Shulou(Shulou.com)11/24 Report--

Stones can be seen everywhere in human daily life. Some of them, such as emerald and marble, are loved by human beings because of their "outstanding appearance". They often appear as decorations in halls, studies, walls and human beings. The other part, such as limestone and sandstone, has become human engineering and building materials, contributing to human society.

However, outside human society, stones are still everywhere, but they are not called by that name, and people often turn a blind eye to them. "Rock", as earth scientists call them. With regard to the formation of rocks, some people may have seen volcanic eruptions, and the flowing magma solidified into rock very intuitively. But what many people don't know is that the silt lying quietly at the bottom of the lake, like the gravel washed by the waves, will become rocks in the future.

Before tectonics and plates began to force, conglomerate sandstone was buried under the river, the mud at the bottom of the lake was turned into shale, and sea water and organisms were shaping a sea of limestone. Underground, temperature and pressure are processed to transform the rocks, some with unique patterns on the "paint", while others are broken and reorganized. Moreover, magmatic rocks that are reluctant to erupt into volcanoes can often be seen in the depths of the ground, and the minerals and gems that accompany them may be their "tears". But it doesn't matter, as the tectonics become active and the plates begin to move, the rocks of different "birthplaces" usher in a "trip". Some go underground, some rise into mountains, some go from the sea to land, and some have experienced changes from tropics to cold zones. Eventually, the rocks are scattered all over the place, waiting for earth scientists to find and share with them what they have seen and the secrets of the earth.

Next, let's go into nature and see what kind of magical stones exist there.

The first stop: the Rift Valley is one of the birthplaces of volcanic rocks. Hot magma invades along the cracks in the earth's crust and rises under the action of its own thermal force and buoyancy. At this time, the earth opened a crack. So under the "attraction" of low pressure, the magma rises until it erupts, cools and solidifies into volcanic rock on the surface, and the magma left underground forms intrusive rock.

Different types of magma form different kinds of volcanic rocks. As we all know, the earth is composed of the crust, the mantle and the core. Magma derived from the crust is richer in SiO2 than that derived from the mantle. In the classification of magmatic rocks, one of the most basic classification basis is the content of SiO2 in rocks, which can be divided into ultrabasic rocks, basic rocks, neutral rocks and acid rocks. Basalt is a typical basic volcanic rock with a SiO2 content of 45-52%, which is formed by magma evolution produced by the melting of mantle material. Granite is a typical acid intrusive rock with a SiO2 content of more than 65%, and its magma is formed by partial melting of the earth's crust. The two are easy to distinguish from their appearance: basalt is mainly composed of plagioclase and pyroxene, and also contains a certain amount of olivine, so it usually shows dark tones such as black, dark green and gray; granite is mainly composed of light-colored feldspar and quartz. A small amount of biotite or hornblende is dark "spots" distributed on the surface of light-colored rocks.

Figure 1. Gray-black basalt (image source Aether)

Figure 2. Light-colored granite (Tu Yuan Aether) rifts in different environments can also produce different kinds of volcanic rocks. Magma evolves and differentiates under the influence of temperature, pressure and other substances. It is also basalt, the continental rift mostly produces alkaline basalt, and the mid-ocean ridge produces mid-ocean ridge basalt.

The continental rift is a tectonic region similar to the Great Rift Valley of East Africa. The crust slowly split here, and perhaps a few million years later, when sea water poured in, the continental rift was transformed into the mid-ocean ridge of the new ocean. (this process has also been introduced in our past tweets. ~ Please stamp for details: "the beginning" is over, but the loop is not over yet.)

The mid-ocean ridge is the weakest part of the earth's crust. The asthenosphere underneath drags the oceanic crust to both sides, and mantle material continues to spew out to form a new oceanic crust. It is in such a rapid extensional environment that mantle material can melt relatively thoroughly under low pressure and high temperature near the surface environment, resulting in the formation of mid-ocean ridge basalt (MORB). On the other hand, the continental rift belongs to a slow extensional environment, and the partial melting of the mantle material is deeper and its degree is not complete, but formed in alkaline basalts with great differences in mid-ocean ridge basalts.

Formed on the seafloor, mid-ocean ridge basalts often have special pillow-like structures. When flowing magma encounters cold sea water, it is often the surface layer that solidifies first, while the interior is still flowing, so it can form a pillow-like ellipsoid. Under the condition of uniform and slow cold shrinkage, the magma erupted on land is separated by cold shrinkage fissures to form regular polygonal columnar joints. The famous scenic spot "Giant Road" in Northern Ireland is a spectacular sight made up of tens of thousands of basalt columns.

Figure 3. Pillow structure (Picture Source Internet)

Figure 4. Columnar joints of columnar basalts that make up "Giant Road" (figure source K. Mitch Hodge on Unsplash) are common in "overflow basalts". 250 million years ago, a super volcanic eruption ushered in in the southwest, where magma spewed out along cracks in the earth's surface and flooded the earth, resulting in an overflowing basalt layer hundreds of meters thick. Igneous rocks like this, covering more than 100000 square kilometers, are known as "igneous provinces" and are generally thought to be related to mantle plumes. (see mantle plume legend for details) some scientists believe that five mass extinctions in geological history seem to be related to the eruption of overflow basalts.

