Are there rocks originally on the earth? Where do all kinds of rocks come from?

There are many types of rocks on the earth, but they can be roughly divided into three categories: igneous rocks, metamorphic rocks and sedimentary rocks. Here’s a look at the origins of some common rocks in each category. First of all, from a geological point of view, the surface of the earth can be roughly simplified into a model like the one below:

Let’s now take a look at what rocks are in each area in the model and how they came from of.

It is necessary to clarify two confusing concepts here. First, the lithosphere and the crust are not the same thing. The lithosphere includes the uppermost layer of the earth's crust and mantle, and below it is the asthenosphere. The earth's crust is divided into oceanic crust (also called oceanic crust) and continental crust (continental crust, also called continental crust). When we usually discuss "plate", we are actually talking about the lithosphere. If most of the area above a plate is oceanic crust (such as the area between the mid-ocean ridge and the trench in the picture above), the plate is an oceanic plate. A plate is a continental plate if there is significant continental crust above it (such as the plate on the far right in the image above, and the plate from the far left of the image to the mid-ocean ridge). Second, the difference between rock and mineral needs to be noted. A rock is a mixture of one or more minerals, and a mineral is a naturally occurring solid pure substance (including elements and compounds). Next we start talking about the formation of rocks.

Initially, the Earth and the other planets in the solar system (at least the other terrestrial planets) formed at the same time. When the Earth was first formed, it was a chaotic planet, similar to the structure of chondrites (chrondrites) in the current universe. There was no layering of the core, mantle, and crust. However, about 50 million years after the formation of the earth, a Mars-sized planet Theia hit the earth, generating huge energy and almost melting the entire earth into liquid (part of the material was knocked away from the earth, formed the moon). In the process, the liquid Earth began to stratify. The heavier substances in the liquid, such as iron and nickel, began to settle toward the center of the earth and differentiated to form an iron core. The remaining materials composed of lighter elements such as magnesium, aluminum, silicon, carbon, oxygen, calcium, and sodium float outside the core, forming the primitive mantle. During the differentiation process of heavier materials sinking toward the center of the earth, gravitational potential energy is released, so the mantle can continuously absorb energy, maintain temperature, and convection, which also provides certain conditions for the formation of geomagnetism.

After that, the earth entered a cooling period, and heat was sent to space in the form of long-wave radiation. The fastest cooling is in the outermost layer, and the temperature drops below the melting point of some minerals (such as garnet, spinel, olivine, pyroxene, etc.). These minerals begin to form solid crystals, and these crystals form lherzolite, peridotite, dunite, pyroxenite, and orthorhombic peridotite in the upper part of the mantle. Mantle rocks such as harzburgite and websterite formed the earliest lithosphere. The thickness of these rocks is not uniform, and the weak spots later become the growth boundaries of the plates. In the process, comets brought water to Earth, and oceans began to form on top of the rocks.

With these initial rocks, various rocks were later formed. The first is the various igneous rocks created by volcanic activity (of course, the previous rocks are also igneous rocks, and because they contain very little silicon, they are classified as Ultramafic). Volcanic activity can be roughly divided into three categories: Mid Ocean Ridges, Volcanic Islands and Volcanic Arcs. First let's look at mid-ocean ridges:

In some areas, the plates (lithosphere) drift outward, causing the lithosphere to thin and the asthenosphere to rise. The pressure under the rock becomes smaller, causing the melting point of the rock to lower, thus producing a large amount of magma. Mid-ocean ridges are formed when magma surges upward, flows out of weak spots on the Earth's surface, and cools.

Igneous rocks formed at mid-ocean ridges make up the oceanic plates. The bulk of the oceanic plate is gabbro, an intrusive igneous rock that formed as the interior of the plate rock gradually cooled. A small amount of magma was immersed in seawater at the top of the mid-ocean ridge and cooled rapidly, forming extrusive basalt (basalt), which is the surface of most oceanic plates.

The second type of volcanic activity is volcanic islands (Oceanic Islands or Volcanic Islands, the most typical one is the Hawaiian Islands, others include Tahiti, Mauritius, Faroe Islands and Cape Verde Islands, etc.).

This volcanic activity is located at the center of the plate. In those places, hot mantle plumes rise upward from deep within the mantle, forming hot spots. Because of its extremely high temperature (about 200 degrees Celsius higher than normal magma), it can pass through the lithosphere of the ocean plate and form a volcanic island in the middle of the ocean after cooling. Because its magma comes from the deeper mantle, its chemical composition is different from that of rocks formed at mid-ocean ridges. For example, it contains more potassium, barium, zirconium, titanium and other elements. The igneous rocks formed by these magma on the earth's surface are accumulated and raised above the sea level, forming volcanic islands.

