I have been writing on the geological origins of India's two famous island chains. Coincidentally, I am reading David Quammen's classic book - The Song Of The Dodo- Island Biogeography In An Age Of Extinctions. The focus is about the importance of islands in understanding evolution and extinction. Darwin and Wallace independently got their insights regarding the formation of new species through studying the geographic distribution of fauna on island groups.
Yesterday being World Environment Day it is timely to share this great passage-
A continental island begins with everything, and everything to lose. An oceanic island begins with nothing and everything to gain. Island biogeography, over the past century and a half, has been the scientific record of those gains and losses.
What this implies is that since continental islands are extensions of continents with a low area between them and the main landmass, they already have a terrestrial fauna on them when they become isolated on account of rising seas flooding the low shelf and severing the land connection between. Oceanic islands on the other hand rise from the ocean floor anew, made up either of lava piling up from the sea floor, or by coral growth on undersea volcanic summits. They get colonized by birds flying in from the mainland an reptiles and small mammals arriving with flotsam. These are pioneer fauna which diversify in their unique way on this new land.
Highly Recommended.
Yesterday being World Environment Day it is timely to share this great passage-
A continental island begins with everything, and everything to lose. An oceanic island begins with nothing and everything to gain. Island biogeography, over the past century and a half, has been the scientific record of those gains and losses.
What this implies is that since continental islands are extensions of continents with a low area between them and the main landmass, they already have a terrestrial fauna on them when they become isolated on account of rising seas flooding the low shelf and severing the land connection between. Oceanic islands on the other hand rise from the ocean floor anew, made up either of lava piling up from the sea floor, or by coral growth on undersea volcanic summits. They get colonized by birds flying in from the mainland an reptiles and small mammals arriving with flotsam. These are pioneer fauna which diversify in their unique way on this new land.
Highly Recommended.
Hi Suvrat,
ReplyDeleteI had not heard of this writer before, but after reading your blog I grabbed a copy of the book, Song of the Dodo from library. It is a superb book indeed. Big in scope and so richly detailed. Thanks for recommending it!
I have a question that comes to mind while reading. Say during ice age, land bridge connection occurs between mainland and an island (Case in point Australia and Tasmania or India/Srilanka). Animals live all over the land area. When ice age recede or sea level rise, water floods in separating the land masses. Is this process quick or does it happen over decades/centuries? Does animals living in the soon to be submerged area get time to move gradually to higher ground (either mainland or future island) or do they perish in a sort of biblical deluge? I am trying to understand how fast the sea water floods in and how animals react to that.
I can imagine even humans living at those times and area encountering this situation. Imagine a family living exactly between present day Australia and Tasmania. Are they doomed or they get time to "relocate"?
Thank you, Mohan
Hi Mohan- yeah, its a great book.
ReplyDeleteregarding your question. Sea level rise due to melting of polar ice caps is quite gradual enough, say few cms per year. So flooding of the shelf and isolation of land as islands will take place over a few centuries. Plenty of time for resident animals to migrate.
Thank you for the answer. My house is in a beach town, I have seen the sea at the same distance from my home all my life. It is hard for me to even imagine that sea getting closer (or getting farther) by even say 100m..
ReplyDeleteMohan
There have been deep sea fossils found high in the Alps and deep sea rocks found high in the Rockies. Then, how could Western North America have been comprised of continental crust when those deep sea fossils and rocks formed? The rocks were in the Interior Seaway. Then how could ocean crust have been overlying continental crust? Is there anywhere today where this is found?
ReplyDeleteLaurence- deep sea need not mean oceanic crust. it can just mean deeper water (and deep sea sediments)on subsided continental crust, conditions which existed periodically in the Paleozoic in the Rocky mountain region. regarding your second question, slices of oceanic crust can be thrust over continental crust in collisional settings. This is observed in many places for example in Himalayas and Cypress..
ReplyDeleteThank you Suvrat Kher. That would be epicontinental. Still, I think that the idea of plate tectonics has been overplayed as a Deus ex machina unnecessarily and incorrectly. For example, exotic terannes need not require plate tectonics to explain them.It would be sufficient to understand that the climate fluctuates more frequently than currently thought and periodically. When the climate changes one degree at the equator it changes about five degrees at the poles. There are also a very large number of finds of continental crust throughout the modern oceans. See NCGT Journal.New Concepts In Global Tectonics.
ReplyDeleteExotic terrain means a crustal block has been transported from somewhere else. Climate fluctuations cannot move crustal blocks. It does require plate movements.
DeleteWhy must the continental crust all form at one time early in Earth history? I suspect that new continents continually arise. For example, why should we not think the mid-ocean ridges won't eventually become new continents? Look at the mountain chains down the Western sides of the Americas. They look like they could have originated as a mid-oceanic ridge.If continental crust is granitic, then why can't that continue to form anew from a volcanic island in the middle of oceanic crust? Granite is made of sedimentary rock that descends and heats up, so why couldn't it still be formed today? This whole idea that continents do not continually form seems like something from early science.
ReplyDeleteContinental crust is forming today too. No geologist thinks otherwise. And yes a small fraction by partial melting of sedimentary rocks. But the bulk of granite forms in the upper mantle by partial melting and fractional crystallization of basalts.
DeleteMid oceanic ridges are oceanic crust which eventually gets subducted. It doesn't become new continents. The mountains down the western sides of the Americas have formed by thrusting of American continental crust and by magmatism and volcanism along a convergent boundary. They did not originate as a mid oceanic ridge.