Figure 5. Black almond-shaped plagioclase basalt collected from Emeishan area (figure source Aether)

Figure 6. Red is the eruption age of overflow basalt and gray is the percentage of biological extinction. Several prominent extinction peaks almost all coincide with the red band (image source Posts about flood basalts on) the second stop: if the ocean wants to understand sedimentary rocks, it has to mention the ocean, the hotbed of sedimentary rocks. In that era when the land had not yet formed, the whole earth was like a boiling hot pot, except that the bottom of the soup was magma. With the decrease of temperature, the magma gradually condensed into the primitive crust, and the crystalline water in the magma precipitated and accumulated. As a result, the ocean formed. By the late Archean (28,500 million years ago), the amount of water on the earth was now 7 to 10, and sedimentary rocks were common.

According to their genesis, sedimentary rocks can be divided into two types-epigenetic sedimentary rocks and authigenic sedimentary rocks. The premise of sedimentary rock formation is matter. The minerals that form epigenetic sedimentary rocks come from other rocks, volcanic eruptions or cosmic materials, while the minerals that form authigenic sedimentary rocks are newly formed in a chemical or biochemical way. The former includes terrigenous clastic rocks (sandstone, siltstone, mudstone, etc.) and volcanic clastic rocks, while the latter includes carbonate rocks (limestone, dolomite, etc.), siliceous rocks, iron rocks, evaporite and so on.

Figure 7. Siltstone (image source watermark)

Figure 8. Pyroclastic rock (image source watermark)

Figure 9. Limestone (image source watermark)

Figure 10. Iron rock (map source watermark) on land, a ubiquitous piece of granite is exposed to the air, its hornblende and biotite decompose under weathering, and the remaining feldspar, quartz and other minerals are carried away by wind and running water. until it's deposited somewhere. Over time, the thickness of the sediment gradually increases, the bottom sediment receives a gradual increase in pressure and temperature, and begins to dehydrate, and the pores between minerals begin to shrink, and finally become stable sedimentary rocks. This is the formation process of terrigenous clastic rocks.

But in the ocean, most sedimentary rocks belong to authigenic sedimentary rocks, and they are mainly carbonate rocks, and their formation mode is quite different from that of terrigenous clastic rocks. There are a large number of Ca2 + and CO32- ions dissolved in sea water, and when they reach saturation, CaCO3 will be precipitated. If other factors are not taken into account, these white deposits "plaster" accumulate, crystallize into calcite, and then undergo the same burial-dehydration process as terrigenous clastic rocks to form pure limestone. But there are many kinds of limestone in nature. why is that?

Perhaps one of the most important and wonderful factors is biology. Stromatolites formed by cyanobacteria bear witness to the oxygen-free age of the earth. Cyanobacteria is a single-celled prokaryote capable of photosynthesis and secretes mucus, captures and binds plaster, which is common in shallow intertidal zones. The periodic reproduction of cyanobacteria has created a layered structure of limestone. During the period of high tide or storm, cyanobacteria can capture a large amount of plaster brought into the intertidal zone to form a "bright layer" of laminated limestone, while a calm environment is more suitable for cyanobacteria to breed, forming a "dark layer" rich in organic matter in stromatolites. In ancient times, the proliferation of cyanobacteria greatly increased the oxygen content of the atmosphere. Since then, the rise of aerobic organisms has led to the disappearance of stromatolites, which exist only in a few areas.

Figure 11. Stromatolites (graphic source Aether) are also one of the sources of limestone. In addition to forming fossils, the calcareous bones and shells of organisms can be weathered and eroded into fine plaster, and can also exist in limestone in the form of debris. Different kinds of creatures have different shapes of debris, such as spindle-shaped spindles, hook-shaped trilobite bones, spiral foraminifera, circular sea lily stems. This kind of rock is collectively referred to as bioclastic limestone, referred to as bioclastic limestone.

Secondly, the external force acting all the time has a great influence on the structure of limestone. The unconsolidated limestone is broken by waves or storm currents, and large fragments become oblong ellipsoids shaped like "bamboo leaves" under the erosion of running water. When the water is quiet again, these "bamboo leaves" accumulate with the plaster to form bamboo leaf limestone.

Figure 12. Bioclastic limestone (image source watermark)

Figure 13. Sea lily limestone (figure source watermark) as the consolidated limestone is buried deep underground, the pressure brought by the overlying strata increases, and some soluble materials in the limestone dissolve and transfer due to pressure, and undulating sutures are formed in the rock. On the other hand, some insoluble substances such as clay minerals, iron and organic matter remain in the fine seams of the sutures, making the sutures look darker. This kind of limestone is suture limestone.

Fig. 14 Bamboo Leaf Limestone (Source Watermark)

Figure 15. Suture limestone (figure source Aether) in addition to limestone, common marine rocks include dolomite, manganite, phosphorite and so on. These rocks are of different genesis and have their own characteristics, which are distributed in tidal flat, lagoon, coastal, shallow sea and even deep sea, providing considerable mineral resources for human beings and silently recording the changes of the earth's history.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), written by Aether, edited by Pepper and Salt Mushroom

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