Earth's hard outer layer is called the crust. It is made up of large interlocking slabs called tectonic plates. The plates fit together like puzzle pieces far beneath our feet. Fold mountains are created where two or more plates are pushed together. At these boundaries, rocks are warped and folded into hills and mountains.
Fold mountains are created through a process called orogeny. It takes millions of years to create a fold mountain, but you can mimic it in seconds. Cover a table with a tablecloth, or place a rug flat on the floor. Now push the edge of the tablecloth or rug. You will see wrinkles develop and fold on top of each other.
There is one huge difference between rock folds and cloth folds. In the tablecloth experiment, the table itself does not fold. In the creation of mountains, Earth's crust itself is warped into folded forms.
Himalayas, Andes and Alps Are Fold Mountains
Fold mountains are the most common type of mountain in the world. Some of the most famous active ranges are the Himalayas, Andes, and Alps.
The Himalayas stretch through the borders of China, Bhutan, Nepal, India, and Pakistan. The crust beneath the Himalayas is still in the process of being folded. Here, the Indian tectonic plate is pushing into the Eurasian plate.
The Andes are the world's longest mountain chain. They stretch along the entire west coast of South America. Here, the Nazca plate is moving down below the South American plate. The Andes are mostly being folded up from the thicker rocks of the South American plate.
The Alps stretch across Italy, Austria, Germany, Switzerland, and France. Here, the tiny Adriatic microplate is crashing into the much larger Eurasian plate. The mountains include rocks that were once part of the ocean floor. They were lifted up over time in the process of folding.
Not all fold mountains are soaring peaks. The Appalachians, stretching along North America's East Coast, are generally low, gentle slopes. Long ago, the Appalachians were taller than the Himalayas. However, millions of years of erosion have taken their toll. Today, some of the highest peaks of the Appalachians are less than a third the height of the Himalayas' Mount Everest.
Domes and Basins Are Folds, too
Fold mountains are defined by complex geologic forms known as folds. There are many different types of folds. Scientists usually categorize them by their shape. Do they have sharp turns or gentle curves? Do they fold inward or outward?
A fold mountain usually has more than one type of fold. Anticlines and synclines are the most common up-and-down folds. An anticline is shaped like a question mark, with the oldest rocks in the center of the fold. A syncline is shaped like the letter "U," with the youngest rocks in the center of the fold.
Domes and basins are often considered types of folds. A dome is a series of symmetrical anticlines, roughly shaped like half a sphere. Like an anticline, the oldest rocks in a dome are found in the center. A basin is a dip in Earth's surface. Like a syncline, a basin has its youngest rocks in its center.
Other types of fold include:
- monoclines. A monocline is a type of fold in which all rock layers dip in the same direction.
- chevron. A chevron is a fold where rock layers look like zig-zags.
- slump. A slump fold is the result of slope failure (a type of landslide). The slope failure happened when sediments were soft. This occurred before they became a single mass of rock. As the sediments transformed into stone, they became a slump.
- ptygmatic. Ptygmatic folds are a type of slump fold. They are created where the folding material is less solid than the material around it. Many ptygmatic folds are created as rock melts and pushes into another rock layer.
- disharmonic. Disharmonic folds describe mixed rock formations. Here, different rock layers have different fold shapes.
Photograph by James P. Blair, National Geographic
very old.
layers of rock that have folded into a hill or crest.
to arrange by specific type or characteristic.
zig-zag or V-shaped pattern.
crash.
complicated.
to press together in a smaller space.
curving inward.
thick layer of Earth that sits beneath continents.
area where two or more tectonic plates bump into each other. Also called a collision zone.
curving outward.
having parts or molecules that are packed closely together.
rock formation in which different rock layers have different fold shapes.
areas of the Earth's crust that have been bent and forced up by movement of tectonic plates.
person who studies the physical formations of the Earth.
study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.
land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).
slant, slope, or dip.
to thrust or bring into.
to change into stone or rock.
downward movement of rock, soil, and other material.
rock that has transformed its chemical qualities from igneous or sedimentary.
to copy another organism's appearance or behavior.
inorganic material that has a characteristic chemical composition and specific crystal structure.
step-shaped fold in a rock formation in which all rock layers gently dip in the same direction.
landmass that forms as tectonic plates interact with each other.
series or chain of mountains that are close together.
large mass or fold of rock that has been thrust from its position by tectonic activity. Also called a thrust sheet.
thin layer of the Earth that sits beneath ocean basins.
process of a specific mountain range or ranges being formed.
the way mountains are formed.
layer of rock visible above the surface of the Earth.
(300 million years ago) ancient supercontinent that contained all present-day continents and began to break up about 200 million years ago.
the very top.
rock formation (fold) created where the folding material is much more viscous than the material surrounding it.
something that is left over.
natural substance composed of solid mineral matter.
having an irregular or jagged surface.
(sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.
rock formed from fragments of other rocks or the remains of plants or animals.
slant, either upward or downward, from a straight or flat path.
rock formation (fold) formed by the collapse of soft sediments on the edge of a continental boundary.
round object.
physical or mental factor (or set of factors) that disturbs the body's normal state of functioning or ability.
to pull downward or beneath something.
ancient, giant landmass that split apart to form all the continents we know today.
able to be influenced to behave a certain way.
having the same arrangement of parts on either side.
layers of rock that have folded to create a dip or area between hills.
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
amount of loss or suffering from an event.
elevation of the Earth's surface due to tectonic or other natural activity.
liquid that is thick and sticky.
necessary or very important.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
to bend out of shape.
