Why are some rocks as old as the Earth, while others may be just born?

We often lament the importance of air and water to life, but little do we know that rocks, like them, are an important part of life. Rocks make up the outermost crust of the earth. They are not only the skeletons of mountains and valleys, but also the record book of the earth's 4.6 billion years of evolution. Some rocks are almost the same age as the earth, with a history of 4.3 billion years! However, some rocks are even very young, perhaps less than an hour old. So how did these ancient and young rocks form? Why are some "young" and some "old"? What wonders do these rocks contain? Let's reveal these questions one by one today.

(banded iron formation, a type of sedimentary rock)

1. What are rocks?

First of all, what exactly is a rock? From a scientific definition, a rock is a collection of minerals or natural glass. Imagine putting together Lego blocks of different shapes, colors, and sizes to form models of various shapes. Rocks are like these models, and minerals are like the building blocks that make up the models.

(mineral)

2. Types of Rocks

You may know that animals and plants come in different categories, and the same is true for rocks. Generally speaking, rocks are divided into three categories: igneous rocks, sedimentary rocks, and metamorphic rocks. These three types of rocks are formed differently and have their own unique appearances.

1. Magmatic rock

Seeing this name, you must have known its origin. That's right, igneous rock is a type of rock formed by the cooling of magma. This cooling process can occur on the surface of the earth or inside the earth. The igneous rock formed by cooling on the surface of the earth is called "explosive rock", while the igneous rock formed by cooling inside the earth is called "intrusive rock". Let's take a look at the specific differences between these two types of rocks.

Extrusive rocks are formed by volcanic eruptions. After the magma is ejected from the volcano, it cools rapidly when it comes into contact with the air. This cooling process causes some minerals in the magma to crystallize rapidly. Because the crystallization speed is too fast, the formed minerals are also very small. Basalt is an extrusive rock. You may see the magical hexagonal rock geological wonders on many social media. These rock pillars are basalt. The reason why this polygonal shape is formed is that they slowly shrink when cooling.

In addition to extrusive rocks such as basalt, which are composed of minerals, there is another extrusive rock that contains almost no minerals - obsidian. Obsidian is a kind of rock with a glassy texture. If you look closely at obsidian, you can also see circles of texture on it - if you happen to break a glass, you will also see circles of texture on the broken surface of the glass - this texture is called "conchoidal fracture".

In addition to extrusive rocks, there is another type of magmatic rock called "intrusive rocks". During a volcanic eruption, some magma does not erupt onto the surface, but cools and solidifies deep underground, forming intrusive rocks. Due to the slow cooling rate, the crystals of intrusive rocks are generally coarse, such as granite. Most of the large spherical stone pillars on the roadside are made of granite.

2. Sedimentary rocks

Sedimentary rocks are like a jigsaw puzzle, except that they are not made up of individual pieces of paper, but rather of fragments of other rocks. Rocks undergo weathering, transportation, deposition, and diagenesis to form sedimentary rocks.

In the first step of weathering, rocks are constantly eroded and broken down into debris, such as small pieces of rock fragments, sand, gravel, clay, etc., by natural forces such as sunlight, rain, wind, and glaciers. In this process, very fine rock fragments will form soil with some plant humus.

In the second step of transportation, the already formed rock fragments will be transported to distant places by wind, flowing water and glaciers. In this process, the fragments will continue to be rounded and sorted, just like running water washing stones, making them round.

In the third step of sedimentation, some of the transported rock fragments will settle in the oceans and rivers, stacking up layer by layer.

Finally, as time goes by, new rock fragments accumulate layer by layer, and the rock fragments in the lower layer will be compressed more and more tightly, just like compressed biscuits. The originally loose fragments are gradually compressed. Under the action of pressure, the minerals in the groundwater will cement the fragment particles together, just like cement, and finally consolidate the sediment into hard rock. This is also the final diagenesis.

On the surface of the earth, about 75% of the area is covered by sediments (rocks), and almost all of the seabed is covered by sediments (rocks). The nut-bread-like rocks we see around rivers are sedimentary rocks, and they are called "conglomerates".

In addition to conglomerate, coal is also a sedimentary rock, which is formed when ancient plants are buried.

3. Metamorphic rocks

Metamorphic rocks are a type of rock that is formed by metamorphism. Igneous rocks, sedimentary rocks, and even metamorphic rocks themselves can all be formed by metamorphism. Metamorphism can be understood as heating in a pressure cooker. Under the action of high temperature and high pressure, the mineral structure and composition of the rock will change, thus forming new rocks.

In our lives, marble used for decoration is a kind of metamorphic rock. Pure marble is pure white, such as the Jinshui Bridge in front of Tiananmen Square, which is made of pure white marble. The marble with patterns used in home decoration is often caused by some impurities.

Currently, the oldest mineral crystal found on Earth is a zircon found in a metamorphic rock called "metamorphic conglomerate" in Jack Ridge, Australia. It is 4.4 billion years old, almost as old as the Earth. This ancient zircon crystal is like a time capsule from ancient times, opening the door to the early history of the Earth for us.

3. Rock Cycle

Magmatic rocks, sedimentary rocks, metamorphic rocks, these rocks on the earth are not static, they will transform into each other. Magmatic rocks are formed by the cooling of magma, and all the formed rocks can be weathered or metamorphic to form sedimentary rocks or metamorphic rocks. And these magmatic rocks, sedimentary rocks, and metamorphic rocks will return to the ground under the geological action of the earth, and will be "melted" into magma again by the earth, waiting for the next eruption and rock formation. This series of processes is called the "rock cycle."

For example, the 4.4 billion-year-old zircon mineral mentioned above is from a metamorphic conglomerate that has not yet entered the final step of the rock cycle—melting into magma. Rocks are witnesses of the history of the Earth, and each rock contains the secrets of the Earth's evolution. Next time you see a stone, stop and take a closer look. Perhaps you will be able to read a few words written by Mother Earth.

References:

[1] Crystallography and Mineralogy, Li Shengrong, Geological Publishing House

Author: Shi Jianyi Popular Science Creator

Reviewer: Dong Hanwen, Associate Researcher, Institute of Geology, Chinese Academy of Geological Sciences