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rock that can float in water?
Hint: There are three types of rocks. Igneous rocks are formed when magma or lava cools down and hardens. Sedimentary rocks are formed due to accumulation or deposition of organic particles or minerals followed by cementation. Metamorphic rocks are formed when existing rocks are converted by heat, pressure, or active liquids, such as hot mineral carrying water.
Complete answer: Option A: Shale is a fine-grained sedimentary rock composed of clay minerals and mud mixed with other minerals, especially quartz and calcite. It is used to make brick, pottery, tiles, etc. Shale cannot float on water.
Option B: Obsidian is a naturally occurring igneous glass formed by the rapid cooling of lava from volcanoes. It is black or dark greenish in color and is extremely rich in silica. Obsidian does not float on water.
Option C: Limestone is a hard sedimentary rock which is composed mostly of calcium carbonate or dolomite and is used to make building material and for making cement. Limestone does not float on water.
Option D: Pumice is a light colored igneous rock that is formed during volcanic eruptions. Many pieces have a high enough porosity which makes them float on water until they gradually become soaked.
Therefore the correct answer is Option D: Pumice.
Note: A very similar rock to pumice is scoria. They must not be confused with each other. Scoria is always dark in color while pumice is always light grey in color. The major difference is that scoria does not float in water, while pumice does.
- 2″ x 2″ block of pumice
- 2″ x 2″ block of granite
- Tiny pebbles the size of your baby fingernail
- Hand towels
- Notebook and pencil
- Are you ready to rock? First, collect your rocks. If you don’t live near a beach, you can find sand at craft stores as well. Pumice stones are common fish tank ornaments, and they’re also used for personal grooming. Granite is common in landscaping gravel. If you’d like to add other rocks and compare them to the pumice and granite, make sure that they’re around the same size so that you can compare them easily.
- Now that you’re rocking, fill a dishpan with water. Keep your hand towels handy in case of spills. Put the dishpan in a waterproof place.
- Hold each of the rocks in your hand. What does each look like? What does each feel like? Write down your observations in your notebook, or get an adult to help.
- Create a hypothesis, your best guess about what is going to happen. Will each rock sink or float? Do you think that rocks can float?
- Now, test your rocks. Start with the big ones and move to the smaller ones. Gently place the piece of granite on top of the water. As you let it go, get a helper to start the stopwatch. What happens? If it sinks, how long does it take to sink to the bottom?
- Do the same thing with the pumice, the tiny pebbles, and the sand. Make sure that you experiment with the sand last.
- As you experiment with each rock or set of rocks, write down whether your rock floats or sinks, the time it takes to sink, and any other observations you may have about the rocks and the water.
The pumice will float for a while and then gradually sink. The granite will sink. Small pebbles will float momentarily and then sink. Sand will float, then move into the water, and gradually drift down to the bottom of the container.
If you’ve ever thrown a rock into a pond, you know that rocks usually sink. So why do some rocks float?
The reason that most rocks sink is because of the law of buoyancy, which is all about how things float or sink. This law is also called Archimedes’ Principle. Archimedes was a scientist who lived in ancient Greece.
Archimedes’ Principle says this: An object will float if it displaces as much water as it weighs. This sounds complicated, but what it means is that everything you put into water pushes the water away from itself. If the amount of water it pushes away is the same as the weight of the object, then the object will float. Light things float, but heavy things can also float if they’re designed to push enough water away. Think about big ships on the ocean!
So what does this have to do with your experiment? Granite and small pebbles generally sink to the bottom right away. When it comes to rocks, pumice is a bit of an oddball. Pumice has hundreds of tiny air bubbles in it. When it was made in a volcano, the volcano’s force pushed air into the rock. This makes pumice very light. It usually floats for a while, but then water gets into it and it starts to sink.
What about sand? If you use dry sand and a small container, the sand might be able to sit on the surface for a short amount of time. This is because of the surface tension of the water. Surface tension means that the water has a tough “skin” of water molecules that allow some things to float on the surface, like leaves. After a while, the sand gets wet and the water usually moves, and the sand will sink to the bottom.
Can you think of anything else that might float on water? Armed with your new knowledge, try to find out why it can!
Background: Floating Rocks
Synopsis: In August 2019, several groups of sailors were surprised to find themselves sailing through rocks as far as the eye could see. It was pumice that had been extruded from a subsea volcanic vent near Tonga and had accumulated into a raft the size of Manhattan. The raft is headed toward the Great Barrier Reef carrying organisms that may bring new life to the reef. Some scientists suggest that rafts such as this one were instrumental in the distribution of life-forms around the planet.
- Pumice is a light-colored volcanic rock that floats.
- Pumice forms when molten igneous rock is ejected at high pressure from a volcano, similar to the whipped cream that comes from a pressurized can.
- When the explosion releases the pressure, the foamy mass cools suddenly, freezing the air bubbles in place, creating a lightweight, low-density rock filled with air bubbles.
- Pumice is usually rhyolitic in composition, rich in silica and aluminum.
- Like frothed milk on cappuccino, frothed rocks float on water because of their entrained air bubbles.
- When volcanoes extrude their foamy eruptions under water, the magma solidifies into pumice, which breaks up into chunks that float to the sea surface.
- As the pumice collects, it forms accumulations called pumice rafts. Sailors in the South Pacific have reported pumice rafts for centuries.
- In 2012, a raft up to 154 mi2 (400 km2) near New Zealand could be seen by satellites and was reported by sailors. It was caused by the underwater eruption of Havre submarine seamount in the Kermadec Islands.
- Many of the pumice raft sightings have been attributed to volcanoes along the Tonga Trench, north of New Zealand.
- Rafts include marble- to basketball-sized chunks, although some larger pieces have been reported that stick up 2 ft (60 cm) above the water’s surface.
- In 2019, a raft larger than Manhattan Island (23 mi2 or 59 km2) surprised sailors and was mapped using remote sensing.
As pumice stone rafts float across the ocean, they pick up and drop off life along the way
Do you have a piece of pumice on a string in your shower? Do you know where it actually came from?
When an underwater volcano spurts molten rock, the lava rushing out into the water column quickly cools, fills with gas, hardens and rises to the surface.
This bubble-filled rock is called pumice, which often washes together to form a raft at the mercy of the wind and waves.
And while you might use your pumice to scrape away dead skin, as an ocean raft, it’s a safe haven for new life to attach to and grow.
A rocky raft of life
While the geological processes of bubbling lava are undoubtably intriguing, it’s the biological processes that rapidly take over the buoyant pumice that fascinate scientist Eleanor Velasquez.
“When they wash up, they are literally chock-a-block full of animals and plants,” said Dr Velasquez, who studies pumice rafts at Griffith University.
“The beauty of the pumice raft is that it gets washed around past all these different shallow marine reef systems and has the opportunity to pick up all of these hitchhiking corals and oysters and barnacles,” she said.
Many species of marine animals and plants reproduce by dispersing eggs and sperm or larvae into the open water.
But they can’t exist in that form forever and sedentary animals like corals and oysters need to find a hard surface to settle down on.
The thousands of tiny holes in the pumice that formed when the hot lava cooled not only enable the rock to float, but also make the perfect home for microscopic animals and plants.
“We call those safe sites,” Dr Velasquez said.
“With the pumice, suddenly you’ve got trillions of mini habitats that allow planktonic species to colonise something that wasn’t previously there.”
But Dr Velasquez has found that there’s one particular animal that’s needed to really kickstart the colonisation and allow the pumice raft to become a floating marine hotel.