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We all see those smooth, round rocks in rivers, but have you ever wondered what they’re made of and how they get so smooth?
These river rocks are actually any kind of rock that’s been tumbled and polished by the rushing water in a river. Over time, the water rubs the rocks together and against other things, smoothing out all their sharp edges. Granite, gneiss, quartzite, and sandstone are all common types of rocks you might find as river rocks.
Glaciers are amazing! They’re like giant ice rivers that can carve away even the toughest rocks. Water, whether it’s frozen or flowing, is super strong and can break down even the most compacted rocks. The results are incredible smooth pebbles and big boulders!
Nature’s a powerful force! It can turn giant rocks into all sorts of sizes, from big chunks perfect for building roads to smooth pebbles great for decoration. Let’s find out what kind of rocks and minerals you can find in river rocks and how these rocks actually get shaped.
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Where Do River Rocks Come From
River rocks are cool, right? They’re so smooth and round; it’s hard to imagine how they got that way! It makes you wonder how nature can take giant, rough rocks and turn them into these smooth pebbles. It just shows how patient nature can be!
River rocks are natural formations and come from rivers and streams. Initially, river rocks were just common rocks that got into rivers or streams, became a part of the water flow, and began to abrade, losing their sharp ages. Eventually, the hard rock gets its smooth form and becomes a river rock.
The river rock formation is a vital part of the rock cycle. A combination of weathering and abrasion processes disintegrates any primary rock (igneous, metamorphic, or sediment), creating sediments that are not still sedimentary rock but only at the beginning of new sedimentary rock formation.
Is River Rock a Natural Stone
Looking at a perfectly smooth river rock, it is hard to believe it is made exclusively by nature. It crystallized a rock from the melt, crushed it, and smoothed and polished it to create magnificent landscapes of river rock beaches.
River rock is a natural stone. It is formed of any common hard rock (mostly igneous or magmatic) by erosion and abrasion as water flows over and shapes rock chunks. The stones are not manufactured or processed artificially or with the help of any human intervention.
River rocks are natural stones. Because of their smooth surface and similar sorted sizes, they are widely used in construction, paving, and decoration.
River rocks are famous among people who prefer natural and eco-friendly materials. Nature tried to break these rocks, but they were harder to withstand. So, these rocks are highly preferable in construction. They are safe and durable.
TIP: Rivers are often great places for rockhounding. There you can find a lot of beautiful rare rocks. Check out the most common rocks you can find in or near rivers in the article below:
10 Most Common Types of Rocks You Can Find In Rivers
What Kind of Stone is River Rock
River rocks come in all shapes and sizes, depending on the rocks around them. They’re mostly found in hilly or mountainous areas where there are lots of tough rocks and steep slopes. Gravity helps the water tumble the rocks downhill, making them smooth and round. Flat, dry areas don’t have many river rocks because there aren’t many rocks to start with, and the land isn’t sloped enough for the water to move around.
River rocks can be made from many different types of tough rocks, like granite, gneiss, and sandstone. These rocks are packed with hard minerals like quartz, which helps them resist getting worn down too quickly by the water.
Most river rocks are extremely hard. Crystalline igneous rocks, microcrystalline metamorphic rocks, and fine-grained sedimentary rocks commonly represent them.
Some common types of rock that can be found in river rocks include:
- Granite – a hard, crystalline igneous rock,
- Basalt – a dark-colored, fine-grained igneous rock,
- Quartzite – a hard metamorphic rock that was originally pure quartz sandstone,
- Sandstone – a sedimentary rock that is composed mainly of sand-sized grains,
- Shale – a fine-grained sedimentary rock,
- Schist – a medium- to coarse-grained metamorphic rock,
- Conglomerate – a sedimentary rock made of cemented rounded pebbles and gravel.
You will rarely meet river rocks made of limestone because of the low hardness of minerals and the fragility of the shells it is made of.
The main factor that defines river rock composition is the hardness of the minerals they are composed of. Hardness is the ability to withstand scratching. You can refer to the Mohs scale and check what minerals are soft and what are the hardest.
There are super hard minerals out there, like topaz and diamond, but they’re not very common in rocks. Quartz, on the other hand, is a pretty tough mineral (rated a 7 on a special scale), and it’s also really common. That’s why you almost always find pieces of quartz of different sizes in river rocks!
Minerals that are commonly found in river rocks are:
- Quartz
- Minerals of the feldspar group (albite, oligoclase, andesine, labradorite, bytownite, anorthite, and potassium feldspars)
- Pyroxenes
- Amphiboles
TIP: A Mohs hardness test is one of the most valuable tricks that should be in all rockhounds’ playbooks. Check out my 3 top picks for Mohs hardness test kits in the article below:
3 Best Mohs Scale Test Kits: Test the Hardness of Your Gemstones
What Makes River Rocks Smooth and Round
River rock smoothing and rounding is a long-term process that requires several agents. They are essentially water, wind, ice, and gravity.
A process called abrasion makes rocks smooth and round. This complex process occurs when rocks and other materials are transported and worn down by the movement of water, ice, wind, or gravity. Other vital factors are a water stream’s speed and abrasion duration.
Rocks become smooth and round through a process called abrasion. Abrasion occurs during rock transportation via river stream.
Rocks, driven by water flow, collide with each other and the bottom and sides of the river. These collisions cause small pieces of the rock to break off, gradually making the rocks smaller and smoother.
