岩の地質

物理的特性

岩石の物理的特性は、地質学、岩石物理学、地球物理学、材料科学、地球化学、地盤工学などの多くの作業分野で重要かつ有用です。調査の規模は、分子および結晶から地球および他の惑星体の陸域研究にまで及びます。地質学者は、鉱物堆積物の起源を再構築するために、岩石の放射年代測定に興味を持っています。地震学者は、事前の物理的または化学的変化を使用して、予想される地震予測を策定します。結晶学者は、特別な光学的または物理的特性を持つ鉱物の合成を研究します。探査地球物理学者は、地下の岩石の物理的特性の変化を調査して、石油やガス、地熱エネルギー、金属の鉱石などの天然資源の検出を可能にします。地質工学のエンジニアは、建物、ダム、トンネル、橋、地下貯蔵庫などの構造物を、その上に、またはその中に構築する材料の性質と挙動を調べます。固体物理学者は、電子デバイス、コンピュータコンポーネント、または高性能セラミックの材料の磁気的、電気的、および機械的特性を研究します。石油貯留層のエンジニアは、坑井の丸太で、または高温高圧での深部掘削のプロセスで測定された応答を分析します。または高性能セラミック。石油貯留層のエンジニアは、坑井の丸太で、または高温高圧での深部掘削のプロセスで測定された応答を分析します。または高性能セラミック。石油貯留層のエンジニアは、坑井の丸太で、または高温高圧での深部掘削のプロセスで測定された応答を分析します。

Since rocks are aggregates of mineral grains or crystals, their properties are determined in large part by the properties of their various constituent minerals. In a rock these general properties are determined by averaging the relative properties and sometimes orientations of the various grains or crystals. As a result, some properties that are anisotropic (i.e., differ with direction) on a submicroscopic or crystalline scale are fairly isotropic for a large bulk volume of the rock. Many properties are also dependent on grain or crystal size, shape, and packing arrangement, the amount and distribution of void space, the presence of natural cements in sedimentary rocks, the temperature and pressure, and the type and amount of contained fluids (e.g., water, petroleum, gases). Because many rocks exhibit a considerable range in these factors, the assignment of representative values for a particular property is often done using a statistical variation.

Some properties can vary considerably, depending on whether measured in situ (in place in the subsurface) or in the laboratory under simulated conditions. Electrical resistivity, for example, is highly dependent on the fluid content of the rock in situ and the temperature condition at the particular depth.

Density

Density varies significantly among different rock types because of differences in mineralogy and porosity. Knowledge of the distribution of underground rock densities can assist in interpreting subsurface geologic structure and rock type.

In strict usage, density is defined as the mass of a substance per unit volume; however, in common usage, it is taken to be the weight in air of a unit volume of a sample at a specific temperature. Weight is the force that gravitation exerts on a body (and thus varies with location), whereas mass (a measure of the matter in a body) is a fundamental property and is constant regardless of location. In routine density measurements of rocks, the sample weights are considered to be equivalent to their masses, because the discrepancy between weight and mass would result in less error on the computed density than the experimental errors introduced in the measurement of volume. Thus, density is often determined using weight rather than mass. Density should properly be reported in kilograms per cubic metre (kg/m³), but is still often given in grams per cubic centimetre (g/cm³).

Another property closely related to density is specific gravity. It is defined, as noted above, as the ratio of the weight or mass in air of a unit volume of material at a stated temperature to the weight or mass in air of a unit volume of distilled water at the same temperature. Specific gravity is dimensionless (i.e., has no units).

The bulk density of a rock is ρ_B = W_G/V_B, where W_G is the weight of grains (sedimentary rocks) or crystals (igneous and metamorphic rocks) and natural cements, if any, and V_B is the total volume of the grains or crystals plus the void (pore) space. The density can be dry if the pore space is empty, or it can be saturated if the pores are filled with fluid (e.g., water), which is more typical of the subsurface (in situ) situation. If there is pore fluid present, where W_fl is the weight of pore fluid. In terms of total porosity, saturated density is

and thus

where ρ_fl is the density of the pore fluid. Density measurements for a given specimen involve the determination of any two of the following quantities: pore volume, bulk volume, or grain volume, along with the weight.

A useful way to assess the density of rocks is to make a histogram plot of the statistical range of a set of data. The representative value and its variation can be expressed as follows: (1) mean, the average value, (2) mode, the most common value (i.e., the peak of the distribution curve), (3) median, the value of the middle sample of the data set (i.e., the value at which half of the samples are below and half are above), and (4) standard deviation, a statistical measure of the spread of the data (plus and minus one standard deviation from the mean value includes about two-thirds of the data).

