Question
I have an oak tree and I have rock around the base around 15 feet in diameter.  
Small marble rock.  How close can I come up to the tree and how thick can I
have the rock? I'm just trying to keep weed growth down.

Answer
The use of rock around trees should be avoided. While rock has some limited applications as a ground cover, most rock creates an alkaline environment and most trees prefer an acidic soil. Volcanic rock is particularly detrimental to trees; not only is it alkaline, but it抯 darker color causes it to hold excessive heat.

Rock over landscape fabric is a common mulching material often considered as low maintenance. It is the preferred material for non-plant areas. Rock mulch has the advantage over wood/bark chips that it does not blow, float, or require additional amounts every few years as it decomposes.

Rock mulch can become a heat sink, creating a significantly warmer afternoon, evening, and nighttime microclimate. Outdoor people space adjacent to rock mulch will be warmer and less comfortable for summertime afternoon and evening use. In planting beds, rock mulch increases temperatures and may increase water requirement,

Tree Mulching recommendations:

Use organic mulch such as wood chips or compost. Use wood chips that have been composted for at least 3 months. A mix of fine and coarse material is best.
Apply composted material to the soil surface and top with coarser and fresher material. Incorporation into the soil is not recommended or necessary.
Mulch should be about 2-4 inches deep. Two inches for poorly drained soils. Do not exceed 6 inches in depth.
The larger the mulched area the better, but it should reach at least to the tree's drip line (the outer perimeter of the branches).
Do not use geotextile landscape fabrics beneath the mulch, as this prevents the decomposing mulch from mixing with the soil. Plus the weeds just grow on top and through the fabric.
Keep mulch at least 6 inches away from the tree trunk.
Avoid disrupting the mulch with annual flowers. If plants are desired, use perennial ground covers such as vinca or English ivy.