Over time, the constant movement of water and the repeated collisions with other rocks and sediment wear the rocks down even more, rounding their edges and smoothing their surfaces.
Another process that can make rocks smooth and round is called attrition. Attrition happens when rocks rub against each other and wear down the rough edges.
You can see this grinding process happening in rivers and streams, where rocks get carried downstream by the current. It even happens in the ocean, where waves and currents toss rocks around!
This grinding and wearing down, called abrasion and attrition, takes a long time, but it can happen to any rock. Softer rocks like limestone and sandstone smooth out faster, but even tough rocks get rounder over time.
TIP: Gemstones are rare treasures that can be found in natural environments worldwide. Check out valuable tips on finding beautiful rocks in nature in the article below:
8 Tips On Finding Gemstones In Nature (Rivers & Creeks)
Why Does Moving Water Make Rocks Smooth
Moving water makes rocks smooth through the process of abrasion. During abrasion, rocks constantly interact with water flow, transporting other smaller sediments and particles. These particles collide with a rock, like sandpaper, and wear away sharp edges, making a rock smooth.
Water primarily provides energy for rock movement. It rubs rock boulders with each other, with smaller debris, and with a river bed. Water without other rocks added is much slower than water with sediments. Water flows over rocks and carries sediment and other particles with it.
As the water moves, it collides with the rocks, and the sediment and particles in the water act like tiny hammers, chipping away at the surface of the rocks. Over time, this constant collision and erosion wears down the rough edges of the rocks and makes them smooth.
Three essential factors influence smoothing. They are:
- Speed of water
- Volume of water
Faster-moving water, such as in a river or stream, can erode rocks more quickly than slower-moving water, such as in a lake or pond.
Similarly, a larger volume of water can erode rocks more quickly than a smaller volume.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
How River Rocks Are Formed
The river rock formation is an integral part of the rock cycle. It takes several steps and comprises several factors.
The natural erosion and abrasion processes in rivers and streams form river rocks. At first, rocks on hills are weathered. Then rocks come to rivers and are smoothed due to abrasion. When a stream slows down, river rocks of similar size and level of smoothness are deposited together.
- The first step is to disintegrate rock massifs. The process begins with the weathering of rock on hills and mountains, caused by factors such as heat, cold, rain, and freeze-thaw cycles.
- Transportation is the second step. Rocks come to a river or stream because of gravity’s influence. Being in active water flow, rocks are severely abraded and smoothed.
- Deposition. As the river flows, the smaller rocks, sand, and sediment are carried downstream and deposited along the riverbed. Additionally, during the last step, sorting takes place. The larger rocks (boulders) that are too heavy to be carried away by the water are left behind. The smaller rocks (gravel) are still moved down the stream. Over time, these deposits can build up and form a rock, gravel, and sand layer.
TIP: Rock collectors know how hard it is to find those perfect specimens for tumbling. Check out the best places in nature to find rocks for tumbling in the article below:
7 Best Places to Find Rocks for Tumbling in Nature
How Long Does It Take for Water to Smooth a Rock
How long it takes for a rock to smooth out in a river is a tricky question! There’s no one-size-fits-all answer because it depends on a bunch of things.
- Rock type: Soft rocks like sandstone smooth out faster than tough rocks like granite.
- Rock size: Bigger rocks take longer to become smooth pebbles.
- Water power: Faster rivers with stronger currents can smooth rocks quickly.
- Weather: Lots of rain and freezing weather can speed up the process.
It can take anywhere from a few years to thousands of years for a rock to become smooth in a river. The tougher the rock, the longer it takes!
The stream speed and the volume of water
The volume and speed of the water play a role in how quickly a rock can be smoothed. Faster-moving water, such as in a river or stream, can erode rocks faster than slower-moving water, such as in a lake or pond. Similarly, a larger volume of water can erode rocks more quickly than a smaller volume.
The type of rock
For softer rocks like limestone and sandstone, it can happen relatively quickly, taking only a few years or decades for the water to smooth rock. The process can take much longer for harder rocks such as granite, possibly hundreds or thousands of years.
The weather conditions
The weather conditions also play a role in the process, as the temperature changes, rain, and freeze-thaw cycles can cause the rock to crack and break, making it easier for the water to erode the surface.
Chemical weathering
Chemical weathering can also have an impact on the surface of the rocks. Limestone, marble, and gypsum are the most susceptible rocks to chemical weathering.
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Conclusion
River rocks are a cool product of nature’s work! Rain and strong currents in rivers slowly wear down all sorts of rocks, big and small. This is called erosion. As the water tumbles the rocks together (abrasion!), they bump into each other and the riverbed, chipping away bits and pieces. Over a long time, this grinding polishes the rocks, making them smooth and round.
These smooth rocks, mostly made of tough stuff like granite and sandstone, settle along the riverbed as the smaller pieces get carried further downstream. Over time, these pebbles and boulders can build up into big layers of rock, gravel, and sand!
Minerals that makeup river rocks are represented mainly by:
- quartz,
- feldspar group minerals (potassium feldspar, albite, labradorite, oligoclase),
- pyroxenes, and
- amphiboles.
The time it takes for water to smooth a rock can vary widely, depending on the type of rock, the volume and speed of the water, the weather conditions, and other factors, but it can take from a few years to several thousands of years.
TIP: The most common situation all rock hunters and mineral collectors deal with is at-home mineral identification. Find out the step-by-step guide in the article below:
Identifying Minerals and Rocks at Home (Step-by-Step Guide)