I'm designing a lower gravity planet that's got enough mass to hold onto an atmosphere but around 75% of Earth's gravity. I read that lower-gravity planets will have more pronounced geological features - so, higher mountains, for example. Does this also mean the crust would be more uneven, with higher and lower elevations and deeper oceans?
Let's use our Solar System as sample to verify if planets with lower gravity have higher mountains, looking at the highest mountains in it.
|Name||Height||Location (surface gravity $m/s^2$)|
|Olympus Mons||72,000 ft (22,000 m)||Mars (3.69)|
|Equatorial Ridge||65,617 ft (20,000 m)||Iapetus (0.22)|
|Boösaule Mons||59,711 ft (18,200 m)||Io (1.79)|
|Ascraeus Mons||49,000 ft (15,000 m)||Mars (3.69)|
|Ionian Mons||41,667 ft (12,700 m)||Io (1.79)|
|Elysium Mons||41,338 ft (12,600 m)||Mars (3.69)|
|Arsia Mons||38,386 ft (11,700 m)||Mars (3.69)|
|Limb Mountain||36,089 ft (11,000 m)||Oberon (0.35)|
|Skadi Mons||35,105 ft (10,700 m)||Venus (8.87)|
|Euboea Montes||34,449 ft (10,500 m)||Io (1.79)|
|Mauna Kea||33,464 ft (10,200 m)||Earth (9.81)|
|Haleakala||29,856 ft (9,100 m)||Earth (9.81)|
|Mount Everest||29,029 ft (8,848 m)||Earth (9.81)|
As you can see Earth is out of the top-10, so it looks like indeed gravity affect the elevation of the relieves on a celestial body: the lower the first, the higher can the second get, as expected from the mountains having to "fight" against the gravity to stay in place and not crumble.
Looking into your question, I found this at the Astronomy Stack Exchange: Does a planet's mass or gravity affect the height of it's mountains?.
There's some mathematical and scientific things in there, if that's what you're searching for. But it seems the general answer is yes. :) Hope that link helps!
(Edit: I think this would be better as a comment, but I don't have the rep yet, sorry)
Looking at Mars, which has lower gravity than Earth, the crust does seem to be more uneven. It's home to both Valles Marineris, the deepest canyon in the solar system, and Olympus Mons, the highest mountain. Io's mountains are higher than Earth's as well. In general, lower gravity allows for the crust to be more uneven and have more extreme features. So, short answer, yes. Higher elevations will be higher, and lower elevations will be lower.
It can do. There are a couple of other variables you may want to consider as well as gravity: temperature and level of volcanic activity.
Anything large enough to be classed as a dwarf planet by definition has enough gravity to bend the matter it is made of approximately into a sphere. How imperfect that is depends on whether the planet is actively working against gravity: Volcanic activity. This comes in two main forms: volcanoes, which give you peaks in their own right (including the mighty Olypmpus Mons on Mars or e.g. Iceland or Gran Canaria on Earth), and tectonic activity, which whilst very slow, is sufficiently powerful to overcome gravity and creates mountain ranges. Apart from volcanic activity, the other way, the other possibility for creating "peaks" is craters formed by bombardment from space, but since you world has oceans I'm assuming it has a fairly dense atmosphere protecting it so only very large asteroids would get through.
The second thing to bear in mind is the temperature. A planet's ability to hold on to water at all is determined by a combination of how strong the gravity is and how warm it is. Given that you want oceans at lower gravity than Earth, it will have to be a cold planet. Notice that Io and Mars do not have oceans as they don't have enough gravity to keep hold of water, so although created by volcanic activity, it leaks away into space. Although you'll here reports of there being water ice on Mars, it is in fact far drier than the driest desert on Earth, and although Io has the strongest surface gravity of any moon, it also has the least amount of water of any known astronomical object in the Solar System.
For the relationship between gravity, temperature and the ability to hold on to water (or indeed an atmosphere) see this wikipedia diagram. To hold on to water your planet needs to have a temperature / gravity somewhere near the top of the blue band (like earth), or in the green band, to the left of Earth (not too far though or your oceans will freeze). This will also ensure you have enough gravity to hold on to oxygen etc. which I assume your inhabitants will breath :-)