How Are Rocks Formed

What are rocks and how they are formed?

At the Youth Science Institute we encourage children to take a closer look at all aspects of the world around them, starting with the ground under their feet. Through our Moving Exploding Earth program, children learn about the constant process of forming and breaking down rocks called the Rock Cycle.

Soil is one of the elements of this process. Soil consists of both mineral and organic materials. The organic component comes from the death and decay of plant parts that form humus in the soil. Decaying plants help supply the soil with nitrogen, which is important in forming the green parts of new plants. The mineral component of soil comes from rocks. All rocks are made up of minerals, which are naturally occurring, inorganic substances. Minerals are also important in building plant cells. We get minerals for our own bodies, like calcium, iron, magnesium, potassium, and sulfur from the soil by eating plants or eating other animals that eat plants.

Rocks get broken down into small mineral pieces through a process called weathering. Weather (wind, rain, heat and cold temperatures) gradually break small pieces off rocks and carry these pieces downhill, breaking them up even further as they travel. One of the reasons why the Santa Clara Valley is such an important agricultural area is thousands of years of soil that has built up on the valley floor from the surrounding mountains.

The pieces of rock that form soil differ in size as well as mineral components. Soil scientists call the largest pieces, sand. Medium size pieces are called silt and the smallest soil granules are those of clay. Most soils are a mixture of different size pieces. Over time as these pieces, sediments, build up in layers they become heavy enough to exert enough pressure on the layers below to form rocks. These rocks are called sedimentary rocks. Common examples of sedimentary rocks found in the bay area include sandstone, shale, chert, and limestone. In some cases larger pieces of rock, pebbles get compressed in also an the resulting rocks are call a conglomerate. Concrete is a manmade conglomerate. Shells, bones, or plant parts can also get incorporated into rocks forming fossils. There are many visible shell fossils in the rocks along the river in Alum Rock Park.

Layers of sediment are only one of the parts of the rock cycle. As we in the bay area are too well aware, the earth beneath or feet is not one solid mass. It is made of large pieces, called plates that move around. The reason that the plates move is that they are sitting on a layer of the earth with molten rock, called magma. Occasionally the magma finds a crack or hole in or between plates and makes it’s way to the surface. Magma that has escaped to the surface, we call lava. As lava cools it forms another type of rock called igneous. Some igneous rocks many people are familiar with are obsidian and basalt.

Sometimes lava is forcefully ejected from a volcanic vent and the resulting rocks are full of air pockets like pumice. Pumice is so full of air that pieces of pumice will float in water. Sometimes the magma does not make it all of the way to the surface before it cools and forms rocks. This is how many areas of the Sierra Nevada Mountains were formed. Over time erosion exposes these buried igneous rocks called, granite.

A final piece of the rock cycle also commonly occurs in our area and other areas of the world where oceans meet land. The plates under the ocean tend to be denser than the adjacent land plates. When ocean plates bump up against land plates they tend to push under the land plates. This downward movement is called subduction. Rocks that are on the plate boundaries are subjected to a great deal of pressure and heat during the subduction process. This heat and pressure changes the rocks resulting in the formation of metamorphic rocks. Sandstone rocks become smoother in texture and darker in color and form quartzite rocks. Shale also becomes darker and harder forming slate. Limestone develops the large beautiful crystal structure of marble. Granite crystals melt together forming the wavy lines and larger crystals of gneiss.

So you can see that the earth beneath our feet is involved in a constantly changing process, called the Rock Cycle. Any type of rock can get broken down and pressed to together to form new sedimentary rocks. Any type of rock can get pushed into the earth melted and subjected to pressure to form metamorphic rocks. Any type of rock and get buried even further to the point where it melts completely and then rise to the surface again to form new igneous rocks.

To see examples of all of the these types of rocks and to learn more about the changing nature of our earth, visit the YSI Nature Centers in Alum Rock Park and Vasona Lake County Park.