydrated kaolinite) can form in volcanic ash, while imogolite has been found in spodic horizons formed in crystalline parent materials.
Most soils form in unconsolidated materials. Because they have been
transported, any variety or combination of minerals may initially
be present in unconsolidated materials, even combinations that would not
occur in geologic materials. Examples include the presence of both acidic
and basic mineral types, volcanic ash and crystalline igneous minerals,
and any other conceivable combinations. Consequently, the properties of
the soils formed in the sediments may be difficult to predict. Clearly,
the mode of transport and the composition of the source materials determines
the overall composition of the parent materials.
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Mode of Deposition | Rock Type | Rock pH | Grain Size | Properties | Organic Parent Materials