Concept of Geosynclines by Geographers
Think of a book that different authors have written over centuries. Some ideas remain unchanged, while others evolve as new evidence emerges. The concept of geosynclines is similar—various geographers have contributed their theories to explain how these long, narrow water depressions transform into majestic mountains.
So, let’s take a journey through the minds of some great geographers and see how their ideas shaped our understanding of geosynclines.
1. The Foundation: Hall and Dana’s Contribution
📌 James Hall & James Dwight Dana—two pioneering geologists—were the first to recognize geosynclines as precursors to folded mountains.
🟢 Hall’s Contribution:
- He provided evidence that geosynclines and folded mountains are directly related.
- He noticed that continuous sedimentation leads to subsidence, but surprisingly, the depth of the geosyncline remains the same.
- This means sedimentation and subsidence occur at the same rate, maintaining a shallow water depth despite accumulating thick layers of sediments.
🟢 Dana’s Contribution:
- Dana confirmed that the rocks in folded mountains are marine in origin (formed in water bodies).
- He named these water bodies ‘geosynclines’—long, narrow, shallow seas where sediments accumulate before being compressed into mountains.
💡 Analogy: Imagine you are pouring sand into a glass of water. Even though you add more sand, the water level doesn’t rise much because the bottom keeps sinking under the weight. This is exactly what Hall and Dana described!
2. Haug’s Perspective: Deep vs. Shallow Debate
📌 The French geologist Émile Haug expanded on this idea but introduced a key difference:
- He believed geosynclines were deep water bodies, not shallow ones.
- He even drew paleogeographical maps, showing long, narrow oceanic tracts eventually turning into folded mountains.
🚨 Counterargument:
Haug’s idea of deep water geosynclines was later refuted because the marine fossils found in folded mountains belonged to shallow-water organisms.
🔍 This means geosynclines could not have been deep oceans but shallow, subsiding basins where sediments accumulated.
According to Haug there is a systematic sedimentation in the geosynclines where the littoral margins have coarser sediments, the finer sediments are deposited in the centre. Moreover not all geosynclines get converted into folded mountains.
3. J.W. Evans: Geosynclines Are Not Fixed in Shape
📌 Evans took a flexible approach, stating that geosynclines can change shape and form depending on environmental conditions.
- They can be narrow or wide.
- They can exist between two landmasses, in front of mountains, or near plateaus.
✅ However, one thing remains constant—geosynclines always undergo sedimentation and subsidence, eventually leading to mountain formation through compression.
4. Schuchert’s Classification of Geosynclines
📌 Schuchert categorized geosynclines based on their size, location, and evolutionary history into three types:
1️⃣ Monogeosyncline:
- Long, narrow, and shallow.
- It undergoes only one cycle of sedimentation and mountain building.
- Example: The Appalachian Mountains.
2️⃣ Polygeosyncline:
- Longer and wider than monogeosynclines.
- It undergoes multiple cycles of mountain building.
- Example: The Himalayas.
3️⃣ Mesogeosyncline:
- Very long and narrow ocean basins, surrounded by continents.
- They are characterized by great abyssal depth and may pass through several geosynclinals phases of folding.
- Example: The Tethys Sea (which later formed the Himalayas).
5. Arthur Holmes: The Force Behind Geosynclines
📌 Arthur Holmes moved beyond just describing geosynclines—he explained why they form using internal geological processes. You must have gone through the Convection current Hypothesis by Arthur Holmes in the chapter of Continental Drift.
🔥 Four Main Causes of Geosyncline Formation
1️⃣ Migration of Magma:
- According to Holmes the crust of the earth is composed of 3 shells of rocks viz. outer layer of granodiorite (10 to 12 km), intermediate layer of amphibolites (20-25km), and the lower layer of eclogite and some peridotite- Migration of magmas from intermediate layers to neighboring areas causes collapse and subsidence of upper or outer layer and thus is formed a geosyncline.
2️⃣ Metamorphism:
- The rocks of the lower layers of the crust are metamorphosed due to compression caused by converging convective currents. This metamorphism increases density of the rocks, with the result the lower layer of the crust is subjected to subsidence and thus a geosyncline is formed.
3️⃣ Compression:
- Some geosynclines are formed due to compression and resultant subsidence of outer layer of the crust caused by convergent convective current
4️⃣ Thinning of Sialic Layer:
- If the sialic layer is stretched apart due to tensile forces exerted by diverging convective currents. This process causes thinning of sialic layer which results into creation of geosyncline.
💡 Analogy: Imagine pressing on a soft sponge—it compresses, bends, and deforms. This is what happens when tectonic forces act on the Earth’s crust!
6. Duster’s Classification: Location-Based Geosynclines
📌 Duster classified geosynclines based on where they are found:
1️⃣ Intercontinental Geosynclines:
- Always situated between two continents or landmasses
- Example: The Tethys Sea (which existed between the Indian and Eurasian plates).
2️⃣ Circum-Continental Geosynclines:
- Located along the margins of continents.
- Example: The Andes and Rockies, formed along the Pacific margins.
3️⃣ Circum-Oceanic Geosynclines:
- Found along the margins of oceans.
- Example: The Japan Trench.
7. Stille’s Classification: Volcanic vs. Non-Volcanic Geosynclines
📌 Hans Stille divided geosynclines based on the presence of amount of volcanic materials:
1️⃣ Eugeosyncline:
- Found far from the continental crust.
- Contains thick deposits of volcanic rocks like basalt and lava flows.
- Eugeosynclinal rocks include thick sequence of greywacke, chert, slate and submarine lavas.
- Here the deposits are more deformed and metamorphosed and intruded by small to large igneous plutons
- Example: The Pacific Ocean margins.
2️⃣ Miogeosyncline:
- Found along continental margins on continental curst.
- Mostly contains sedimentary rocks like sandstone, limestone, and shale.
- Example: The Atlantic Coast of North and South America.
