Continental Drift Theory

Continental drift describes one of the earliest ways geologists thought continents moved over time. According to Alfred Wegener, the proponent of the Continental Drift Theory, continental landmasses were "drifting" across the Earth, sometimes plowing through oceans and into each other. He called this movement continental drift. Wegener was convinced that all of Earth's continents were once part of an enormous, single landmass called Pangaea.

Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their positions as the continents we recognize today. Modern day scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth's lifespan. These include Pannotia, which formed about 600 million years ago, and Rodina, which existed more than a billion years ago.

Tectonic activity scientists did not accept Wegener's theory of continental drift. One of the elements lacking in the theory was the mechanism for how it works- why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. But this is not the case. Today, we know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. The continents are still moving today. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. The processes of seafloor spreading, rift valley formation, and subduction where heavier tectonic plates sink beneath lighter ones, were not well established in the 1960s. These processes were the main geologic forces behind what Wegener recognized as continental drift.

Theory of Plate Tectonics

Major Tectonic Plates Map

Plate tectonics is a theory which suggests that Earth's outer shell is divided into several plates which hover over the mantle, the stony inner layer above the core. The reason for plate tectonics moving is due to the convection in the mantle, where hot material near the Earth's core rises and colder mantle rocks sinks. The movement forms three types of tectonic boundaries which affect Indonesia:

convergent plates which move into each other;
divergent plates which move apart; and
transform plates which slide past each other horizontally

Convergent Boundaries

Convergent boundaries form when an oceanic plate goes under a landmass. As a result, the edge of the continental plate may fold to form a mountain range, while the oceanic plate subducts and the rock which melts in the Earth's hot interior may cause earthquakes on its way up and finally forming volcanic eruptions as it reaches the surface. The bend between the oceanic plate and the continental plate tends to form a trench. With all the pressure, folding and bending the plates undergo, the break and slip in the plates cause earthquakes.

The Krakatoa eruption of 1883 was an example of convergent plates movement, where the volcano erupted due to the subduction of the Indo-Australian tectonic plate as it moved northwards towards Asia. A subduction zone is the area where two tectonic plates converge, where one bends and slides under the other, going down the mantle, a layer under the crust.

Transform Boundaries

A transform boundary is where two tectonic plates slide past each other horizontally, thus creating stress when parts of these plates gets stuck at areas where they touch and the rest of the plates continue to move. The stress causes the rock to break or slip meaning they suddenly lurch the plates forward and cause earthquakes. These areas of breakage are called faults, with the majority of Earth's faults found along the transform boundaries in the Ring of Fire.

The Sumatran Fault is a strike-slip fault which extends the entire length of Sumatra and coincides geographically with the volcanic arc. The fault is heavily segmented, with most segments less than 100 km long. The importance of these short segments are that they limit the area that can be affected by one event, and magnitudes tend to be limited to 7.5 or less.

Indonesia's Tectonic Plate Boundaries

Indonesia is considered to be one of the most geologically active regions on Earth. This comes as no surprise as the archipelagic nation is situated within the boundaries of 4 major tectonic plates. Indonesia is located between two continental plates: the Indo-Australian Plate (Sahul Shelf) and the Eurasian Plate (sunda Shelf); and between two oceanic plates: The Pacific Plate and the Philippine Sea Plate.

The subduction of the Indian Plate beneath the Eurasian Plate formed the volcanic arc in Western Indonesia, one of the most seismically active areas on Earth with a long history of powerful eruptions and earthquakes. This chain of active volcanoes formed Sumatra, Java, Bali, and the Lesser Sunda Islands, most of which, particularly Java and Bali, emerged within the last 2-3 million years. The Pacific and Australian plate movements controlled the tectonics of the eastern portion of Indonesia.

Tectonic setting of Indonesia and the surrounding region. Indonesia is highlighted in green. Locations of plates, boundaries and faults are shown. Indonesia lies at the point of convergence between the Indo-Australian, Sunda, Philippine Sea, and Caroline plates. Subduction is prevalent.