1. A 2-Billion-Year-Old Pebble Inside a Conglomerate: Rocks Are Reused
The discovery in the Hida River can be read as a sign that rocks are not finished products. They become the raw materials for other rocks. Because a conglomerate is made of piled-up grains, the pebble inside it must already have been a rock before it was buried and cemented [8].
2. The Three Types of Rock and How Each One Forms
Rocks are broadly divided into three types based on how they form. Educational materials from the U.S. National Park Service describe these same three categories [2]. The first type is igneous rock, which forms when magma cools and hardens. The second is sedimentary rock, formed when sand, mud, or pebbles pile up in layers and get buried and hardened. The third is metamorphic rock, which happens when existing rocks change their minerals due to heat and pressure [2].
The conglomerate found in the river is a sedimentary rock because its pebbles piled up and hardened [3]. The pebble inside it was sillimanite gneiss, which is a type of metamorphic rock [6]. This suggests a specific journey: the pebble was metamorphic rock before it became a pebble, piled up, and became part of a sedimentary rock. The sources used here do not explain where the pebble was transported from.
3. How Sedimentary Rocks Form From Grains and Are Sorted by Grain Size
Sedimentary rocks are classified by the size of the grains that make them up. A rock made of grains 2 mm or larger is called conglomerate. If the grains are between about 1/16 mm and 2 mm, it is sandstone. If they are smaller than about 1/16 mm, it is mudstone [3]. The lower limit for conglomerate (2 mm) is 32 times larger than the upper limit for mudstone (1/16 mm).
The process of loose grains turning into rock is called diagenesis. As more material piles on top, the weight squeezes the spaces between grains tighter (compaction). Then, minerals deposit in those spaces, gluing the grains together (cementation) [4].
Some sedimentary rocks form from the remains of living things. Limestone can form from accumulated coral, shells, or foraminifera. Chert can form from accumulated radiolarian shells. Coal forms from plants. Tuff forms from volcanic ash [1][5]. However, limestone can also form through chemical precipitation, so not all limestone is biological in origin [5].
4. How Metamorphic Rocks Form Without Melting
Metamorphic rocks form when sedimentary or igneous rocks change due to temperature and pressure changes. Crucially, metamorphism happens while the rock remains solid; it does not melt [6]. If a rock melts and then hardens, it becomes igneous rock. During metamorphism, reactions occur between minerals, creating new minerals and textures.
According to materials from Shimane Prefecture, contact metamorphic rocks, formed by the heat of magma, typically form at temperatures around 500 to 600°C. Regional metamorphic rocks, formed over wide areas by plate collisions, generally form at temperatures of 300 to 800°C with strong pressure [1]. Some types of metamorphic rock can form at lower temperatures, around 200 to 300°C [6].
Marble is a metamorphic rock formed when limestone changes and the crystals of calcite grow larger [5]. This means rocks made from accumulated coral or shells can transform into a completely different rock when heated.
5. How Cooling Speed Changes the Grain Size of Igneous Rocks
Igneous rocks form when magma cools and hardens. This includes lava that hardened after erupting from volcanoes, but also magma that hardened underground [1].
The speed of cooling determines the size of the grains. Rocks that cool quickly near the surface are called volcanic rocks and have fine grains. Rocks that cool slowly deep underground are called plutonic rocks, and their mineral grains are coarser than about 1 mm [1]. For reference, a 1-yen coin is about 1.5 mm thick, so 1 mm is about two-thirds of that thickness.
The size of the grains in an igneous rock gives clues about where it formed and how long it took to cool.
6. The Rock Cycle: How Any Rock Can Become Another Type
Any rock can become sediment if it undergoes weathering and erosion. It can become metamorphic rock if it experiences pressure and heat. It can become magma and then igneous rock if it melts [7]. The U.S. National Park Service describes this as a "long, slow journey" [2].
This journey does not always follow a fixed order like igneous to sedimentary to metamorphic. Sedimentary rock can become the material for another sedimentary rock without melting. Metamorphic rock can undergo metamorphism again. There are shortcuts and dead ends in this cycle [7]. The sources used here do not specify how many years this cycle takes.
The pebble from Shichiso Town can be seen as a snapshot of this journey: it was born as metamorphic rock, broke into a pebble, piled up, and entered a conglomerate rock [8].
7. How to Observe Rocks in Your Town
You can look at stone walls, gravestones, or floors in stations and buildings in your town. You might find marble floors. When observing rocks, look for three things: Can you see individual grains? Are the grain sizes uniform? Do you see shiny crystal grains?
Seeing grains alone does not prove a rock was formed by accumulation, because crystals can also be present. Grain size and shape are clues. If you see large crystal grains, it might be a rock that cooled and hardened (igneous) or one that changed (metamorphic). While you cannot determine the rock type by appearance alone, asking "Where in its journey is this rock?" is a form of observation.
For safety, do not get too close to roads, rivers, or cliffs. There are places where you must not break rocks or take them home. If you have the chance to visit the Oldest Stone Museum in Shichiso Town, you can see the 2-billion-year-old stone in person. Check official information for opening days before you go [9].