The rocks of the volcanic island can be divided into two series based on the content of potassium, sodium, iron and other elements. The first series is typical of the Hawaiian Islands and contains more iron. It is collectively called the tholeiitic basalt series (tholeiitic trend). Its source magma is divided into olivine basalt (olivine normative basalt), quartz basalt (quartz normative basalt), and Icelandic basalt. (Basaltic icelandite), Icelandic rock (icelandite) and other types. The second series can be found on most other volcanic islands. Compared with tholeiitic basalt, this series has a smaller degree of partial melting of the mantle, contains more alkali metals, and has calcium carbonate participating in the reaction. This series is called the alkaline basalt series (alkaline trend), which can be divided into olivine rough andesite (mugearite), Hawaiian rock (hawaiite, although its content in the Hawaiian Islands is very small, it is named after Hawaii) and trachyte ) and other rocks. Because the plates move but the position of the mantle plume does not move with them, volcanic islands often appear in clusters (similar to a ticking timer). As for the cause of the formation of mantle hot plumes, there is still controversy in academic circles. Some people think it is a purely thermodynamic principle, some people think it is related to the rotation of the earth, and some people think it is caused by the plate subducting into the mantle stirring the mantle. In special cases, mid-ocean ridges and mantle thermal plumes coincide (such as the Galapagos Islands and Iceland). In this way, igneous rocks formed by hot mantle plumes are melted again by mid-ocean ridges. Since the silicon content of the original igneous rock has increased during the initial melting and solidification process (the silicon content of typical Icelandic rock is 60%-70%), melting and solidification again will produce rhyolite (rhyolite) with a higher silicon content. , the silicon content can reach 74%).

The third type of volcanic activity is volcanic arcs, which are related to plate subduction.

Volcanic arcs are divided into two types: when one oceanic plate subducts under another oceanic plate, island volcanic arcs are formed, typically such as the Aleutian Islands, Scott Islands and Mariana Islands etc.; when an oceanic plate subducts under a continental plate, a continental volcanic arc will be formed, typically such as the Andes, Kamchatka Peninsula, and the Cascade Mountains of the United States (including the mountains of Mount St. Helens and Mount Rainier), etc. . Among them, the igneous rocks produced by continental volcanic arcs constitute the existing continental plates. Its layering and formation mechanism is as follows:

The ocean plate subducted into the ground is heated and melted (the water contained in the plate also lowers its melting point) to form magma. The minerals in the magma have different melting points and begin to crystallize in batches during the cooling process to form rocks. Of these, the least silicon-containing magmas form gabbro in the lowermost parts of the continental crust. However, unlike the gabbro in the ocean plate, due to the action of high-temperature supercritical water, the gabbro here will undergo varying degrees of metamorphism and thus contain hornblende, so it is called hornblende gabbro. gabbro).

Magma containing slightly more silicon forms a layer of diorite on top of the gabbro, which is the middle layer of the continental crust. Sometimes, hot magma invades the diorite through cracks in the lower crust, partially melting the diorite to form a melt that contains more silicon. This melt will flow above the diorite, where it will crystallize and solidify, forming granodiorite and tonalite, which contain more silicon, and form the top layer of the continental crust. If the lava erupts from the volcanic crater and cools rapidly, it will form corresponding extrusive igneous rocks (volcanic rocks), including andesite and dacite.

In some areas, the continental crust becomes thinner due to stretching, so magma has the opportunity to invade the granodiorite and tonalite in the uppermost layer of the crust

The diorite formation, and Partially melt them. In this way, after the melt slowly crystallizes again, granite, which contains the most silicon, will be formed. If this melt erupts to the surface and solidifies quickly, it forms rhyolite. Because rhyolite contains large amounts of silicon and is highly viscous, it often forms destructive volcanic eruptions. This series of igneous rocks is called the calc-alkaline trend.

After the formation of igneous rocks, they do not remain unchanged. Igneous rocks undergo changes and become metamorphic rock or sedimentary rock. Let’s briefly talk about metamorphic rocks here. On the seafloor, especially near mid-ocean ridges, seawater enters the ocean plate through hydrothermal alternation (you can read about black smokers and other knowledge). When the ocean plate subducts into the mantle, the seawater will form supercritical water under high temperature and pressure and enter the upper mantle (lower layer of the lithosphere). There, supercritical liquid, high temperature, and high pressure caused the original mantle rocks to undergo metamorphic reactions, forming eclogite (formed under higher pressure conditions, containing minerals such as garnet and omphacite) and blueschist (pressure The temperature is lower than that of eclogite, and it often contains minerals such as blue amphibole, calcite, chlorite, epidote, garnet and muscovite). The olivine in some gabbro also reacts with supercritical liquids to form metamorphic rocks containing minerals such as serpentine.

