Kids Ask Mikey
Sometimes when students are studying Earth Science, the class sends questions to Mike the geologist for an answer. Here are some of the questions and answers.
Is sand a kind of rock? Erich.
If it is loose sand grains then it is not a rock, but is called a sediment.
Where do opals come from? Casey
They form from chemicals dissolved in water, kind of like sugar dissolved in tea. As the water dries up, the chemicals are left behind as films of opal.
How is marble made? Michael
Marble is simply limestone, a kind of sedimentary rock, that has been under heat and pressure for awhile. The heat and pressure cause the minerals in limestone to change form, resulting in marble.
Are bricks rock? Cady
No, rocks are made by nature and bricks are made by man. However, bricks are made by mixing clay and sand and then heating them in a big furnace to cause them to get hard and bind together.
How are rocks and minerals such different colors? Taylor
Color of any mineral depends on the internal or atomic structure of that mineral and how light reacts to that structure. It is very complicated and several books have been written about color of rocks and minerals.
How are rocks made? Kevin
From three basic processes in the rock cycle: from molten liquid (igneous rock) that cools and hardens, from particles or grains that get bound together with some type of material (sedimentary rocks), or from the application of heat and pressure to pre-existing rocks (metamorphic) which causes changes in the minerals that make up the original rock.
Why are they different shapes and sizes? Virginia Because there are many different types of rocks and they react differently to weathering and erosion.
How long does it take for bones to turn into fossils? Peter
It depends on the conditions underwhich the bones were buried and how much water flows through the sediment. An easy way to tell if a bone is fossilized is to heat it with a lighter. If you smell a burning stinky organic smell, then the bone is younger than 10,000 years old and is not completely fossilized. I use this test all the time if the bone is white and looks porous (has a lot of tiny holes in it).
How do rocks get crystals in them? Jasmine
I think you might be talking about geodes, which are a special type of rock that is hollow and has minerals or crystals pointing to the inside when you break them open. First you have to have a hole in some type of rock (usually limestone or the igneous lava rocks, basalt or rhyolite). Then water with dissolved minerals can move through the rock and begin to coat the walls of the holes. After a while, crystals begin to form and grow from the coatings toward the center. Then the host rock weathers away and you find a rounded rock that when you break it open is lined with crystals!
Are geodes metamorphic, igneous, or sedimentary rocks? Caroline That's a good question! Let's first look at a definition of what a geode is: A hollow rounded rock nodule filled with an inner lining of mineral matter. Sand, water, or petroleum may fill the cavity, or the lining may consist of crystallized calcite, quartz, or other minerals.
We should also consider the definitions of the 3 types of rock:
Igneous - rock formed from a melt.
Sedimentary - rock formed from the deposition of particles of minerals in layers or beds.
Metamorphic - rock formed by the effects of added heat or pressure to another rock.
Not one of these definitions really fits because these definitions define processes! What we find is that with some types of stones it is that the forms that they take (like geodes, concretions, nodules, and many others) that we as geologist use to describe them. So a geode is an aggregate of minerals that do not quite fit the definitions we geologists so neatly give concerning the 3 rock types. This is because the terms we use, like geode, describe a particular form of mineral matter, not the process by which it formed.
Now, how do geodes form?
There are many different types of geodes, but they may be looked at in two groups. those that form in igneous rock, particularly lava. And those that form in sedimentary rock, particularly limestone. What do these two rocks have in common to be the typical hosts for geodes? They both often have holes or cavities in them. Lava may have holes formed by expanding gases and limestone may have holes formed by the leaching of minerals by water passing through the rock.
So....we know we must have a hole in a rock for a geode to begin to form. Another thing we must have is some water moving through the rock that contains dissolved minerals, like silica. The host rock must be porous enough for water to move through it, kind of like a sponge. As the water passes through the rock and gets into the holes, it may begin to deposit its minerals on the sides of the hole. If the mineral does not completely fill the hole, then we have a geode. In a geode the minerals are deposited from the outside in! This is how some agates form if the hole becomes completely filled. The temperature of the water may be warm, but often is somewhat cool.
The chemicals dissolved in the water determine what the geode contains. Most contain quartz of some type, either agate or crystals, so the water must contain silica, but often we see other minerals, like calcite, dolomite, pyrite, etc. In those instances, the water must have also contained elements like calcium, magnesium, iron, sulfur, and other groups of elements, like carbonate, sulfate, etc.
But back to your question: What kind of rocks are geodes? They are rocks formed by the seepage of water and deposition of minerals in holes that existed in certain types of rocks. They don't really fit into the neat categories of igneous, metamorphic, or sedimentary rocks.
How many different kinds of rocks are there ? Kelly
Kelly, there are many different kinds if you look at the different names as being different types, but there are 3 major types of rocks if you group them by the processes by which they form. Igneous, Metamorphic, and Sedimentary.
How long do rocks last? Virginia
Virginia, it really depends on whether they stay buried in the ground or if they are exposed to weathering and erosion on the surface of the earth. The oldest rocks dated are over 3 billion (yes, 3,000,000,000 !) years old. They are now exposed on the earth's surface and depending on how rapid weathering and erosion is may last several 1000 more years.
How big can rocks get? Peter+Kevin
Peter and Kevin, How big do you want them? Rocks in the ground cover large areas, but geologists do not necessarily consider them rocks. We term them rock formations when they cover large areas. There is a neat State Park in Missouri that has large rounded masses of granite (an igneous rock) the size of elephants sitting on the rock outcrop. This is at Elephant Rock State Park. Normally, the rocks that we see weathering out of a rock outcrop vary in size from very small stones to pieces as large as your family car.
Ch 12
Contact the authors of Rockhounding Arkansas revised Feb 2000
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