The Rock cycle

Igneous rocks A rock "sight-seeing" tour around the world might well begin in the Hawaiian Islands with a visit to the world's most active and best-studied volcano, Kilauea. This famous volcano, like others across the globe, is the birthplace of thousands of tons of rocks.

The rocks are "born" as volcanic eruptions eject massive amounts of magma onto Earth's surface. Remember that magma is nothing more than a large body of hot liquid rock and minerals. As the magma cools into a solid rock around the base of the volcano, it can be collected as samples of rocks such as basalt, pumice, or obsidian.

Mount Rushmore is made of Granite

Metamorphic rock What does it mean to be "rock solid?" The phrase is used to describe something that cannot easily be changed. But metamorphic rocks are rocks that have been changed within the Earth. Because rocks are not easily changed, metamorphic rocks must develop in environments where heat and pressure are intense and extreme. These extreme conditions are only present deep inside the Earth.

Contact Metamorphism CONTACT METAMORPHISM occurs when a rock is exposed to hot magma inside the Earth. The intense heat of the magma alters the rock, often causing its minerals to recrystallize. Thus, the new rock has new or larger mineral crystals than the older rock. Sometimes, the hot magma will even introduce new minerals and modify the entire chemical composition of the original rock. The area of rock affected by contact metamorphism is appropriately known as the baked zone.

Regional Metamorphism REGIONAL METAMORPHISM, on the other hand, occurs during the formation of mountain ranges. As tectonic plates collide and converge, intense pressure deforms and alters sedimentary and igneous rocks already buried in the Earth. The mountain ranges of east Greenland are an example of where this has taken place. Often, folds or curves in the rocks indicate the direction of the intense pressure.

Sedimentary Rock Traveling east to Zion National Park in Utah, tourists would find the well-known Navajo Sandstone Formation. It looks like a towering sand dune that has been permanently solidified against the landscape. The sandstone rocks that make up the Navajo Sandstone Formation began millions of years ago as tiny grains of sand in a prehistoric desert. Over time, winds carried the sand and deposited it into giant sand dunes. The pressure of each new sand deposit caused the sand underneath to compact and cement together into solid rock.

Sedimentary Rock

shale A common clastic sedimentary rock is shale, the most abundant of all sedimentary rocks. Shale forms when mud and clay harden. Because the clay sediments are extremely small, they settle out slowly. In fact, shale formations can take about 5 million years to form.

limestone The most common chemical sedimentary rock is limestone. Most limestone rocks are organic — that is, they develop from the remains of organisms. Much limestone is made up of marine animals and contains pieces of shells, corals, and mollusks. Another chemical sedimentary rock is coal, which is often found in layers with other sedimentary rocks. As plant remains are deposited in layers and slowly alter into carbon over millions of years, they form coal.

Limestone cliffs, limestone with fossils

Although these sandstone rocks began as a collection of individual grains of sand, the sand grains themselves had to come from somewhere and something else. Where might these sand grains may have originated?

Geologists theorize that the sand grains may have broken off a larger rock formed earlier by a volcanic eruption. Or they may have broken off older sandstone rocks. Because the Earth is so dynamic, rocks are always changing. Minerals that make up rocks are constantly moved and acted upon by the environment. Environmental agents change one type of rock into an entirely new rock. The web of environmental processes that forms and changes rocks is known as the ROCK CYCLE.

The Rock Cycle

Another view of the Rock Cycle

Rock cycle To gain an idea of how the rock cycle works, the world rock tour that began in Hawaii might end in east Greenland. Millions of years ago there, magma deep inside the Earth forced its way into another rock and solidified. This created an igneous rock. Then, intense pressure in the Earth caused the rock to up-heave, fold, and crumple, until it became an entirely new rock: a metamorphic rock. That new rock now makes up a mountain formation in Greenland.

Activity The rock cycle never ends. I, Rock: Sandy Stone said to her son, "Rocky, I remember when you were just a cute little grain of sand..." Tell the story of a rock's progression through the rock cycle.