Metamorphic rocks are generally produced in high-temperature and high-pressure environments. In addition to subduction zones, another place where metamorphic rocks are produced is in high mountain areas, especially mountainous areas formed by the collision of two continental plates, such as the Himalayas and the Alps. This kind of metamorphic rock formed on a large scale is called regional metamorphism, and it is divided into many metamorphic phases (facies) based on the temperature and pressure conditions during formation and the minerals contained.

The two phases mentioned above, eclogite and blueschist, are called extremely high-pressure metamorphic phases because they are both produced in the ultra-high-pressure environment of the earth's mantle. Metamorphic phases whose pressure is slightly lower than theirs are called medium- and high-pressure metamorphic phases, and these phases are formed at the base of mountains. According to differences in temperature and pressure, they can be divided into zeolite (zeolite, containing minerals such as zeolite, chlorite and albite), prehnite & pumpellyite (containing chlorite, chlorite, sodium feldspar and other minerals) Minerals such as feldspar, muscovite and quartz), greenschist (greenschist, containing albite, potassium feldspar, quartz, biotite, muscovite, chlorite, calcite and actinolite, etc. Minerals), amphibolite (amphibolite contains minerals such as biotite, muscovite, staurolite, quartz, kyanite and plagioclase) and granulite (granulite, contains minerals such as kyanite, plagioclase, potassium feldspar etc. ) and other five phases. The distribution of these five phases in the alpine belt is from shallow to deep, and the temperature is also from low to high.

Metamorphic rocks may also occur in high-temperature but low-pressure environments. For example, magma invades the earth's crust through cracks. Around the magma chamber, the rock is heated but does not melt, and metamorphism can occur. Such metamorphic rocks are called contact metamorphisms, and their corresponding metamorphic phases are low-pressure metamorphic phases.

The low-pressure metamorphic phase can be subdivided into albite-epidote hornfels, hornblende hornfels, pyroxene hornfels and sanidine according to different temperatures. ) and other four phases. Since the pressure when these phases are generated is smaller, morphologically, compared with the rocks of the medium-high pressure or extremely high-pressure phases mentioned above, the foliation of these low-pressure phases (foliation, that is, the foliation of the rocks due to high pressure) The layer-by-layer texture of mineral extrusions) is much weaker.

There is another major type of rock called sedimentary rock. Unlike metamorphic rocks, which are often produced in geologically active plate subduction zones, sedimentary rocks are generally formed in shallow seas and seafloors on continental shelves where crustal activity is less active, and are widely distributed on the surface of land. After physical or chemical erosion (flowing water, glaciers, plants, wind, tides...), igneous rocks and metamorphic rocks will detach or fragment and be carried elsewhere by flowing water or air. In places with lower energy (plains, lakes, beaches, shallow seas, alluvial fans, deltas...), these debris settle. Larger particles are deposited first, generally at the bottom of the sediment layer, or closer to their source. Smaller particles can be carried further to places, such as the deep ocean. They are generally deposited in shallow layers. After lithofication, these sediments will become sedimentary rocks. Sedimentary rocks can be divided into different types based on particle size, morphology and depositional environment. Take the beach as an example:

Due to the influence of ocean tides, the nearshore part has higher energy and larger particles, which is called sandstone. Sometimes there is also larger conglomerate on the shore. Far offshore, below the continental slope, where the water is calmer, smaller sediments accumulate to form shale. In deeper seas, limestone (containing minerals such as calcite, dolomite, and aragonite) containing a variety of carbonic compounds will be formed. Biological remains are buried in sediments, forming fossils (fossil) or ooze (ooze). There are also some special sedimentary rocks, such as clastic rocks formed by volcanic eruptions.

The three major rock types (igneous rock, metamorphic rock and sedimentary rock) can be transformed into each other. For example, metamorphic and sedimentary rocks may melt as plates subduct into the Earth's mantle. When they erupt again to form rocks on the surface, they become igneous rocks. Igneous and metamorphic rocks can form sedimentary rocks through erosion and deposition. Igneous rocks and sedimentary rocks can also form metamorphic rocks under conditions of high temperature and pressure.