AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Question why do igneous intrusions occur in specific areas related to geosynclines and orogenesis.
Answer Ivan,
Early geologists attempted to explain the origin of mountain ranges and igneous activity by proposing an hypothesis called geosynclinal theory and the orogenic cycle. Many of the proponents of this hypothesis were European and naturally they found most of their evidence for their thinking in the Alps. In general, the hypothesis envisioned a long period of crustal sinking to form long linear troughs that filled with sediments. When this trough became deep enough, the 'geosyncline' became unstable and a final phase of orogenesis started. In this phase, the sediments in the deepest part of the trough melted and the margins of the trough began to close like a vise causing the whole thing to fold and buckle. With nowhere to go but up, a mountain range was born. The new mountain range had a core of highly metamorphosed rocks and granite sheathed in folded sedimentary rocks.
Failure to identify the driving force of all of this activity was a serious flaw in the hypothesis and geologists worked for a century to try to resolve this last big question. At the time, it was widely believed that the crust could rise and fall but certainly not move laterally from place to place. There were a few doubters of the accepted explanation of orogenesis but these geologists were generally ignored or worse. Early in the 20th century German geologist proposed a 'crazy' idea that the continents 'floated' on the mantle and shifted around with time. He had an even wilder idea that continents could actually break apart or aggregate together to form new continents.
Then, in the 1960s, everything changed. Geologists and geophysiscists began to recognize that old Alfred Wegener was right and the new theory of plate tectonics displaced the older geosynclinal theory. This did not happen without a fight of course. Generations of geologists had been trained to interpret igneous activity and mountain building through the lens of the old theory. The last large geologic community to hold onto these older theories was in the Soviet Union. Eventually, even they had to admit that plate tectonic theory explained the origin of mountains and igneous activity better than geosynclinal theory.
So the short answer to your question is, there is no such thing as a geosyncline. The more general term orogenesis may still have some validity in the sense that it simply refers to mountain building.
The longer answer is that mountain ranges form by at least three major mechanisms. First, the longest and tallest mountain ranges on earth form where two oceanic plates diverge. The mid-Atlantic ridge is a good example of a mountain range that is thousands of kilometers long, many kilometers tall, and composed mainly of primitive rocks called basalt and diabase. Second, when two plates collide, either an oceanic and a continental or two continental plates, the leading edges of the plates buckle into mountain ranges and the high heat flow causes the deepest rocks in these linear collision zones to melt. Finally, as plates pass over stationary 'hot spots' in the mantle, mountains are formed by thermal uplift and intrusion of igneous rocks. The Hawaiian Island chain is a good example of this.
