Since the Cretaceous period to the Eocene epoch of the Paleogene period, about 96-45 million BC, Australia became separated from Antarctica, slowly moving north across the Pacific Ocean towards Asia. Where the two continental plates met, one was pushed below the other, creating a subduction zone to the southeast of the Asian landmass. As the ocean lithosphere - the rigid outer layer of Earth - was drawn down into the mantle and melted, new magma was produced, resulting in large amounts of volcanic activity.
Now, in 100 million AD, Australia's short life as a single continent is over, and it has finally fused with the southeastern edge of Asia and later the northeastern edge (combining with a bit of northwestern North America as well). Seafloor sediments and rock between the two landmasses have been compressed, sheared, ground together and thrust up into a massive mountain chain. This new chain exceeds the proportions of the Himalayas, the highest mountain range of the Quaternary.
Like the Himalayas in their time, these new mountains continue to rise. As the tectonic plates crush against one another, they simultaneously compress the rock downwards into Earth's mantle and upwards into the sky. Further compression has raised a large block of Southeast Asia to form the Great Plateau, the broadest tract of uplands on the surface of the planet. This immense plateau, surrounded by mountains, towers over the shallow shelf seas which cover much of the landmass.
Newly-formed mountains are sharp and jagged. It takes time for the constant assault of rain, wind, frost and running water to erode them into rounded shapes. In 100 million AD, the Himalayas are mere hills - undulation in the center of the continent. The Great Plateau, on the other hand, consists of ranges of pointed pinnacles and knife-edged crests dropping away into slopes of fragmented rock and scree. The valleys and basins between the ridges have filled with newly-eroded debris and formed upland plains, surrounded by peaks reaching up to 33,000 feet (10,000 meters) - higher than any mountains of the past.
How will life survive at this altitude? The climate of the weather-beaten peaks of the Great Plateau will certainly be harsh, but Earth during 100 million AD is warm and volcanic activity has thrown large amounts of carbon dioxide into the atmosphere, making survival easier. There are ample resources for life to flourish.
The Great Plateau, this system of high plains and basins, hemmed in by the highest mountains in the world, is not the dry, cold desert one might expect. Back during the reign of humanity, high-altitude mountain systems such as the Himalayas were home to little more than hardy desert herbs, shrubs and small rodents. Not so the valleys and plains of the Great Plateau, 100 million years on. These are rolling grasslands.
At the outer edges of the Great Plateau, the steep, debris-covered slopes are swept by winds bringing seasonal rains up from the Shallow Seas. The heavy rainfall and loose soil make for an unstable surface, prone to mudslides and rock falls. However, in many areas the surface is stabilized by plant life evolved to cope with just such conditions.
The oceanward slope of the Great Plateau is green with true grasses. Ridges and banks of vegetation undulate down into the layer of cloud drifting up from the sea. Beyond a narrow coastal plain, sunlight glints on the crest of the waves.
Earthquakes are common, as the plateau is still being pushed up, and it is on the center of the the meating-point of multiple techtonic plates.