In which era of geological time scale, there were two phases of orogenesis experienced on earth surface ?

Question

Accepted Answer

The correct answer is Option 3: Palaeozoic. Here's why:

*   **Why Palaeozoic is Correct:** The Palaeozoic Era is characterized by significant tectonic activity and mountain-building events (orogenesis). Specifically, two major orogenic phases are recognized within the Palaeozoic:

*   **Caledonian Orogeny:** This occurred primarily during the Silurian and Devonian periods. It resulted in the formation of mountain ranges in regions like Scandinavia, Scotland, and eastern North America.
    *   **Variscan (or Hercynian) Orogeny:** This occurred during the late Carboniferous and Permian periods. It led to the formation of mountain ranges across Europe (e.g., the Hercynian Mountains) and parts of North America.

The presence of these two distinct and well-documented orogenic phases firmly establishes the Palaeozoic Era as the period with two major orogenic events.

*   **Why the other options are incorrect:**

*   **Azoic:** The Azoic Eon is a largely obsolete term referring to the earliest period of Earth's history, before the development of life. Due to the lack of significant geological records from this time, it's impossible to identify specific orogenic phases. The Earth's crust was still forming and solidifying, but distinct, well-defined orogenies as seen in later eras are not applicable to the Azoic.

*   **Mesozoic:** While the Mesozoic Era did experience tectonic activity, including the early stages of the breakup of Pangaea and the formation of the Rocky Mountains (Laramide Orogeny, which extended into the Cenozoic), it doesn't have the same clear distinction of *two* major, separate orogenic phases that characterize the Palaeozoic. The tectonic activity was more continuous and less clearly defined into two distinct events.

*   **Cenozoic:** The Cenozoic Era is known for the Alpine orogeny (formation of the Alps, Himalayas, etc.) and the continued development of the Rocky Mountains. While significant orogenesis occurred, it's generally considered a single, ongoing phase of mountain building rather than two distinct, separate events comparable to the Caledonian and Variscan orogenies of the Palaeozoic.