There are a lot of reasons why people of color don't go into the Earth Sciences and other disciplines. I'm going to outline the reasons I know. Some of these reasons have been touched on already--so I don't feel like I'm writing the bible on this. Probably too long . . .
The nerd factor is a strong deterrent--both culture and lifestyle.
While not all scientists and historians have poor social skills, enough do to make the idea of working with these folks unattractive. Also its a luxury to not have to care much about personal appearance in the sense of being reasonably fashionable or kept. In that generally people will not think less of you because of this. Also a lot of people are uncomfortable with smart people of color. Want a pop culture confirmation?
Study Weird Al's video "White and Nerdy"
http://www.youtube.com/watch?v=-xEzGIuY7kw&feature=relatedThe black dudes come into his obviously upscale, though "white bread' world at the start of the video. Every thing the title character does is upscale. Who plays badminton? Who can afford a Segway? Who can afford scads of action figures? Yeah, White and Nerdy wants to be friends with the gangstas ie black dudes who make vulgar hand signs. This video shows that being nerdy is a positive thing because even though the gangstas flip W&N off W&N is financially successful, extremely smart and seemingly secure. One thing that bothers me about this video is that I know Native Americans who play ping-pong, Latina/os who are into D&D, African-Americans who know pi to a thousand places . . . but they don't fit into the reality here. Only white guys in American mythology/popular culture can be smart and successful and it doesn't matter if they're clueless, cuz they make the rules, run the world. Nor does it matter that the minority people in the video are gangstas drinking 40 oz malt liquor out of a paper bag, instead of any number of other depictions of black or whatever people.
Most people of color I know put a lot of value on family and extended family. Which is not to say others don't but it's my white friends who are most apt to write off their family--not visit them or speak to them for years, not know what family members are doing, etc. The lifestyle of sciences often asks that people give up close family ties. And the culture supports this. So you'd be asking a person of color to give up a cultural value to become a scientist.
While poor people often don't have enough to eat, that's not always the case. Both my parent grew up poor but always had food. What's more evident with a lot of poor is lack of resources, access to resources and experiences that expose a person to knowledge and ability to negotiate different environments. While a person who is well off can have this lack of exposure also, it's less detrimental to them because they can choice to 'slum' or not. A poor person generally does not have that choice. And a person of color is going to be more likely to have to negotiate more different class and cultural worlds because of family and work worlds if they are to be 'successful' in the wider world.
The minority students you know and teach may be middle class but chances are they are one generation away from being working class or poor. Chances are they have working class or poor relatives. And just because they went to the same schools as the white kids does not mean that they necessarily have the same access to resources or have the same comfort level with school, education and the cultural at large.
Another issue is paying for school. I don't know for all other minorities but for Native American students there are some pretty wonderful grant programs for health careers.
Another issue is what the education means to the family and community. Many communities ask their best and brightest to take on careers that give back to the community in some way--one reason why health careers are popular--or to be a role model--lawyer, doctor. If you could show how a degree/career in Earth Sciences could help a person 'give back' to the community that you'd have a leg up.
Rock Prof if you have no idea why your recruitment methods work, here's some idea. Although I always felt intellectually prepared for school, culturally I felt like standing in the threshold. Even with professors I liked. Part of this is because I always had to meet my professors in their cultural comfort zone. They never had to venture out of theirs to meet me in mine. Or I would have been happy meeting in the middle. If you want to just recruit people who have dark skin or a different ethnicity, without looking at and adapting the culture of your discipline, department or school, I can't see that the situation will change.
Even though minority students might not 'do' science differently, when we look at the discipline discretely, they are often going to bring in a different set of values. If y'all can't adjust somewhat they won't come. But don't take my word for it. You should read some of the literature on why minority student stay out of the sciences, or why companies can't retain minority employees and/or poll the students you try to recruit.
Here's a big reason, when we study science in this country, its white men that get credit for discoveries, explorations. They are the big name scientists. This is disheartening, when one knows for example without the contributions of women, computers would not exist as they do today. Here's one example
http://abcnews.go.com/Technology/Story?id=3951187&page=1This study generally also ignores the contributions of other cultures. What this does is create a disconnect for people of color/women. I know in my home and community there are smart capable people, yet my reality is not reflected in the education system, where at best my people are bit players at worse they are invisible.
This subject is such a hard one to write about as it's easy for some one to say--well white people are poor too, or well all white people are not nerdy. All I can stay to that is yes, but you're missing the point. If the ticket for inclusion in the sciences is that a person of color has to look like (ie dress, mannerisms, . . .), act like, think like the people who populate most of the sciences, it's just not going to happen. And what's even more insidious is that professors can say, in all honesty, that they don't try to keep people of color out. Yet the problem is that they don't try to create a culture that is welcoming.