Question 6 through 11
We currently believe that the earth’s crust the rocky part of the earth is composed of several large, rigid plates. These plates are being created at some edges and being destroyed at others. They’re also moving across the earth. This theory is called plate tectonics. It was first put forth in 1963 by a Canadian geophysicist by the name of Tuzo Wilson.
Tuzo Wilson was instrumental in advancing the theory of plate tectonics. He suggested that the Hawaiian and other volcanic island chains might have formed as a result of the movement of a plate over a motionless “hotspot” in the earth’s mantle. Hundreds of studies have proved that Wilson was right. However, in the early 1960s, his idea was considered so radical that his “hotspot” manuscript was initially rejected by all the major international scientific journals.
Basically, plates are areas of the earth’s crust that move as a unit. At the present time, there are eight large plates, as well as a similar number of smaller plates.
According to the theory of plate tectonics, a plate has three kinds of boundaries with other plates: oceanic ridges, oceanic trenches, and transform faults. Most of the world’s earthquakes and volcanoes occur at plate boundaries. This is what you’d expect because plate boundaries are where a great deal of friction and stress occur.
At plate boundaries, a couple of things can happen. One is that rock is forced up from the mantle in molten form as lava at ridges. Another is that rock is melted and forced back into the mantle at trenches. This process of rock being “swallowed” or forced back into the earth’s mantle is called subduction. During subduction, as a plate dives into the depths, we think part of it finds its way back to the surface in the form of volcanoes.
The theory of plate tectonics and the discovery of sea floor spreading have confirmed the theory of continental drift, the movement of continents. Sea floor spreading was discovered in the North Atlantic, and soon afterward in all other oceans.
What we found is that in the areas around oceanic ridges the deep sea floor is formed by rising lava, which then spreads out sideways in both directions.
So, does the spreading of the ocean floor mean that the surface of the earth is increasing? No, not in the least. Sea floor spreading doesn’t cause an increase in the earth’s surface. And why not? Because the lava that rises and spreads from the oceanic ridges sinks again elsewhere in subduction zones, which are nearly identical with the ocean trenches.
Subduction zones are areas of frequent earthquakes and are usually associated with the rows of volcanic islands that accompany the oceanic trenches. Subduction is currently happening beneath island arcs, like Japan. Subduction is also taking place on the Pacific Northwest Coast of North America, beyond the end of the San Andreas Fault. This is where a subducted plate is thought to have disappeared beneath the North American plate in recent geological time, leaving the volcanoes of the Cascade Range as evidence of its past existence.
Questions 12 through 17.
Listen to part of a talk in an anthropology class.
Human beings, like all animals, are territorial. The center of our territory is the home. Think about it. The home is where we spend most of our time. We begin and end our day there.
We eat and sleep and bathe … and relax … and play with our children there … and are most comfortable there. We keep our possessions there. We furnish our homes as an expression of our taste. We say and generally believe that “There’s no place like home” and “Home is where the heart is.”
The human habit of building homes has a long history. Anthropologists think home building began with very simple round huts, similar to the shelters still built in parts of Africa today. Round huts probably started out like this: tree branches were leaned up against one another like the ribs of an umbrella; then the frame was covered with leaves or animal skins.
It’s from these round, tent–like structures that the first solid dwellings probably evolved. As the tents became larger and more elaborate, they became the center of family life, and the place where the family’s possessions could be kept. In other words, these structures became home, the heart of the human territory. Round huts progressed from being temporary shelters, made of wood and skins and leaves, into stronger, more permanent structures built of stone.
But even though stone structures were stronger, there was still a problem: the round shape made it difficult to combine it with other structures. This difficulty was overcome with the development of the cube, or box shape. The box shape was a major development in home construction. By making the sides of the house rectangular, and then covering the four walls with a roof, it was possible to place structures next to one another, and to join them with doorways. Thus, the room was invented.
After the room came the multi-unit dwelling: the apartment house. Nearly ten thousand years ago, this method of building led to the construction of clusters of rectangular buildings that made up the first complex human settlements. Today the box shape not only survives but also remains the basis of our domestic dwellings.
The homes of today still contain some ancient features. Around the house itself there’s an outer perimeter the symbolic boundary of the ancient home territory. Today the boundary is often marked by a barrier, like a fence, a wall, or a hedge. Inside the boundary, we find the yard where we keep our dogs and the garden, where we like our ancestors grow a few fruits and vegetables. We surround our home with a grass lawn, like our ancestors surrounded theirs with pastures for their livestock.
Today, humans are more sociable than in ancient times, and so we allow others to enter the home territory, especially the outer boundary. For example, we let people bring letters and packages to our door. Sometimes we let outsiders enter the first of the private areas, the hallway, which is inside the solid wall of the home itself. But beyond the hallway, there are more boundaries. Each room you come to becomes more private and less available to outsiders.
Guests are allowed to enter the living room. Closer friends can go farther. We tell our closest friends to “make yourself at home” and allow them into the rooms where we cook and eat and pursue our hobbies. But … up the stairs … or down the hall … somewhere less accessible in our home this is where we find the bedrooms and bathrooms, the most private rooms of our home. These rooms are where we, the owners of the territory, feel most secure. This is where we retreat, like any animal to its den, whenever we are at our most vulnerable for example, when we’re sleeping, bathing, or when we’re sick.