Week IIB. The Rock Cycle Stages

(Teachers guide)

The major processes that turn rocks into soil and again into rock are:

- mountains are born; the movement of the Earth's crust up or down;

- mountains are worn down; the erosion, or "weathering," of mountains into boulders, rocks, sand, and eventually (with the addition of organic material) into soil;

- particles are carried by wind and water and laid down in layers.

- material returns to the interior of the Earth where it is reheated and returned to the surface as magma (liquid rock)

STEP ONE: Mountains are Born

Mountains are born when two continents collide or when magma rises up from the Earth's core. As plates move, the Earth's crust stretches and is crushed, cracked, folded, and crumbled. These movements also open new faults through which magma from the mantle can come out and create a layer of igneous rock.

Boulders fallen from the mountains are slowly worn away by the stream.

STEP TWO: Mountains are Worn Down (weathering)

As soon as they are formed, mountains begin to be worn down, "weathered" by ice, water, and wind into smaller particles. We tend to think of large mountains as eternal, but whole mountain chains have come and gone in Earth's history. The Appalachian Mountains in the United States were once as tall and large as the Rockies, but have been weathered down to their present gentle rises.

The speed at which weathering occurs depends on a number of factors.

Some types of rock are harder than others. Hot and humid climates break down rocks more quickly. Some land is shaped so that water and wind can get at the rock more easily, and areas vary in the presence of organisms and plants that get into cracks in the rocks or live on the surface and make soil. Some soil-making processes called "weathering" are shown in these photographs.

The hard volcanic core of this mountain remains after the surrounding material has eroded away.

Weather and water grind rocks down boulders into silt, sand, and clay.

Physical (or mechanical) weathering occurs when temperature changes cause cracking. Water then gets into the cracks, freezes into ice, and expands to break the rock further. Cycles of cooling and warming are among the most powerful forces for breaking down rocks. But sometimes the material is so hard\to break down that it remains solid while the material all around it has been eroded. This is true of the cores of old volcanoes.

Chemical weathering creates the smaller particles of soil by changing rock's chemical nature. Water from rain and rivers, combined with certain atmospheric gases, forms weak acids that wear down rocks. For example, water combines with carbon dioxide in the air and produces carbonic acid. Carbonic acid can react with some minerals inside rocks to cause disintegration and decomposition.

Biological weathering. Biological organisms promote disintegration too. Lichens and moss are known as true pioneers because they are the first plants to attach to rock surfaces. Later, complex plants send their roots deep into the rocks, causing them to crack. This allows other types of weathering to follow. The decayed remains of these plants become integrated into soil and create a medium for the growth of more microorganisms.

Macroorganisms (for example, insects and rodents) also play a role in disintegrating rock into soil. They burrow or tunnel among small rocks, creating openings in which physical and chemical weathering can take place. Earthworms process tiny particles of rock through their guts and excrete them as soil, changing the pH of the soil as they go. The remains of these animals are eventually decomposed into the soil as well. This disintegration of rocks caused by the action of organisms (micro and macro) and plants is called biological weathering.

STEP THREE: Particles are carried by wind and water and laid down in layers. (Sedimentation)

Glaciers, moving water, and wind carry pieces of rocks that collide and rub against each other, breaking off smaller and smaller particles. These tiny particles are then deposited elsewhere as soil. Small particles and organic matter are carried away by wind and water and set down in layered sediments.

There are a few useful rules of sedimentation that help us "read" the Earth.

- Younger layers are laid down on top of older layers. For example, as we travel into the Grand Canyon in the United States, we go back in history, layer-by-layer, step by step.

- Layers are generally horizontal, and when we see abrupt breaks in the layers, or folds, we know that strong geological forces have shaped the breaks. We can piece together history by finding continuities and discontinuities in the rock formations.

* Intrusion is younger than the stratified layers. Soil formation is occurring all around the globe, but the rate of formation is greater in regions where high temperatures, rainfall, and humidity are prevalent. These conditions promote both chemical weathering because of the abundance of water and biological weathering by supporting plant and animal life.

- Younger layers are laid down on top of older layers. For example, as we travel into the Grand Canyon in the United States, we go back in history, layer by layer, step by step.

The Grand Canyon in the USA

STEP FOUR: Mountains are reborn. Material returns to the interior of the Earth where it is reheated and returned to the surface as magma (liquid rock

This step is described in detail in the next week's work.

For a brief reference, see:

http://neic.usgs.gov/neis/qed/