Notes on Basic Geology
Notes created & information organization based on the book:
The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at for additional resource information
Plate Tectonics - Page 5

OLD Structures

Cratons & Orogens are the large structural units within continental crust. A Craton is any portion of the earths crust which has attained tectonic and isostatic stability. (This is another way of saying that the rocks are no longer undergoing change by tectonic forces.) The deformations present must be of ancient origin.

Orogens typically surround cratons and are elongated regions of intensely folded and faulted rock. They are the remains of ancient mountain ranges that were formed when cratons collided. They are typically thicker than the cratons they surround and many have not obtained isostatic equilibrium.

Continental Shields

This is a group of cratons and orogens that has reached isostatic equilibrium. In North America has a huge continental shield. It happens to show itself in areas of Canada so it is sometimes called the Canadian Shield. In the United States it is mostly covered with sedimentary layers of rock and not visible.

When we measure the age of the cratons in the shield they are all over 2 billion years old. The small cratons in this shield were like "mini-continents" and about 1.6 billion years ago they welded themselves together forming their outer orogens and creating the current shield structure.

Basically when two orogens collided they formed their central craton.

Continental Margins
active margin
("divergent" margin)

There are 5 defined types of continental margins today.

1.) Passive Margin: these are margins isolated by distance from plate boundaries. they form in the middle of a plate. They form as a spreading event occurs. When a new ocean basin forms by the rifting of continental crust, the margins on each side are passive margins.

The Red Sea is such a margin today. It probably looks much like a very young Atlantic Ocean did millions of years ago. Down its length is a spreading center, and it is pushing Arabia in one direction and the continent of Africa in the other.

1.) At some time in the past it was formed across the land.

2.) Created a rift valley.

3.) The ends sank below sea level and water entered the rift.

4.) There was a short time of salt precipitation as the ocean evaporated in its shallow days, followed by normal sedimentation.

5.) This is the stage of the Red Sea today.

active margin
("convergent" margin)

ocean - continent

2.) Continental Convergent Margins: this is a margin where a continental plate was in contact with an oceanic subducting plate. The Andes mountains and the southern coast of South America are a good example. The wet subduction zone fosters the formation of andesitic magma which rise under the continent and produces a chain of volcanoes. (A continental volcanic arc.)

This type of zone produces a very distinctive "finger print". Intense deformation at the margin, specific metamorphism and deformation of the sediment in the trench (burial metamorphism). This produces a melange lower in the zone. (high pressure - low temperature metamorphics (blue schist). Further inland the heated magma thickens the crust and adds heat. Hence a second metamorphic region is also created, but here it is regional metamorphism.

Thus a Continental Convergent Margin produces two nearly parallel lines of metamorphic rock. You will also find the remains of the old feed stock magma (batholiths). After the overburden has weathered away, the metamorphics and batholiths will be visible and providing a clear picture of what used to be a continental volcanic arc.

active margin
("convergent" margin)

continent - continent

3.) A Continental Collision Margin: this what remains when two continents, each on its own plate, collide. Continents are already thick bodies, so when two collide there is even more uplift, producing highlands that can create new weather patterns.

Just before two continental plates collide, they have margins filled with "wash-off" sediment. As they come together this trapped sediment is pushed up forming a mountain system that is highly folded and thrust faulted (because these are compressive forces and not tensional).

These complex shaped systems form orogens that are folded, bent, mangled, mutilated, and spent. Metamorphic and igneous rocks are always present from the wet initial sediment, to the intense pressure of the collision.

The mountains get most of their material from the shallow continental shelf and the deeper continental slope and rise. Hence the sediments often show fossils of both deep water and shallow water types.

A final tale rock found in some of these collisions is serpentine. This is a alteration products of basalt oceanic crust. Some of this material can be swept up as the two continental plates collide.

A distinctive feature is that the new mountains lie inland, often surrounded by a large land mass.

active margin
(transform margin)

4.) Accreted Terrane Margins: these types of margins are sort of the "junk yards" of what remains. They include bits and pieces of island arcs, torn fragments of continents created by transform faulting, and any other bits a pieces of crust not already part of a continent.

They are by far the most complex margins. They can also be described as pieces of continental material that are too buoyant to be lost in a subduction zone.

Examples today include Taiwan, the Philippines, and Indonesia. These are all ancient parts of old continents with complex structures. Each fragment is called a terrain.

The recognition of this type of margin is fairly new in geology. One of the most studied areas in the coastal region along the Pacific Ocean and the Canadian-Alaskan shore. There are a number of such margins in this region.