The Myanmar Earthquake

The tragic, 7.7 magnitude earthquake in Myanmar this week occurred along fault lines between the Indian and Eurasian Plates.  The former continues to push NNE into the latter, forcing up the Himalayas and producing frequent earthquakes in that area.  Adding to tectonic pressure within that region is the subduction of the Indo-Australian Plate beneath the southeast portion of the Eurasian Plate.

While these powerful earthquakes seem to have a predilection for impoverished countries, where the houses and buildings do not meet modern codes, this is a false narrative.  That assumption is derived from the fact that much of the planet remains relatively "underdeveloped."  Of course, our brief time on Earth also fosters that conclusion.

Here in the U.S., most quakes have been relatively mild over the past Century but major, destructive events lie ahead, especially along the West Coast (an Active Margin).  We can contribute to relief efforts and, perhaps, improve our quake prediction accuracy but we will never control tectonic activity. 

Eruption of Mount Agung

Mount Agung, 9944 feet, is in western Bali.  One of 78 active volcanoes in Indonesia, its last major eruption occurred in 1963 but another is expected within the next few weeks.  Earthquakes developed on and near the volcano back in September and the release of smoke and ash has been intermittent throughout October and November.

The volcanic islands along the western and southern edge of Indonesia have formed as the Indo-Australian Plate has been subducting beneath a southeastern extension of the Eurasian Plate.  As it is forced downward, toward the Earth's mantle, the leading edge of the subducting plate melts and plumes of magma push up through the crust of the overriding plate.  Initially producing a volcanic island arc, the ongoing subduction and volcanism has culminated in the large islands that we observe today (Sumatra, Java, Bali and others to their east).

Another major eruption of Mount Agung will likely produce widespread devastation but will not come close to the effects of Mount Toba's eruption, on Sumatra.  That supervolcanic eruption, which occurred 74,000 years ago, temporarily cooled the Earth's climate and had a major impact on the human population of our planet; indeed, a significant percentage of early humans had left Africa and colonized the southern rim of Asia by that time (see The Toba Winter).

Mauritius: A Hybrid Island

Like many volcanic islands across the globe, Mauritius, in the Indian Ocean east of Madagascar, was thought to have formed above a hotspot.  In such cases, a mantle plume pushes upward into the oceanic crust, igniting volcanism and the formation of land that gradually emerges from the sea.  If the oceanic plate is moving above the hotspot, a chain of islands is formed (see The Hawaiian Ridge).

The volcanism that produced Mauritius has occurred on and off for about 9 million years.  Recently, however, as reported in the New York Times this week, a geologist has discovered the presence of continental crust beneath the volcanic rock.  In essence, it appears that a hotspot formed adjacent to or beneath the continental fragment or that oceanic crust has been subducting beneath that fragment to produce the volcanism; the latter seems less likely.

The continental fragment itself apparently broke away from Gondwana as it split up to form the Southern Continents (Africa, South America, Antarctica, India and Australia) some 200 million years ago.  The Madagascar-Seychelles platform, north of Mauritius, is also a continental fragment of Gondwana; such fragments move about the globe as oceans open and close and these exotic terranes may eventually fuse to the edge of the major Continents.

Formation of the Indian Ocean

About 200 million years ago (MYA), the Tethys Sea split Pangea into Laurasia (the northern continents) and Gondwanaland (the southern continents); the latter included Africa, India, Antarctica, South America and Australia.  Since that time, the Indian Ocean has formed as these Southern Continents split from one another.

In the mid Jurassic Period, 150 MYA, the Atlantic Ocean began to form, splitting Europe from North America and Africa from South America.  By the early Cretaceous Period, some 130 MYA, a landmass including Madagascar, the Seychelle Islands and India rifted from the rest of Gondwanaland and, by 100 MYA, Africa split from Antarctica as the Southwest Indian Ridge began to form.  About 85 MYA, India rifted from the Madagascar-Seychelles land mass and Australia began to split from Antarctica along the Southeast Indian Ridge.  Finally, the Mid Indian Ridge (which trends NNW to SSE and connects the Southwest and Southeast Indian Ridges) began spreading as well, accelerating India's movement toward southern Asia; its northwestern extension, known as the Carlsberg Ridge and ending in the Gulf of Aden, began to open about 62 MYA and India finally slammed into Asia 55 MYA, lifting the Himalayas (a process that continues today).

As these four spreading zones continued to open, Africa drifted to the NNW, Antarctica moved southward, India plowed northeastward into Asia and Australia drifted eastward and then northeastward, all surrounding the vast Indian Ocean which has an average depth of 12,900 feet; the portion of the Indian Ocean south of the Southwest and Southeast Indian Ridges is often referred to as the Southern Ocean.  While the Carlsberg Ridge is no longer active, the Southwest, Mid and Southeast Indian Ridges continue to produce oceanic crust; in concert, the African Plate continues to move NNW (lifting the Alps and igniting volcanism in southern Europe) and Australia is drifting NNE.  Most evident are the earthquakes and subduction volcanoes along the western and southern rims of Indonesia, where the Australia Plate dips below the Eurasian Plate.