Power Tools for Technical Communication: Headings Practice

In this lab, you compose headings for a technical document:
  1. Copy the text below this box, and paste it into your preferred word-processing software.
  2. Add headings at the appropriate points in the text and at the appropriate levels (use first-, second-, or third-level headings).
  3. You are welcome to use other fonts or other typographical effects, but bold on these headings works just fine.
  4. Put your name, Headings Practice: Print, and the date on this document, and either print it out or show it on screen to your instructor, or send it be e-mail attachment to your instructor.

Continental drift is the scientific theory which explains how continents have moved in geologic time. It also explains how mountains form and why volcanoes and earthquakes occur. The positions of the continents affect climate and the ability of species to move from one place to another.

The earth is composed of three layers: (1) a thin, rigid, brittle crust; (2) a hotter, denser, semi-solid mantle; and (3) a core made up of molten rock surrounding a very dense, solid center. There are two types of crust: oceanic and continental. Oceanic crust is made of heavy basaltic rock; it is relatively thin about 3 miles (5 km) thick. Continental crust is made of light granitic rock; it is 20 miles (32 km) thick on average and can be up to 120 miles (200 km) thick under high mountains or the centers of old continents. Its deep roots under mountains allows them float high.

Tectonic plates are massive irregularly shaped slabs of crust that move together with the cool, rigid upper part of the mantle; plates are 50 miles (80 km) thick on average. They are often composed of both continental and oceanic crust. The earth today is divided into 15 large and small plates, from hundreds to thousands of miles across (see Figure 1). South America and the South Atlantic Ocean make up one large plate. Central America and the Caribbean Sea make up a small plate.

Tectonic plates move in relation to one another. How they do this is not fully understood. The rising, cooling off and sinking of molten rock in the lower mantle probably propels the plates above. Most of the action in geology, including volcanoes and earthquakes, takes place at the boundaries between plates. There are three types of boundaries: (1) divergent, (2) convergent, and (3) transform.

A divergent boundary occurs between plates that are pulling apart from one another. As they do, they leave a weak line in the crust where magma can erupt through, creating a ridge of new crust. The new crust continues to pull away from the boundary in both directions, allowing more crust to form along the ridge. Most of the earth's divergent boundaries are below the oceans. The undersea spreading ridges that they form circle the globe like the stitches on a baseball. The Mid-Atlantic Ridge is a spreading ridge that runs the length of the Atlantic Ocean. A divergent boundary that occurs between continental plates forms a rift. Eventually the two land masses break apart. The Rift Valley of East Africa is a continental rift. Eventually, it will break off East Africa from the rest of Africa.

A convergent or colliding boundary occurs between plates that are moving toward one another. When one or both of the plates boundaries are made of oceanic crust, one plate sinks under the other in a process called subduction. When spreading oceanic crust collides with continental crust, it sinks into a trench under the continental crust, forcing it up into mountains. The Nazca plate of oceanic crust is colliding with the western boundary of the South American plate, forcing up the Andes Mountains. When spreading oceanic crust collides with another plate of oceanic crust, one of the two sinks into a trench under the other. Undersea volcanoes are often spawned in the process. If they grow large enough, they may become an arc of volcanic islands at the plate boundary. The Australian plate is subducting under the Eurasian plate, forming the volcanic Indonesian Archipelago. When continental crust collides with another plate of continental crust, both plates buckle, forming mountains. The Indian plate is colliding with the Eurasian plate, forming the Himalayan Mountains.

A transform boundary occurs between plates that are sliding past one another. They are most common on the ocean floor, though the San Andres fault in California is a transform boundary between the Pacific and North American plates. Earthquakes are common along transform boundaries.

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