Mountains of Io can be divided into five general morphologic types:

MESA
Elevated plain with a relativelyt smooth (at a scale of ~1 km), flat-topped surface.
Type example: Ethiopia Planim
Mesas may be difficult to distinguish from eroded layered plains, which appear separately in this database.

PLATEAU
Elevates plainwith a rugged surface but without a steep or prominent peak.
Type example: Iopolis Planum
Pateaus are the most common morphologic type, making up ~50% of all mountains.

PEAK
Isolated promontory with a relatively simple morphologz rising to a sharp point or a short ridge.
Type example: Western Hi'iaka Montes
Peaks may be asymmetric in shape and are always less than 100 (an usually less than 50) km across. Some peaks could be the partially buried tops of once larger structures.

RIDGE
Elevated structure dominated by one or more prominent linear or arcurate rises.
Type examples: Zaal Montes or Ionian Mons
Ridges are often asymmentric and can have planar flanks. The second most common morphologic type, maing up ~25% of all mountains.

MASSIF
Elevated structure with a rugged and complex surface morphology rising to one or more peaks.
Type examples: Boösaule Montes and Tohil Montes
They have a rugged irregular appereance and can be asymmetric in cross section.

MIXED TYPES
At least two mountains are complex structures featuring two or more of the above morphologies. These are classified on their dominant morphology.

UNCLASSIFIED
Several mountains are not classified due to low image resolution or inability to discern their morphology.

VOLCANIC MOUNTAINS
Few mountains are identified as having primarz volcanic morphologies.
Type example: unnamed at 30S 245W
Other candidate volcanic mountains are the twoo unusual large circular mesa-like plateaus Inachus and Apis Tholus. They could represent volcanic material extruded from the central vents or they could represent erosional remnants of volcanic plains.

CANDIDATE MOUNTAINS
Numerous features are not classified as mountains due to low image resolution or inability to discern their morphology, but suspected to be elevated areas. These are listed separately and require further study, or, further, better resolution images.

LAYERED PLAINS AND FRACTURED PLAINS
These features are smooth topped elevated areas with basal scarps. Such features can be eroded remains of older lava flow layers or could be created by other processes. Layered plains are less then 1 km high.