Location and Setting: Plate Tectonics and Mountains

Concepts and learning outcomes

Students will understand that:

• Mountains tend to form near plate boundaries in long ranges that parallel the boundaries.
• Volcanoes form over subduction zones as continuous arc-shaped chains of volcanoes of different sizes.
  • Mountains composed entirely of volcanic rocks might be, but are not necessarily, volcanoes.
• As plates move around over geologic time, plate boundaries change, resulting in modern day mountain belts that are far from present-day plate boundaries.

Time requirements

Vocabulary

Background for teachers

The resource, Mountains, provides excellent information on mountain-building processes.

Batholiths are made up of large blocks of igneous rock that formed and cooled under the earth's surface (a pluton). Plutons are commonly made up of hard, granite type rocks. Over millions of years the softer surrounding rocks may erode away, leaving a massive block of exposed granite called a batholith. Yosemite's famous Half Dome mountain is just one piece of the massive batholith that makes up almost the entire Sierra Nevada mountain range in California. Batholiths are recognizable by two important features:

• uniform rock types (no layers -- a single piece or mixture of the same type of igneous rock that intruded into the crust as basically a single blob and slowly cooled. It turns out that most batholiths like the Sierra Nevada actually consists of dozens of smaller blobs that intruded in a complicated way over several million years.)
• impressive erosional features (steep, sharp, knife-edge-like faces can only happen in uniformly hard rock. Soft rock can't have such steep slopes and sides and would collapse like a pile of sand.)

Mesas and buttes are landscape features that rise up above the surrounding material, but they are not uplifted in the way that volcanic or folded mountains are. They are erosional features – the remains of higher ground that eroded away around them. Mesas and buttes are almost always “capped” by a dense and difficult to erode layer such as basalt, and their formation is not associated with plate boundaries. These geologic features are pretty easy to identify simply by observation. Mesas and buttes are isolated, steep-sided and usually flat-topped areas of land. Their formation generally begins with a high flat area of land, called a plateau. As the plateau erodes it may gradually form mesas and buttes. Mesas are usually larger than buttes and smaller than plateaus but the distinction between the terms is subtle and not very important.

A mountain’s shape, composition and structure provide important clues to how it formed. Two other aspects of mountains, location and proximity to an active plate boundary can also provide key information about their origins and history. Location places a mountain in relation to geologic processes that are occurring in the earth’s crust. New mountains almost always form at or near plate boundaries and different kinds of boundary interactions produce different kinds of mountains. While new mountains form near plate boundaries, plate boundaries can move. Determining whether a mountain is near or far from the boundary that gave rise to it can provide insights into the movement of the plates over time. Modern day mountain belts that are far from present-day plate boundaries indicate that plate boundaries had been located near by and have moved over geologic time.

Potential Student misconception:

• Plate boundaries are not quite as clear and clean as they are drawn on maps. Often, the boundary between two plates can be more like a broad transition zone. This is true in the Western United States where there is plate boundary deformation throughout the entire area from California all the way to Colorado. The active portion of the plate boundary is currently located along the west coast but historically was much further east. Earthquakes and active mountain building are scattered throughout this entire region.
• Students may think that deformation only occurs at plate boundaries. In fact, deformation caused by interaction between plates occurs in zones that are typically a few hundred kilometers wide, parallel to the boundaries. Mountains form NEAR plate boundaries, but not AT plate boundaries.

Some of the lessons use the terms compress, shear, and tension. If you have not yet introduced these ideas to your class and would like to, you can look them up at: Forces in the Earth. The terms are not necessary for student understanding.

Students record their ideas in the Mountain Building Journal and we provide suggested answers in the Mountain Building Journal: Teacher's Guide.

Activities

Resources used