地球の上部地殻で見つかったさまざまな種類の岩石の乾燥かさ密度をまとめたものを表に示します。密度の関数としてサンプルのパーセントを示すこれらのデータのヒストグラムプロットを図3に示します。指定されたパラメーターには、（1）サンプルの分割、1つのデータ列の密度の範囲（例：0.036 g / cm ³）が含まれます。図3の場合、（2）サンプル数、および（3）標準偏差。小さな挿入図は、「mode-x」から「mode + x」までの間隔内にあるサンプル（縦軸）のパーセンテージです。ここで、xは横軸です。

さまざまな種類の岩石の乾燥かさ密度

ロックタイプ	サンプル数	平均（立方センチメートルあたりのグラム）	標準偏差	モード（立方センチメートルあたりのグラム数）	中央値（グラム/立方センチメートル）
Source: After data from H.S. Washington (1917) and R.J. Piersol, L.E. Workman, and M.C. Watson (1940) as compiled by Gary R. Olhoeft and Gordon R. Johnson in Robert S. Carmichael (ed.), Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984).
all rocks	1,647	2.73	0.26	2.65	2.86
andesite	197	2.65	0.13	2.58	2.66
basalt	323	2.74	0.47	2.88	2.87
diorite	68	2.86	0.12	2.89	2.87
dolerite (diabase)	224	2.89	0.13	2.96	2.90
gabbro	98	2.95	0.14	2.99	2.97
granite	334	2.66	0.06	2.66	2.66
quartz porphyry	76	2.62	0.06	2.60	2.62
rhyolite	94	2.51	0.13	2.60	2.49
syenite	93	2.70	0.10	2.67	2.68
trachyte	71	2.57	0.10	2.62	2.57
sandstone	107	2.22	0.23	2.22	2.22

In Figure 3, the most common (modal) value of the distribution falls at 2.63 g/cm³, roughly the density of quartz, an abundant rock-forming mineral. Few density values for these upper crustal rocks lie above 3.3 g/cm³. A few fall well below the mode, even occasionally under 1 g/cm³. The reason for this is shown in Figure 4, which illustrates the density distributions for granite, basalt, and sandstone. Granite is an intrusive igneous rock with low porosity and a well-defined chemical (mineral) composition; its range of densities is narrow. Basalt is, in most cases, an extrusive igneous rock that can exhibit a large variation in porosity (because entrained gases leave voids called vesicles), and thus some highly porous samples can have low densities. Sandstone is a clastic sedimentary rock that can have a wide range of porosities depending on the degree of sorting, compaction, packing arrangement of grains, and cementation. The bulk density varies accordingly.

乾燥かさ密度の他の分布プロットを図5と6に示します。サンプルの区分は、図5と6Aでは0.036 g / cm ³、図6Bでは0.828パーセントです。表には、アメリカの地質学者であるGordon R. JohnsonとGary R. Olhoeftが作成した他のさまざまな種類の岩石の典型的な乾燥かさ密度の範囲を示しています。

他のいくつかの岩石タイプの典型的な密度範囲

ロックタイプ	密度（グラム/立方センチメートル）
出典：RA Daly、GE Manger、およびSP Clark、Jr.（1966）からのデータの後。AFバーチ（1966）; F.プレス（1966）; およびRN Schock、BP Bonner、およびH. Louis（1974）（Robert S. Carmichael（ed。）、Handbook of Physical Properties of Rocks、vol。III、CRC Press、Inc.（1984）。
角閃岩	2.79〜3.14
安山岩ガラス	2.40〜2.57
無水石膏	2.82〜2.93
斜長石	2.64〜2.92
玄武岩ガラス	2.70〜2.85
チョーク	2.23
ドロマイト	2.72〜2.84
ダナイト	2.98–3.76
eclogite	3.32–3.45
gneiss	2.59–2.84
granodiorite	2.67–2.78
limestone	1.55–2.75
marble	2.67–2.75
norite	2.72–3.02
peridotite	3.15–3.28
quartzite	2.65
rock salt	2.10–2.20
schist	2.73–3.19
shale	2.06–2.67
slate	2.72–2.84

The density of clastic sedimentary rocks increases as the rocks are progressively buried. This is because of the increase of overburden pressure, which causes compaction, and the progressive cementation with age. Both compaction and cementation decrease the porosity.

Representative densities for common rock-forming minerals (i.e., ρ_G) and rocks (i.e., ρ_B) are listed inthe

テーブル。典型的には、可変多孔性を有する堆積岩バルク密度は、両方ρドライの範囲として与えられる_Bと（水）飽和ρの_B。細孔を満たしている流体は通常ブライニー水であり、岩が堆積または固化されたときに海水の存在をしばしば示します。気孔率によっては、かさ密度が構成鉱物（または鉱物集合体）の粒子密度よりも小さいことに注意してください。たとえば、砂岩（特徴的にはクオートース）の典型的な乾燥かさ密度は2.0〜2.6 g / cm ³で、気孔率は低いものから30％以上までさまざまです。石英自体の密度は2.65 g / cm ³です。気孔率がゼロの場合、かさ密度は粒子密度と等しくなります。

Saturated bulk density is higher than dry bulk density, owing to the added presence of pore-filling fluid.The Table also lists representative values for density of seawater, oil, and methane gas at a subsurface condition—pressure of 200 bars (one bar = 0.987 atmosphere, or 29.53 inches of mercury) and a temperature of about 80° C (176° F).